CHAPTER IX. SENSITIVENESS OF PLANTS TO LIGHT: ITS TRANSMITTED EFFECTS.

 

 

  Uses of heliotropism—Insectivorous and climbing plants not heliotropic—

  Same organ heliotropic at one age and not at another—Extraordinary

  sensitiveness of some plants to light—The effects of light do not

  correspond with its intensity—Effects of previous illumination—Time

  required for the action of light—After-effects of light—Apogeotropism

  acts as soon as light fails—Accuracy with which plants bend to the light—

  This dependent on the illumination of one whole side of the part—Localised

  sensitiveness to light and its transmitted effects—Cotyledons of Phalaris,

  manner of bending—Results of the exclusion² of light from their tips—

  Effects transmitted beneath the surface of the ground—Lateral³ illumination

  of the tip determines the direction of the curvature of the base—

  Cotyledons of Avena, curvature of basal part due to the illumination of

  upper part—Similar results with the hypocotyls of Brassica and Beta—

  Radicles of Sinapis apheliotropic, due to the sensitiveness of their tips—

  Concluding remarks and summary of chapter—Means by which circumnutation

  has been converted into heliotropism or apheliotropism.

NO one can look at the plants growing on a bank or on the borders of a thick wood, and doubt that the young stems and leaves place themselves so that the leaves may be well illuminated⁴. They are thus enabled to decompose⁵ carbonic acid. But the sheath-like cotyledons of some Gramineae, for instance, those of Phalaris, are not green and contain very little starch⁶; from which fact we may infer that they decompose little or no carbonic acid. Nevertheless, they are extremely heliotropic; and this probably serves them in another way, namely, as a guide from the buried seeds through fissures⁷ in the ground or through overlying masses of vegetation, into the light and air. This view [page 450] is strengthened by the fact that with Phalaris and Avena the first true leaf, which is bright green and no doubt decomposes⁸ carbonic acid, exhibits hardly a trace of heliotropism. The heliotropic movements of many other seedlings¹⁰ probably aid them in like manner in emerging from the ground; for apogeotropism by itself would blindly guide them upwards¹¹, against any overlying obstacle.

 

Heliotropism prevails so extensively among the higher plants, that there are extremely few, of which some part, either the stem, flower-peduncle, petiole, or leaf, does not bend towards a lateral light. Drosera rotundifolia is one of the few plants the leaves of which exhibit no trace of heliotropism. Nor could we see any in Dionaea, though the plants were not so carefully observed. Sir J. Hooker exposed the pitchers¹³ of Sarracenia for some time to a lateral light, but they did not bend towards it.* We can understand the reason why these insectivorous plants should not be heliotropic, as they do not live chiefly by decomposing¹⁴ carbonic acid; and it is much more important to them that their leaves should occupy the best position for capturing insects, than that they should be fully¹² exposed to the light.

 

Tendrils, which consist of leaves or of other organs modified, and the stems of twining plants, are, as Mohl long ago remarked, rarely heliotropic; and here again we can see the reason why, for if they had moved towards a lateral light they would have been drawn¹⁵ away from their supports. But some tendrils are apheliotropic, for instance those of Bignonia capreolata

 

* According to F. Kurtz ('Verhandl. des Bot. Vereins der Provinz Brandenburg,' Bd. xx. 1878) the leaves or pitchers of Darlingtonia Californica are strongly apheliotropic. We failed to detect this movement in a plant which we possessed¹⁶ for a short time. [page 451]

 

and of Smilax aspera; and the stems of some plants which climb by rootlets, as those of the Ivy¹⁷ and Tecoma radicans, are likewise apheliotropic, and they thus find a support. The leaves, on the other hand, of most climbing plants are heliotropic; but we could detect no signs of any such movement in those of Mutisia clematis.

 

As heliotropism is so widely prevalent, and as twining plants are distributed throughout the whole vascular¹⁸ series, the apparent absence of any tendency in their stems to bend towards the light, seemed to us so remarkable¹⁹ a fact as to deserve further investigation²⁰, for it implies that heliotropism can be readily eliminated. When twining plants are exposed to a lateral light, their stems go on revolving²¹ or circumnutating about the same spot, without any evident deflection towards the light; but we thought that we might detect some trace of heliotropism by comparing the average rate at which the stems moved to and from the light during their successive revolutions.* Three young plants (about a foot in height) of Ipomoea caerulea and four of I. purpurea, growing in separate pots, were placed on a bright day before a north-east window in a room otherwise darkened, with the tips of their revolving stems fronting the window. When the tip of each plant pointed²² directly from the window, and when again towards it, the times were recorded. This was continued from 6.45 A.M. till a little after 2 P.M. on June 17th. After a few observations we concluded that we could safely estimate the time

 

* Some erroneous statements are unfortunately given on this subject, in 'The Movements and Habits of Climbing Plants,' 1875, pp. 28, 32, 40, and 53. Conclusions were drawn from an insufficient²³ number of observations, for we did not then know at how unequal a rate the stems and tendrils of climbing plants sometimes travel in different parts of the same revolution. [page 452]

 

taken by each semicircle, within a limit of error of at most 5 minutes. Although the rate of movement in different parts of the same revolution varied²⁴ greatly, yet 22 semicircles to the light were completed, each on an average in 73.95 minutes; and 22 semicircles from the light each in 73.5 minutes. It may, therefore, be said that they travelled to and from the light at exactly the same average rate; though probably the accuracy of the result was in part accidental. In the evening the stems were not in the least deflected²⁵ towards the window. Nevertheless, there appears to exist a vestige²⁶ of heliotropism, for with 6 out of the 7 plants, the first semicircle from the light, described in the early morning after they had been subjected to darkness during the night and thus probably rendered more sensitive, required rather more time, and the first semicircle to the light considerably²⁷ less time, than the average. Thus with all 7 plants, taken together, the mean time of the first semicircle in the morning from the light, was 76.8 minutes, instead of 73.5 minutes, which is the mean of all the semicircles during the day from the light; and the mean of the first semicircle to the light was only 63.1, instead of 73.95 minutes, which was the mean of all the semicircles during the day to the light.

 

Similar observations were made on Wistaria Sinensis, and the mean of 9 semicircles from the light was 117 minutes, and of 7 semicircles to the light 122 minutes, and this difference does not exceed the probable limit of error. During the three days of exposure, the shoot did not become at all bent²⁸ towards the window before which it stood. In this case the first semicircle from the light in the early morning of each day, required rather less time for its performance than did the first semicircle to the light; and this result, [page 453] if not accidental, appears to indicate that the shoots retain a trace of an original apheliotropic tendency. With Lonicera brachypoda the semicircles from and to the light differed considerably in time; for 5 semicircles from the light required on a mean 202.4 minutes, and 4 to the light, 229.5 minutes; but the shoot moved very irregularly, and under these circumstances the observations were much too few.

 

It is remarkable that the same part on the same plant may be affected²⁹ by light in a widely different manner at different ages, and as it appears at different seasons. The hypocotyledonous stems of Ipomoea caerulea and purpurea are extremely heliotropic, whilst the stems of older plants, only about a foot in height, are, as we have just seen, almost wholly insensible to light. Sachs states (and we have observed the same fact) that the hypocotyls of the Ivy (Hedera helix) are slightly heliotropic; whereas the stems of plants grown to a few inches in height become so strongly apheliotropic, that they bend at right angles away from the light. Nevertheless, some young plants which had behaved in this manner early in the summer again became distinctly heliotropic in the beginning of September; and the zigzag³⁰ courses of their stems, as they slowly curved towards a north-east window, were traced during 10 days. The stems of very young plants of Tropaeolum majus are highly heliotropic, whilst those of older plants, according to Sachs, are slightly apheliotropic. In all these cases the heliotropism of the very young stems serves to expose the cotyledons, or when the cotyledons are hypogean the first true leaves, fully to the light; and the loss of this power by the older stems, or their becoming apheliotropic, is connected with their habit of climbing.

 

Most seedling⁹ plants are strongly heliotropic, and [page 454] it is no doubt a great advantage to them in their struggle for life to expose their cotyledons to the light as quickly and as fully as possible, for the sake of obtaining carbon. It has been shown in the first chapter that the greater number of seedlings circumnutate largely and rapidly; and as heliotropism consists of modified circumnutation, we are tempted³¹ to look at the high development of these two powers in seedlings as intimately connected. Whether there are any plants which circumnutate slowly and to a small extent, and yet are highly heliotropic, we do not know; but there are several, and there is nothing surprising in this fact, which circumnutate largely and are not at all, or only slightly, heliotropic. Of such cases Drosera rotundifolia offers an excellent instance. The stolons of the strawberry circumnutate almost like the stems of climbing plants, and they are not at all affected by a moderate light; but when exposed late in the summer to a somewhat brighter light they were slightly heliotropic; in sunlight, according to De Vries, they are apheliotropic. Climbing plants circumnutate much more widely than any other plants, yet they are not at all heliotropic.

 

Although the stems of most seedling plants are strongly heliotropic, some few are but slightly heliotropic, without our being able to assign any reason. This is the case with the hypocotyl of Cassia tora, and we were struck with the same fact with some other seedlings, for instance, those of Reseda odorata. With respect to the degree of sensitiveness of the more sensitive kinds, it was shown in the last chapter that seedlings of several species, placed before a north-east window protected by several blinds, and exposed in the rear to the diffused³² light of the room, moved with unerring certainty towards the window, although [page 455] it was impossible to judge, excepting by the shadow cast by an upright pencil on a white card, on which side most light entered, so that the excess on one side must have been extremely small.

 

A pot with seedlings of Phalaris Canariensis, which had been raised in darkness, was placed in a completely darkened room, at 12 feet from a very small lamp. After 3 h. the cotyledons were doubtfully curved towards the light, and after 7 h. 40 m. from the first exposure, they were all plainly, though slightly, curved towards the lamp. Now, at this distance of 12 feet, the light was so obscure that we could not see the seedlings themselves, nor read the large Roman figures on the white face of a watch, nor see a pencil line on paper, but could just distinguish a line made with Indian ink. It is a more surprising fact that no visible shadow was cast by a pencil held upright on a white card; the seedlings, therefore, were acted on by a difference in the illumination of their two sides, which the human eye could not distinguish. On another occasion even a less degree of light acted, for some cotyledons of Phalaris became slightly curved towards the same lamp at a distance of 20 feet; at this distance we could not see a circular dot 2.29 mm. (.09 inch) in diameter made with Indian ink on white paper, though we could just see a dot 3.56 mm. (.14 inch) in diameter; yet a dot of the former size appears large when seen in the light.*

 

We next tried how small a beam of light would act; as this bears on light serving as a guide to seedlings whilst they emerge through fissured³⁴ or encumbered³⁵ ground. A pot with seedlings of Phalaris was covered

 

* Strasburger says ('Wirkung des Lichtes auf Schw?rmsporen,' 1878, p. 52), that the spores³⁶ of Haematococcus moved to a light which only just sufficed to allow middle-sized type to be read. [page 456]

 

by a tin-vessel, having on one side a circular hole 1.23 mm. in diameter (i.e. a little less than the 1/20th of an inch); and the box was placed in front of a paraffin lamp and on another occasion in front of a window; and both times the seedlings were manifestly bent after a few hours towards the little hole.

 

A more severe trial was now made; little tubes of very thin glass, closed at their upper ends and coated with black varnish³⁷, were slipped over the cotyledons of Phalaris (which had germinated³⁸ in darkness) and just fitted them. Narrow stripes of the varnish had been previously³⁹ scraped off one side, through which alone light could enter; and their dimensions were afterwards measured under the microscope. As a control experiment, similar unvarnished and transparent⁴⁰ tubes were tried, and they did not prevent the cotyledons bending towards the light. Two cotyledons were placed before a south-west window, one of which was illuminated by a stripe in the varnish, only .004 inch (0.1 mm.) in breadth and .016 inch (0.4 mm.) in length; and the other by a stripe .008 inch in breadth and .06 inch in length. The seedlings were examined after an exposure of 7 h. 40 m., and were found to be manifestly bowed towards the light. Some other cotyledons were at the same time treated similarly, excepting that the little stripes were directed not to the sky, but in such a manner that they received only the diffused light from the room; and these cotyledons did not become at all bowed. Seven other cotyledons were illuminated through narrow, but comparatively long, cleared stripes in the varnish—namely, in breadth between .01 and .026 inch, and in length between .15 and .3 inch; and these all became bowed to the side, by which light entered through the stripes, whether these were directed towards the sky or to one side of [page 457] the room. That light passing through a hole only .004 inch in breadth by .016 in length, should induce curvature, seems to us a surprising fact.

 

Before we knew how extremely sensitive the cotyledons of Phalaris were to light, we endeavoured to trace their circumnutation in darkness by the aid of a small wax taper⁴¹, held for a minute or two at each observation in nearly the same position, a little on the left side in front of the vertical⁴² glass on which the tracing was made. The seedlings were thus observed seventeen times in the course of the day, at intervals⁴⁴ of from half to three-quarters of an hour; and late in the evening we were surprised to find that all the 29 cotyledons were greatly curved and pointed towards the vertical glass, a little to the left where the taper had been held. The tracings showed that they had travelled in zigzag lines. Thus, an exposure to a feeble light for a very short time at the above specified⁴⁵ intervals, sufficed to induce well-marked heliotropism. An analogous⁴⁶ case was observed with the hypocotyls of Solanum lycopersicum. We at first attributed this result to the after-effects of the light on each occasion; but since reading Wiesner's observations,* which will be referred to in the last chapter, we cannot doubt that an intermittent⁴⁷ light is more efficacious than a continuous one, as plants are especially sensitive to any contrast in its amount.

 

The cotyledons of Phalaris bend much more slowly towards a very obscure light than towards a bright one. Thus, in the experiments with seedlings placed in a dark room at 12 feet from a very small lamp, they were just perceptibly and doubtfully curved towards it after 3 h., and only slightly, yet certainly, after 4 h.

 

* 'Sitz. der k. Akad. der Wissensch.' (Vienna), Jan. 1880, p. 12. [page 458]

 

After 8 h. 40 m. the chords of their arcs were deflected from the perpendicular⁴⁹ by an average angle of only 16o. Had the light been bright, they would have become much more curved in between 1 and 2 h. Several trials were made with seedlings placed at various distances from a small lamp in a dark room; but we will give only one trial. Six pots were placed at distances of 2, 4, 8, 12, 16, and 20 feet from the lamp, before which they were left for 4 h. As light decreases in a geometrical ratio, the seedlings in the 2nd pot received 1/4th, those in the 3rd pot 1/16th, those in the 4th 1/36th, those in the 5th 1/64th, and those in the 6th 1/100th of the light received by the seedlings in the first or nearest pot. Therefore it might have been expected that there would have been an immense difference in the degree of their heliotropic curvature in the several pots; and there was a well-marked difference between those which stood nearest and furthest from the lamp, but the difference in each successive pair of pots was extremely small. In order to avoid prejudice, we asked three persons, who knew nothing about the experiment, to arrange the pots in order according to the degree of curvature of the cotyledons. The first person arranged them in proper order, but doubted long between the 12 feet and 16 feet pots; yet these two received light in the proportion of 36 to 64. The second person also arranged them properly, but doubted between the 8 feet and 12 feet pots, which received light in the proportion of 16 to 36. The third person arranged them in wrong order, and doubted about four of the pots. This evidence shows conclusively⁵⁰ how little the curvature of the seedlings differed in the successive pots, in comparison with the great difference in the amount of light which they received; and it should be noted⁵² that there was no [page 459] excess of superfluous⁵³ light, for the cotyledons became but little and slowly curved even in the nearest pot. Close to the 6th pot, at the distance of 20 feet from the lamp, the light allowed us just to distinguish a dot 3.56 mm. (.14 inch) in diameter, made with Indian ink on white paper, but not a dot 2.29 mm. (.09 inch) in diameter.

 

The degree of curvature of the cotyledons of Phalaris within a given time, depends not merely on the amount of lateral light which they may then receive, but on that which they have previously received from above and on all sides. Analogous facts have been given with respect to the nyctitropic and periodic movements of plants. Of two pots containing seedlings of Phalaris which had germinated in darkness, one was still kept in the dark, and the other was exposed (Sept. 26th) to the light in a greenhouse during a cloudy day and on the following bright morning. On this morning (27th), at 10.30 A.M., both pots were placed in a box, blackened within and open in front, before a north-east window, protected by a linen⁵⁵ and muslin blind and by a towel, so that but little light was admitted, though the sky was bright. Whenever the pots were looked at, this was done as quickly as possible, and the cotyledons were then held transversely with respect to the light, so that their curvature could not have been thus increased or diminished. After 50 m. the seedlings which had previously been kept in darkness, were perhaps, and after 70 m. were certainly, curved, though very slightly, towards the window. After 85 m. some of the seedlings, which had previously been illuminated, were perhaps a little affected, and after 100 m. some of the younger ones were certainly a little curved towards the light. At this time (i.e. after 100 m.) there was a plain difference [page 460] in the curvature of the seedlings in the two pots. After 2 h. 12 m. the chords of the arcs of four of the most strongly curved seedlings in each pot were measured, and the mean angle from the perpendicular of those which had previously been kept in darkness was 19o, and of those which had previously been illuminated only 7o. Nor did this difference diminish during two additional hours. As a check, the seedlings in both pots were then placed in complete darkness for two hours, in order that apogeotropism should act on them; and those in the one pot which were little curved became in this time almost completely upright, whilst the more curved ones in the other pot still remained plainly curved.

 

Two days afterwards the experiment was repeated, with the sole difference that even less light was admitted through the window, as it was protected by a linen and muslin blind and by two towels; the sky, moreover, was somewhat less bright. The result was the same as before, excepting that everything occurred rather slower. The seedlings which had been previously kept in darkness were not in the least curved after 54 m., but were so after 70 m. Those which had previously been illuminated were not at all affected until 130 m. had elapsed, and then only slightly. After 145 m. some of the seedlings in this latter pot were certainly curved towards the light; and there was now a plain difference between the two pots. After 3 h. 45 m. the chords of the arcs of 3 seedlings in each pot were measured, and the mean angle from the perpendicular was 16o for those in the pot which had previously been kept in darkness, and only 5o for those which had previously been illuminated.

 

The curvature of the cotyledons of Phalaris towards a lateral light is therefore certainly influenced by the [page 461] degree to which they have been previously illuminated. We shall presently see that the influence of light on their bending continues for a short time after the light has been extinguished. These facts, as well as that of the curvature not increasing or decreasing in nearly the same ratio with that of the amount of light which they receive, as shown in the trials with the plants before the lamp, all indicate that light acts on them as a stimulus⁵⁶, in somewhat the same manner as on the nervous system of animals, and not in a direct manner on the cells or cell-walls which by their contraction⁵⁷ or expansion cause the curvature.

 

It has already been incidentally shown how slowly the cotyledons of Phalaris bend towards a very dim light; but when they were placed before a bright paraffin lamp their tips were all curved rectangularly towards it in 2 h. 20 m. The hypocotyls of Solanum lycopersicum had bent in the morning at right angles towards a north-east window. At 1 P.M. (Oct. 21st) the pot was turned round, so that the seedlings now pointed from the light, but by 5 P.M. they had reversed their curvature and again pointed to the light. They had thus passed through 180o in 4 h., having in the morning previously passed through about 90o. But the reversal of the first half of the curvature will have been aided by apogeotropism. Similar cases were observed with other seedlings, for instance, with those of Sinapis alba.

 

We attempted to ascertain⁵⁸ in how short a time light acted on the cotyledons of Phalaris, but this was difficult on account of their rapid circumnutating movement; moreover, they differ much in sensibility, according to age; nevertheless, some of our observations are worth giving. Pots with seedlings were [page 462] placed under a microscope provided with an eye-piece micrometer, of which each division equalled 1/500th of an inch (0.051 mm.); and they were at first illuminated by light from a paraffin lamp passing through a solution of bichromate of potassium, which does not induce heliotropism. Thus the direction in which the cotyledons were circumnutating could be observed independently of any action from the light; and they could be made, by turning round the pots, to circumnutate transversely to the line in which the light would strike them, as soon as the solution was removed. The fact that the direction of the circumnutating movement might change at any moment, and thus the plant might bend either towards or from the lamp independently of the action of the light, gave an element of uncertainty⁵⁹ to the results. After the solution had been removed, five seedlings which were circumnutating transversely to the line of light, began to move towards it, in 6, 4, 7 1/2, 6, and 9 minutes. In one of these cases, the apex⁶⁰ of the cotyledon crossed five of the divisions of the micrometer (i.e. 1/100th of an inch, or 0.254 mm.) towards the light in 3 m. Of two seedlings which were moving directly from the light at the time when the solution was removed, one began to move towards it in 13 m., and the other in 15 m. This latter seedling was observed for more than an hour and continued to move towards the light; it crossed at one time 5 divisions of the micrometer (0.254 mm.) in 2 m. 30 s. In all these cases, the movement towards the light was extremely unequal in rate, and the cotyledons often remained almost stationary⁶¹ for some minutes, and two of them retrograded a little. Another seedling which was circumnutating transversely to the line of light, moved towards it in 4 m. after the solution was removed; it then remained [page 463] almost stationary for 10 m.; then crossed 5 divisions of the micrometer in 6 m.; and then 8 divisions in 11m. This unequal rate of movement, interrupted by pauses, and at first with occasional retrogressions, accords well with our conclusion that heliotropism consists of modified circumnutation.

 

In order to observe how long the after-effects of light lasted, a pot with seedlings of Phalaris, which had germinated in darkness, was placed at 10.40 A.M. before a north-east window, being protected on all other sides from the light; and the movement of a cotyledon was traced on a horizontal glass. It circumnutated about the same space for the first 24 m., and during the next 1 h. 33 m. moved rapidly towards the light. The light was now (i.e. after 1 h. 57 m.) completely excluded, but the cotyledon continued bending in the same direction as before, certainly for more than 15 m., probably for about 27 m. The doubt arose from the necessity of not looking at the seedlings often, and thus exposing them, though momentarily, to the light. This same seedling was now kept in the dark, until 2.18 P.M., by which time it had reacquired through apogeotropism its original upright position, when it was again exposed to the light from a clouded sky. By 3 P.M. it had moved a very short distance towards the light, but during the next 45 m. travelled quickly towards it. After this exposure of 1 h. 27 m. to a rather dull sky, the light was again completely excluded, but the cotyledon continued to bend in the same direction as before for 14 m. within a very small limit of error. It was then placed in the dark, and it now moved backwards⁶², so that after 1 h. 7 m. it stood close to where it had started from at 2.18 P.M. These observations show that the cotyledons of Phalaris, after being exposed to a lateral [page 464] light, continue to bend in the same direction for between a quarter and half an hour.

 

In the two experiments just given, the cotyledons moved backwards or from the window shortly after being subjected to darkness; and whilst tracing the circumnutation of various kinds of seedlings exposed to a lateral light, we repeatedly observed that late in the evening, as the light waned⁶³, they moved from it. This fact is shown in some of the diagrams given in the last chapter. We wished therefore to learn whether this was wholly due to apogeotropism, or whether an organ after bending towards the light tended from any other cause to bend from it, as soon as the light failed. Accordingly, two pots of seedling Phalaris and one pot of seedling Brassica were exposed for 8 h. before a paraffin lamp, by which time the cotyledons of the former and the hypocotyls of the latter were bent rectangularly towards the light. The pots were now quickly laid horizontally, so that the upper parts of the cotyledons and of the hypocotyls of 9 seedlings projected vertically⁶⁴ upwards, as proved by a plumb-line. In this position they could not be acted on by apogeotropism, and if they possessed any tendency to straighten themselves or to bend in opposition⁶⁵ to their former heliotropic curvature, this would be exhibited, for it would be opposed at first very slightly by apogeotropism. They were kept in the dark for 4 h., during which time they were twice looked at; but no uniform bending in opposition to their former heliotropic curvature could be detected. We have said uniform bending, because they circumnutated in their new position, and after 2 h. were inclined in different directions (between 4o and 11o) from the perpendicular. Their directions were also changed after two additional hours, and again on the following morning. We may [page 465] therefore conclude that the bending back of plants from a light, when this becomes obscure or is extinguished, is wholly due to apogeotropism.*

 

In our various experiments we were often struck with the accuracy with which seedlings pointed to a light although of small size. To test this, many seedlings of Phalaris, which had germinated in darkness in a very narrow box several feet in length, were placed in a darkened room near to and in front of a lamp having a small cylindrical⁶⁶ wick. The cotyledons at the two ends and in the central part of the box, would therefore have to bend in widely different directions in order to point to the light. After they had become rectangularly bent, a long white thread was stretched by two persons, close over and parallel, first to one and then to another cotyledon; and the thread was found in almost every case actually to intersect the small circular wick of the now extinguished lamp. The deviation⁶⁷ from accuracy never exceeded, as far as we could judge, a degree or two. This extreme accuracy seems at first surprising, but is not really so, for an upright cylindrical stem, whatever its position may be with respect to the light, would have exactly half its circumference⁶⁸ illuminated and half in shadow; and as the difference in illumination of the two sides is the exciting cause of heliotropism, a cylinder⁶⁹ would naturally bend with much accuracy towards the light. The cotyledons, however, of Phalaris are not cylindrical, but oval in section; and the longer axis⁷⁰ was to the shorter axis (in the one which was measured) as 100 to 70. Nevertheless, no difference could be

 

* It appears from a reference in Wiesner ('Die Undulirende Nutation der Internodien,' p. 7), that H. Müller of Thurgau found that a stem which is bending heliotropically is at the same time striving, through apogeotropism, to raise itself into a vertical position. [page 466]

 

detected in the accuracy of their bending, whether they stood with their broad or narrow sides facing the light, or in any intermediate position; and so it was with the cotyledons of Avena sativa, which are likewise oval in section. Now, a little reflection will show that in whatever position the cotyledons may stand, there will be a line of greatest illumination, exactly fronting the light, and on each side of this line an equal amount of light will be received; but if the oval stands obliquely⁷¹ with respect to the light, this will be diffused over a wider surface on one side of the central line than on the other. We may therefore infer that the same amount of light, whether diffused over a wider surface or concentrated on a smaller surface, produces exactly the same effect; for the cotyledons in the long narrow box stood in all sorts of positions with reference to the light, yet all pointed truly towards it.

 

That the bending of the cotyledons to the light depends on the illumination of one whole side or on the obscuration of the whole opposite side, and not on a narrow longitudinal zone in the line of the light being affected, was shown by the effects of painting longitudinally with Indian ink one side of five cotyledons of Phalaris. These were then placed on a table near to a south-west window, and the painted half was directed either to the right or left. The result was that instead of bending in a direct line towards the window, they were deflected from the window and towards the unpainted side, by the following angles, 35o, 83o, 31o, 43o, and 39o. It should be remarked that it was hardly possible to paint one-half accurately⁷², or to place all the seedlings which are oval in section in quite the same position relatively⁷³ to the light; and this will account for the differences in the angles. Five coty- [page 467] ledons of Avena were also painted in the same manner, but with greater care; and they were laterally⁷⁴ deflected from the line of the window, towards the unpainted side, by the following angles, 44o, 44o, 55o, 51o, and 57o. This deflection of the cotyledons from the window is intelligible⁷⁵, for the whole unpainted side must have received some light, whereas the opposite and painted side received none; but a narrow zone on the unpainted side directly in front of the window will have received most light, and all the hinder parts (half an oval in section) less and less light in varying degrees; and we may conclude that the angle of deflection is the resultant of the action of the light over the whole of the unpainted side.

 

It should have been premised that painting with Indian ink does not injure plants, at least within several hours; and it could injure them only by stopping respiration⁷⁶. To ascertain whether injury was thus soon caused, the upper halves of 8 cotyledons of Avena were thickly coated with transparent matter,—4 with gum, and 4 with gelatine; they were placed in the morning before a window, and by the evening they were normally bowed towards the light, although the coatings now consisted of dry crusts of gum and gelatine. Moreover, if the seedlings which were painted longitudinally with Indian ink had been injured on the painted side, the opposite side would have gone on growing, and they would consequently have become bowed towards the painted side; whereas the curvature was always, as we have seen, in the opposite direction, or towards the unpainted side which was exposed to the light. We witnessed the effects of injuring longitudinally one side of the cotyledons of Avena and Phalaris; for before we knew that grease was highly injurious to them, several were painted down one side [page 468] with a mixture of oil and lamp-black, and were then exposed before a window; others similarly treated were afterwards tried in darkness. These cotyledons soon became plainly bowed towards the blackened side, evidently owing to the grease on this side having checked their growth, whilst growth continued on the opposite side. But it deserves notice that the curvature differed from that caused by light, which ultimately becomes abrupt⁷⁷ near the ground. These seedlings did not afterwards die, but were much injured and grew badly.

 

LOCALISED SENSITIVENESS TO LIGHT, AND ITS TRANSMITTED EFFECTS.

 

Phalaris Canariensis.—Whilst observing the accuracy with which the cotyledons of this plant became bent towards the light of a small lamp, we were impressed with the idea that the uppermost part determined⁷⁸ the direction of the curvature of the lower part. When the cotyledons are exposed to a lateral light, the upper part bends first, and afterwards the bending gradually extends down to the base, and, as we shall presently see, even a little beneath the ground. This holds good with cotyledons from less than .1 inch (one was observed to act in this manner which was only .03 in height) to about .5 of an inch in height; but when they have grown to nearly an inch in height, the basal part, for a length of .15 to .2 of an inch above the ground, ceases to bend. As with young cotyledons the lower part goes on bending, after the upper part has become well arched towards a lateral light, the apex would ultimately point to the ground instead of to the light, did not the upper part reverse its curvature and straighten itself, as [page 469] soon as the upper convex surface of the bowed-down portion received more light than the lower concave surface. The position ultimately assumed by young and upright cotyledons, exposed to light entering obliquely from above through a window, is shown in the accompanying figure (Fig³³. 181); and here it may be seen that the whole upper part has become very nearly straight. When the cotyledons were exposed before a bright lamp, standing⁷⁹ on the same level with them, the upper part, which was at first

 

Fig. 181. Phalaris Canariensis: cotyledons after exposure in a box open on one side in front of a south-west window during 8 h. Curvature towards the light accurately traced. The short horizontal lines show the level of the ground.

 

greatly arched towards the light, became straight and strictly⁸⁰ parallel with the surface of the soil in the pots; the basal part being now rectangularly bent. All this great amount of curvature, together with the subsequent straightening of the upper part, was often effected in a few hours.

 

[After the uppermost part has become bowed a little to the light, its overhanging weight must tend to increase the curvature of the lower part; but any such effect was shown in several ways to be quite insignificant⁸¹. When little caps of tin-foil (hereafter to be described) were placed on the summits of the cotyledons, though this must have added considerably to their weight, the rate or amount of bending was not thus increased. But the best evidence was afforded by placing pots with seedlings of Phalaris before a lamp in such a position, that the cotyledons were horizontally extended and projected at right angles to the line of light. In the course of 5 ? h. they were directed towards the light with their bases bent at right angles; and this abrupt [page 470] curvature could not have been aided in the least by the weight of the upper part, which acted at right angles to the plane of curvature.

 

It will be shown that when the upper halves of the cotyledons of Phalaris and Avena were enclosed in little pipes of tin-foil or of blackened glass, in which case the upper part was mechanically prevented from bending, the lower and unenclosed part did not bend when exposed to a lateral light; and it occurred to us that this fact might be due, not to the exclusion of the light from the upper part, but to some necessity of the bending gradually travelling down the cotyledons, so that unless the upper part first became bent, the lower could not bend, however much it might be stimulated⁸². It was necessary for our purpose to ascertain whether this notion was true, and it was proved false; for the lower halves of several cotyledons became bowed to the light, although their upper halves were enclosed in little glass tubes (not blackened), which prevented, as far as we could judge, their bending. Nevertheless, as the part within the tube might possibly bend a very little, fine rigid⁸³ rods or flat splinters of thin glass were cemented with shellac to one side of the upper part of 15 cotyledons; and in six cases they were in addition tied on with threads. They were thus forced to remain quite straight. The result was that the lower halves of all became bowed to the light, but generally not in so great a degree as the corresponding part of the free seedlings in the same pots; and this may perhaps be accounted for by some slight degree of injury having been caused by a considerable surface having been smeared⁸⁴ with shellac. It may be added, that when the cotyledons of Phalaris and Avena are acted on by apogeotropism, it is the upper part which begins first to bend; and when this part was rendered rigid in the manner just described, the upward curvature of the basal part was not thus prevented.

 

To test our belief that the upper part of the cotyledons of Phalaris, when exposed to a lateral light, regulates the bending of the lower part, many experiments were tried; but most of our first attempts proved useless from various causes not worth specifying⁸⁵. Seven cotyledons had their tips cut off for lengths varying between .1 and .16 of an inch, and these, when left exposed all day to a lateral light, remained upright. In another set of 7 cotyledons, the tips were cut off for a length of only about .05 of an inch (1.27 mm.) and these became bowed towards [page 471] a lateral light, but not nearly so much as the many other seedlings in the same pots. This latter case shows that cutting off the tips does not by itself injure the plants so seriously as to prevent heliotropism; but we thought at the time, that such injury might follow when a greater length was cut off, as in the first set of experiments. Therefore, no more trials of this kind were made, which we now regret; as we afterwards found that when the tips of three cotyledons were cut off for a length of .2 inch, and of four others for lengths of .14, .12, .1, and .07 inch, and they were extended horizontally, the amputation⁸⁶ did not interfere⁸⁷ in the least with their bending vertically upwards, through the action of apogeotropism, like unmutilated specimens⁸⁸. It is therefore extremely improbable that the amputation of the tips for lengths of from .1 to .14 inch, could from the injury thus caused have prevented the lower part from bending towards the light.

 

We next tried the effects of covering the upper part of the cotyledons of Phalaris with little caps which were impermeable⁸⁹ to light; the whole lower part being left fully exposed before a south-west window or a bright paraffin lamp. Some of the caps were made of extremely thin tin-foil blackened within; these had the disadvantage of occasionally, though rarely, being too heavy, especially when twice folded. The basal edges could be pressed into close contact with the cotyledons; though this again required care to prevent injuring them. Nevertheless, any injury thus caused could be detected by removing the caps, and trying whether the cotyledons were then sensitive to light. Other caps were made of tubes of the thinnest glass, which when painted black served well, with the one great disadvantage that the lower ends could not be closed. But tubes were used which fitted the cotyledons almost closely, and black paper was placed on the soil round each, to check the upward reflection of light from the soil. Such tubes were in one respect far better than caps of tin-foil, as it was possible to cover at the same time some cotyledons with transparent and others with opaque⁹⁰ tubes; and thus our experiments could be controlled. It should be kept in mind that young cotyledons were selected for trial, and that these when not interfered⁹¹ with become bowed down to the ground towards the light.

 

We will begin with the glass-tubes. The summits of nine cotyledons, differing somewhat in height, were enclosed for rather less than half their lengths in uncoloured or transparent [page 472] tubes; and these were then exposed before a south-west window on a bright day for 8 h. All of them became strongly curved towards the light, in the same degree as the many other free seedlings in the same pots; so that the glass-tubes certainly did not prevent the cotyledons from bending towards the light. Nineteen other cotyledons were, at the same time, similarly enclosed in tubes thickly painted with Indian ink. On five of them, the paint, to our surprise, contracted after exposure to the sunlight, and very narrow cracks were formed, through which a little light entered; and these five cases were rejected. Of the remaining 14 cotyledons, the lower halves of which had been fully exposed to the light for the whole time, 7 continued quite straight and upright; 1 was considerably bowed to the light, and 6 were slightly bowed, but with the exposed bases of most of them almost or quite straight. It is possible that some light may have been reflected upwards from the soil and entered the bases of these 7 tubes, as the sun shone brightly, though bits of blackened paper had been placed on the soil round them. Nevertheless, the 7 cotyledons which were slightly bowed, together with the 7 upright ones, presented a most remarkable contrast in appearance with the many other seedlings in the same pots to which nothing had been done. The blackened tubes were then removed from 10 of these seedlings, and they were now exposed before a lamp for 8 h.; 9 of them became greatly, and 1 moderately, curved towards the light, proving that the previous absence of any curvature in the basal part, or the presence of only a slight degree of curvature there, was due to the exclusion of light from the upper part.

 

Similar observations were made on 12 younger cotyledons with their upper halves enclosed within glass-tubes coated with black varnish, and with their lower halves fully exposed to bright sunshine. In these younger seedlings the sensitive zone seems to extend rather lower down, as was observed on some other occasions, for two became almost as much curved towards the light as the free seedlings; and the remaining ten were slightly curved, although the basal part of several of them, which normally becomes more curved than any other part, exhibited hardly a trace of curvature. These 12 seedlings taken together differed greatly in their degree of curvature from all the many other seedlings in the same pots.

 

Better evidence of the efficiency of the blackened tubes was incidentally afforded by some experiments hereafter to be given, [page 473] in which the upper halves of 14 cotyledons were enclosed in tubes from which an extremely narrow stripe of the black varnish had been scraped off. These cleared stripes were not directed towards the window, but obliquely to one side of the room, so that only a very little light could act on the upper halves of the cotyledons. These 14 seedlings remained during eight hours of exposure before a south-west window on a hazy⁹² day quite upright; whereas all the other many free seedlings in the same pots became greatly bowed towards the light.

 

We will now turn to the trials with caps made of very thin tin-foil. These were placed at different times on the summits of 24 cotyledons, and they extended down for a length of between .15 and .2 of an inch. The seedlings were exposed to a lateral light for periods varying between 6 h. 30 m. and 7 h. 45 m., which sufficed to cause all the other seedlings in the same pots to become almost rectangularly bent towards the light. They varied in height from only .04 to 1.15 inch, but the greater number were about .75 inch. Of the 24 cotyledons with their summits thus protected, 3 became much bent, but not in the direction of the light, and as they did not straighten themselves through apogeotropism during the following night, either the caps were too heavy or the plants themselves were in a weak condition; and these three cases may be excluded. There are left for consideration 21 cotyledons; of these 17 remained all the time quite upright; the other 4 became slightly inclined to the light, but not in a degree comparable with that of the many free seedlings in the same pots. As the glass-tubes, when unpainted, did not prevent the cotyledons from becoming greatly bowed, it cannot be supposed that the caps of very thin tin-foil did so, except through the exclusion of the light. To prove that the plants had not been injured, the caps were removed from 6 of the upright seedlings, and these were exposed before a paraffin lamp for the same length of time as before, and they now all became greatly curved towards the light.

 

As caps between .15 and .2 of an inch in depth were thus proved to be highly efficient in preventing the cotyledons from bending towards the light, 8 other cotyledons were protected with caps between only .06 and .12 in depth. Of these, two remained vertical, one was considerably and five slightly curved towards the light, but far less so than the free seedlings in the same pots. [page 474]

 

Another trial was made in a different manner, namely, by bandaging with strips of tin-foil, about .2 in breadth, the upper part, but not the actual summit, of eight moderately young seedlings a little over half an inch in height. The summits and the basal parts were thus left fully exposed to a lateral light during 8 h.; an upper intermediate zone being protected. With four of these seedlings the summits were exposed for a length of .05 inch, and in two of them this part became curved towards the light, but the whole lower part remained quite upright; whereas the entire length of the other two seedlings became slightly curved towards the light. The summits of the four other seedlings were exposed for a length of .04 inch, and of these one remained almost upright, whilst the other three became considerably curved towards the light. The many free seedlings in the same pots were all greatly curved towards the light.

 

From these several sets of experiments, including those with the glass-tubes, and those when the tips were cut off, we may infer that the exclusion of light from the upper part of the cotyledons of Phalaris prevents the lower part, though fully exposed to a lateral light, from becoming curved. The summit for a length of .04 or .05 of an inch, though it is itself sensitive and curves towards the light, has only a slight power of causing the lower part to bend. Nor has the exclusion of light from the summit for a length of .1 of an inch a strong influence on the curvature of the lower part. On the other hand, an exclusion for a length of between .15 and .2 of an inch, or of the whole upper half, plainly prevents the lower and fully illuminated part from becoming curved in the manner (see Fig. 181) which invariably occurs when a free cotyledon is exposed to a lateral light. With very young seedlings the sensitive zone seems to extend rather lower down relatively to their height than in older seedlings. We must therefore conclude that when seedlings are freely exposed to a lateral light some influence is transmitted from the upper to the lower part, causing the latter to bend.

 

This conclusion is supported by what may be seen to occur on a small scale, especially with young cotyledons, without any artificial exclusion of the light; for they bend beneath the earth where no light can enter. Seeds of Phalaris were covered with a layer one-fourth of an inch in thickness of very fine sand, consisting of extremely minute grains of silex coated with [page 475] oxide⁹³ of iron. A layer of this sand, moistened to the same degree as that over the seeds, was spread over a glass-plate; and when the layer was .05 of an inch in thickness (carefully measured) no light from a bright sky could be seen to pass through it, unless it was viewed through a long blackened tube, and then a trace of light could be detected, but probably much too little to affect any plant. A layer .1 of an inch in thickness was quite impermeable to light, as judged by the eye aided by the tube. It may be worth adding that the layer, when dried, remained equally impermeable to light. This sand yielded to very slight pressure whilst kept moist, and in this state did not contract or crack in the least. In a first trial, cotyledons which had grown to a moderate height were exposed for 8 h. before a paraffin lamp, and they became greatly bowed. At their bases on the shaded side opposite to the light, well-defined, crescentic, open furrows⁹⁴ were formed, which (measured under a microscope with a micrometer) were from .02 to .03 of an inch in breadth, and these had evidently been left by the bending of the buried bases of the cotyledons towards the light. On the side of the light the cotyledons were in close contact with the sand, which was a very little heaped up. By removing with a sharp knife the sand on one side of the cotyledons in the line of the light, the bent portion and the open furrows were found to extend down to a depth of about .1 of an inch, where no light could enter. The chords of the short buried arcs formed in four cases angles of 11o, 13o, 15o, and 18o, with the perpendicular. By the following morning these short bowed portions had straightened themselves through apogeotropism.

 

In the next trial much younger cotyledons were similarly treated, but were exposed to a rather obscure lateral light. After some hours, a bowed cotyledon, .3 inch in height, had an open furrow⁹⁵ on the shaded side .04 inch in breadth; another cotyledon, only .13 inch in height, had left a furrow .02 inch in breadth. But the most curious case was that of a cotyledon which had just protruded⁹⁶ above the ground and was only .03 inch in height, and this was found to be bowed in the direction of the light to a depth of .2 of an inch beneath the surface. From what we know of the impermeability⁹⁷ of this sand to light, the upper illuminated part in these several cases must have determined the curvature of the lower buried portions. But an apparent cause of doubt may be suggested: as the cotyledons are continually circumnutating, they tend to form a minute [page 476] crack or furrow all round their bases, which would admit a little light on all sides; but this would not happen when they were illuminated laterally, for we know that they quickly bend towards a lateral light, and they then press so firmly against the sand on the illuminated side as to furrow it, and this would effectually exclude light on this side. Any light admitted on the opposite and shaded side, where an open furrow is formed, would tend to counteract⁹⁸ the curvature towards the lamp or other source of the light. It may be added, that the use of fine moist sand, which yields easily to pressure, was indispensable in the above experiments; for seedlings raised in common soil, not kept especially damp, and exposed for 9 h. 30 m. to a strong lateral light, did not form an open furrow at their bases on the shaded side, and were not bowed beneath the surface. Perhaps the most striking proof of the action of the upper on the lower part of the cotyledons of Phalaris, when laterally illuminated, was afforded by the blackened glass-tubes (before alluded⁹⁹ to) with very narrow stripes of the varnish scraped off on one side, through which a little light was admitted. The breadth of these stripes or slits¹⁰¹ varied between .01 and .02 inch (.25 and .51 mm.). Cotyledons with their upper halves enclosed in such tubes were placed before a south-west window, in such a position, that the scraped stripes did not directly face the window, but obliquely to one side. The seedlings were left exposed for 8 h., before the close of which time the many free seedlings in the same pots had become greatly bowed towards the window. Under these circumstances, the whole lower halves of the cotyledons, which had their summits enclosed in the tubes, were fully exposed to the light of the sky, whilst their upper halves received exclusively or chiefly diffused light from the room, and this only through a very narrow slit¹⁰⁰ on one side. Now, if the curvature of the lower part had been determined by the illumination of this part, all the cotyledons assuredly would have become curved towards the window; but this was far from being the case. Tubes of the kind just described were placed on several occasions over the upper halves of 27 cotyledons; 14 of them remained all the time quite vertical; so that sufficient diffused light did not enter through the narrow slits to produce any effect whatever; and they behaved in the same manner as if their upper halves had been enclosed in completely blackened tubes. The lower halves of the 13 other cotyledons became bowed [page 477] not directly in the line of the window, but obliquely towards it; one pointed at an angle of only 18o, but the remaining 12 at angles varying between 45o and 62o from the line of the window. At the commencement of the experiment, pins had been laid on the earth in the direction towards which the slits in the varnish faced; and in this direction alone a small amount of diffused light entered. At the close of the experiment, 7 of the bowed cotyledons pointed exactly in the line of the pins, and 6 of them in a line between that of the pins and that of the window. This intermediate position is intelligible, for any light from the sky which entered obliquely through the slits would be much more efficient than the diffused light which entered directly through them. After the 8 h. exposure, the contrast in appearance between these 13 cotyledons and the many other seedlings in the same pots, which were all (excepting the above 14 vertical ones) greatly bowed in straight and parallel lines towards the window, was extremely remarkable. It is therefore certain that a little weak light striking the upper halves of the cotyledons of Phalaris, is far more potent¹⁰² in determining the direction of the curvature of the lower halves, than the full illumination of the latter during the whole time of exposure.

 

In confirmation¹⁰³ of the above results, the effect of thickly painting with Indian ink one side of the upper part of three cotyledons of Phalaris, for a length of .2 inch from their tips, may be worth giving. These were placed so that the unpainted surface was directed not towards the window, but a little to one side; and they all became bent towards the unpainted side, and from the line of the window by angles amounting to 31o, 35o, and 83o. The curvature in this direction extended down to their bases, although the whole lower part was fully exposed to the light from the window.

 

Finally, although there can be no doubt that the illumination of the upper part of the cotyledons of Phalaris greatly affects the power and manner of bending of the lower part, yet some observations seemed to render it probable that the simultaneous stimulation¹⁰⁴ of the lower part by light greatly favours, or is almost necessary, for its well-marked curvature; but our experiments were not conclusive⁵¹, owing to the difficulty of excluding light from the lower halves without mechanically preventing their curvature.

 

Avena sativa.—The cotyledons of this plant become quickly bowed towards a lateral light, exactly like those of Phalaris. [page 478] Experiments similar to the foregoing ones were tried, and we will give the results as briefly¹⁰⁵ as possible. They are somewhat less conclusive than in the case of Phalaris, and this may possibly be accounted for by the sensitive zone varying in extension, in a species so long cultivated and variable as the common Oat. Cotyledons a little under three-quarters of an inch in height were selected for trial: six had their summits protected from light by tin-foil caps, .25 inch in depth, and two others by caps .3 inch in depth. Of these 8 cotyledons, five remained upright during 8 hours of exposure, although their lower parts were fully exposed to the light all the time; two were very slightly, and one considerably, bowed towards it. Caps only .2 or .22 inch in depth were placed over 4 other cotyledons, and now only one remained upright, one was slightly, and two considerably bowed to the light. In this and the following cases all the free seedlings in the same pots became greatly bowed to the light.

 

Our next trial was made with short lengths of thin and fairly transparent quills¹⁰⁶; for glass-tubes of sufficient diameter to go over the cotyledons would have been too heavy. Firstly, the summits of 13 cotyledons were enclosed in unpainted quills, and of these 11 became greatly and 2 slightly bowed to the light; so that the mere⁵⁴ act of enclosure did not prevent the lower part from becoming bowed. Secondly¹⁰⁷, the summits of 11 cotyledons were enclosed in quills .3 inch in length, painted so as to be impermeable to light; of these, 7 did not become at all inclined towards the light, but 3 of them were slightly bent more or less transversely with respect to the line of light, and these might perhaps have been altogether excluded; one alone was slightly bowed towards the light. Painted quills, .25 inch in length, were placed over the summits of 4 other cotyledons; of these, one alone remained upright, a second was slightly bowed, and the two others as much bowed to the light as the free seedlings in the same pots. These two latter cases, considering that the caps were .25 in length, are inexplicable¹⁰⁸.

 

Lastly, the summits of 8 cotyledons were coated with flexible and highly transparent gold-beaters' skin, and all became as much bowed to the light as the free seedlings. The summits of 9 other cotyledons were similarly coated with gold-beaters' skin, which was then painted to a depth of between .25 and .3 inch, so as to be impermeable to light; of these 5 remained upright, and 4 were well bowed to the light, almost or quite as well as [page 479] the free seedlings. These latter four cases, as well as the two in the last paragraph, offer a strong exception to the rule that the illumination of the upper part determines the curvature of the lower part. Nevertheless, 5 of these 8 cotyledons remained quite upright, although their lower halves were fully illuminated all the time; and it would almost be a prodigy¹⁰⁹ to find five free seedlings standing vertically after an exposure for several hours to a lateral light.

 

The cotyledons of Avena, like those of Phalaris, when growing in soft, damp, fine sand, leave an open crescentric furrow on the shaded side, after bending to a lateral light; and they become bowed beneath the surface at a depth to which, as we know, light cannot penetrate¹¹⁰. The arcs of the chords of the buried bowed portions formed in two cases angles of 20o and 21o with the perpendicular. The open furrows on the shaded side were, in four cases, .008, .016, .024, and .024 of an inch in breadth. Brassica oleracea (Common Red).—It will here be shown that the upper half of the hypocotyl of the cabbage, when illuminated by a lateral light, determines the curvature of the lower half. It is necessary to experimentise on young seedlings about half an inch or rather less in height, for when grown to an inch and upwards the basal part ceases to bend. We first tried painting the hypocotyls with Indian ink, or cutting off their summits for various lengths; but these experiments are not worth giving, though they confirm, as far as they can be trusted, the results of the following ones. These were made by folding gold-beaters' skin once round the upper halves of young hypocotyls, and painting it thickly with Indian ink or with black grease. As a control experiment, the same transparent skin, left unpainted, was folded round the upper halves of 12 hypocotyls; and these all became greatly curved to the light, excepting one, which was only moderately curved. Twenty other young hypocotyls had the skin round their upper halves painted, whilst their lower halves were left quite uncovered. These seedlings were then exposed, generally for between 7 and 8 h., in a box blackened within and open in front, either before a south-west window or a paraffin lamp. This exposure was amply sufficient, as was shown by the strongly-marked heliotropism of all the free seedlings in the same pots; nevertheless, some were left exposed to the light for a much longer time. Of the 20 hypocotyls thus treated, 14 remained quite upright, and 6 became slightly bowed to the light; but 2 of these latter cases were not really [page 480] exceptions, for on removing the skin the paint was found imperfect and was penetrated¹¹¹ by many small transparent spaces on the side which faced the light. Moreover, in two other cases the painted skin did not extend quite halfway¹¹² down the hypocotyl. Although there was a wonderful contrast in the several pots between these 20 hypocotyls and the other many free seedlings, which were all greatly bowed down to their bases in the direction of the light, some being almost prostrate¹¹³ on the ground.

 

The most successful trial on any one day (included in the above results) is worth describing in detail. Six young seedlings were selected, the hypocotyls of which were nearly .45 inch, excepting one, which was .6 inch in height, measured from the bases of their petioles to the ground. Their upper halves, judged as accurately as could be done by the eye, were folded once round with gold-beaters' skin, and this was painted thickly with Indian ink. They were exposed in an otherwise darkened room before a bright paraffin lamp, which stood on a level with the two pots containing the seedlings. They were first looked at after an interval⁴³ of 5 h. 10 m., and five of the protected hypocotyls were found quite erect¹¹⁴, the sixth being very slightly inclined to the light; whereas all the many free seedlings in the same two pots were greatly bowed to the light. They were again examined after a continuous exposure to the light of 20 h. 35m.; and now the contrast between the two sets was wonderfully great; for the free seedlings had their hypocotyls extended almost horizontally in the direction of the light, and were curved down to the ground; whilst those with the upper halves protected by the painted skin, but with their lower halves fully exposed to the light, still remained quite upright, with the exception of the one which retained the same slight inclination¹¹⁵ to the light which it had before. This latter seedling was found to have been rather badly painted, for on the side facing the light the red colour of the hypocotyl could be distinguished¹¹⁶ through the paint.

 

We next tried nine older seedlings, the hypocotyls of which varied between 1 and 1.6 inch in height. the gold-beaters' skin round their upper parts was painted with black grease to a depth of only .3 inch, that is, from less than a third to a fourth or fifth of their total heights. They were exposed to the light for 7 h. 15 m.; and the result showed that the whole of the sensitive zone, which determines the curvature of the lower [page 481] part, was not protected from the action of the light; for all 9 became curved towards it, 4 of them very slightly, 3 moderately, and 2 almost as much as the unprotected seedlings. Nevertheless, the whole 9 taken together differed plainly in their degree of curvature from the many free seedlings, and from some which were wrapped in unpainted skin, growing in the same two pots.

 

Seeds were covered with about a quarter of an inch of the fine sand described under Phalaris; and when the hypocotyls had grown to a height of between .4 and .55 inch, they were exposed during 9 h. before a paraffin lamp, their bases being at first closely surrounded by the damp sand. They all became bowed down to the ground, so that their upper parts lay near to and almost parallel to the surface of the soil. On the side of the light their bases were in close contact with the sand, which was here a very little heaped up; on the opposite or shaded side there were open, crescentic cracks or furrows, rather above .01 of an inch in width; but they were not so sharp and regular as those made by Phalaris and Avena, and therefore could not be so easily measured under the microscope. The hypocotyls were found, when the sand was removed on one side, to be curved to a depth beneath the surface in three cases of at least .1 inch, in a fourth case of .11, and in a fifth of .15 inch. The chords of the arcs of the short, buried, bowed portions formed angles of between 11o and 15o with the perpendicular. From what we have seen of the impermeability of this sand to light, the curvature of the hypocotyls certainly extended down to a depth where no light could enter; and the curvature must have been caused by an influence transmitted from the upper illuminated part.

 

The lower halves of five young hypocotyls were surrounded by unpainted gold-beaters' skin, and these, after an exposure of 8 h. before a paraffin lamp, all became as much bowed to the light as the free seedlings. The lower halves of 10 other young hypocotyls, similarly surrounded with the skin, were thickly painted with Indian ink; their upper and unprotected halves became well curved to the light, but their lower and protected halves remained vertical in all the cases excepting one, and on this the layer of paint was imperfect. This result seems to prove that the influence transmitted from the upper part is not sufficient to cause the lower part to bend, unless it be at the same time illuminated; but there remains¹¹⁷ the doubt, as in [page 482] the case of Phalaris, whether the skin covered with a rather thick crust of dry Indian ink did not mechanically prevent their curvature.

 

Beta vulgaris.—A few analogous experiments were tried on this plant, which is not very well adapted for the purpose, as the basal part of the hypocotyl, after it has grown to above half an inch in height, does not bend much on exposure to a lateral light. Four hypocotyls were surrounded close beneath their petioles with strips of thin tin-foil, .2 inch in breadth, and they remained upright all day before a paraffin lamp; two others were surrounded with strips .15 inch in breadth, and one of these remained upright, the other becoming bowed; the bandages in two other cases were only .1 inch in breadth, and both of these hypocotyls became bowed, though one only slightly, towards the light. The free seedlings in the same pots were all fairly well curved towards the light; and during the following night became nearly upright. The pots were now turned round and placed before a window, so that the opposite sides of the seedlings were exposed to the light, towards which all the unprotected hypocotyls became bent in the course of 7 h. Seven out of the 8 seedlings with bandages of tin-foil remained upright, but one which had a bandage only .1 inch in breadth, became curved to the light. On another occasion, the upper halves of 7 hypocotyls were surrounded with painted gold-beaters' skin; of these 4 remained upright, and 3 became a little curved to the light: at the same time 4 other seedlings surrounded with unpainted skin, as well as the free ones in the same pots, all became bowed towards the lamp, before which they had been exposed during 22 hours.

 

Radicles of Sinapis alba.—The radicles of some plants are indifferent, as far as curvature is concerned, to the action of light; whilst others bend towards and others from it.* Whether these movements are of any service to the plant is very doubtful, at least in the case of subterranean¹¹⁸ roots; they probably result from the radicles being sensitive to contact, moisture, and gravitation, and as a consequence to other irritants which are never naturally encountered. The radicles of Sinapis alba, when immersed in water and exposed to a lateral light, bend from it, or are apheliotropic. They become bent for a length of about 4 mm. from their tips. To ascertain whether this movement

 

* Sachs, 'Physiologie Végétale,' 1868, p. 44. [page 483]

 

generally occurred, 41 radicles, which had germinated in damp sawdust, were immersed in water and exposed to a lateral light; and they all, with two doubtful exceptions, became curved from the light. At the same time the tips of 54 other radicles, similarly exposed, were just touched with nitrate of silver. They were blackened for a length of from .05 to .07 mm., and probably killed; but it should be observed that this did not check materially, if at all, the growth of the upper part; for several, which were measured, increased in the course of only 8 -9 h. by 5 to 7 mm. in length. Of the 54 cauterised radicles one case was doubtful, 25 curved themselves from the light in the normal manner, and 28, or more than half, were not in the least apheliotropic. There was a considerable difference, which we cannot account for, in the results of the experiments tried towards the end of April and in the middle of September. Fifteen radicles (part of the above 54) were cauterised at the former period and were exposed to sunshine, of which 12 failed to be apheliotropic, 2 were still apheliotropic, and 1 was doubtful. In September, 39 cauterised radicles were exposed to a northern light, being kept at a proper temperature; and now 23 continued to be apheliotropic in the normal manner, and only 16 failed to bend from the light. Looking at the aggregate¹¹⁹ results at both periods, there can be no doubt that the destruction of the tip for less than a millimeter in length destroyed in more than half the cases their power of moving from the light. It is probable that if the tips had been cauterised for the length of a whole millimeter, all signs of apheliotropism would have disappeared. It may be suggested that although the application of caustic¹²⁰ does not stop growth, yet enough may be absorbed to destroy the power of movement in the upper part; but this suggestion must be rejected, for we have seen and shall again see, that cauterising one side of the tip of various kinds of radicles actually excites movement. The conclusion seems inevitable¹²¹ that sensitiveness to light resides in the tip of the radicle of Sinapis alba; and that the tip when thus stimulated transmits some influence to the upper part, causing it to bend. The case in this respect is parallel with that of the radicles of several plants, the tips of which are sensitive to contact and to other irritants, and, as will be shown in the eleventh chapter, to gravitation. [page 484]

 

CONCLUDING REMARKS AND SUMMARY OF CHAPTER.

 

We do not know whether it is a general rule with seedling plants that the illumination of the upper part determines the curvature of the lower part. But as this occurred in the four species examined by us, belonging to such distinct families as the Gramineae, Cruciferae, and Chenopodeae, it is probably of common occurrence. It can hardly fail to be of service to seedlings, by aiding them to find the shortest path from the buried seed to the light, on nearly the same principle that the eyes of most of the lower crawling animals are seated at the anterior¹²² ends of their bodies. It is extremely doubtful whether with fully developed plants the illumination of one part ever affects the curvature of another part. The summits of 5 young plants of Asparagus officinalis (varying in height between 1.1 and 2.7 inches, and consisting of several short internodes) were covered with caps of tin-foil from 0.3 to 0.35 inch in depth; and the lower uncovered parts became as much curved towards a lateral light, as were the free seedlings in the same pots. Other seedlings of the same plant had their summits painted with Indian ink with the same negative result. Pieces of blackened paper were gummed to the edges and over the blades of some leaves on young plants of Tropaeolum majus and Ranunculus ficaria; these were then placed in a box before a window, and the petioles of the protected leaves became curved towards the light, as much as those of the unprotected leaves.

 

The foregoing cases with respect to seedling plants have been fully described, not only because the transmission of any effect from light is a new physiological¹²³ fact, but because we think it tends to modify somewhat the current views on heliotropic movements. Until [page 485] lately such movements were believed to result simply from increased growth on the shaded side. At present it is commonly admitted* that diminished light increases the turgescence of the cells, or the extensibility of the cell-walls, or of both together, on the shaded side, and that this is followed by increased growth. But Pfeffer has shown that a difference in the turgescence on the two sides of a pulvinus,—that is, an aggregate of small cells which have ceased to grow at an early age,—is excited by a difference in the amount of light received by the two sides; and that movement is thus caused without being followed by increased growth on the more turgescent side.** All observers apparently¹²⁴ believe that light acts directly on the part which bends, but we have seen with the above described seedlings that this is not the case. Their lower halves were brightly illuminated for hours, and yet did not bend in the least towards the light, though this is the part which under ordinary circumstances bends the most. It is a still more striking fact, that the faint illumination of a narrow stripe on one side of the upper part of the cotyledons of Phalaris determined the direction of the curvature of the lower part; so that this latter part did not bend towards the bright light by which it had been fully illuminated,

 

* Emil Godlewski has given ('Bot. Zeitung,' 1879, Nos. 6-9) an excellent account (p. 120) of the present state of the question. See also Vines in 'Arbeiten des Bot. Inst. in Würzburg,' 1878, B. ii. pp. 114-147. Hugo de Vries has recently published a still more important article on this subject: 'Bot Zeitung,' Dec. 19th and 26th, 1879.

 

** 'Die Periodischen Bewegungen der Blattorgane,' 1875, pp. 7, 63, 123, etc. Frank has also insisted ('Die Naturliche w?gerechte Richtung von Pflanzentheilen,' 1870, p. 53) on the important part which the pulvini of the leaflets of compound leaves play in placing the leaflets in a proper position with respect to the light. This holds good, especially with the leaves of climbing plants, which are carried into all sorts of positions, ill-adapted for the action of the light. [page 486]

 

but obliquely towards one side where only a little light entered. These results seem to imply the presence of some matter in the upper part which is acted on by light, and which transmits its effects to the lower part. It has been shown that this transmission is independent of the bending of the upper sensitive part. We have an analogous case of transmission in Drosera, for when a gland¹²⁵ is irritated, the basal and not the upper or intermediate part of the tentacle¹²⁶ bends. The flexible and sensitive filament¹²⁷ of Dionaea likewise transmits a stimulus, without itself bending; as does the stem of Mimosa.

 

Light exerts a powerful influence on most vegetable tissues, and there can be no doubt that it generally tends to check their growth. But when the two sides of a plant are illuminated in a slightly different degree, it does not necessarily follow that the bending towards the illuminated side is caused by changes in the tissues of the same nature as those which lead to increased growth in darkness. We know at least that a part may bend from the light, and yet its growth may not be favoured by light. This is the case with the radicles of Sinapis alba, which are plainly apheliotropic; nevertheless, they grow quicker in darkness than in light.* So it is with many a?rial roots, according to Wiesner;** but there are other opposed cases. It appears, therefore, that light does not determine the growth of apheliotropic parts in any uniform manner.

 

We should bear in mind that the power of bending to the light is highly beneficial to most plants. There

 

* Francis Darwin, 'über das Wachsthum negativ heliotropischer Wurzeln': 'Arbeiten des Bot. Inst. in Würzburg,' B. ii., Heft iii., 1880, p. 521.

 

** 'Sitzb. der k. Akad. der Wissensch' (Vienna), 1880, p. 12. [page 487]

 

is therefore no improbability in this power having been specially⁴⁸ acquired. In several respects light seems to act on plants in nearly the same manner as it does on animals by means of the nervous system.* With seedlings the effect, as we have just seen, is transmitted from one part to another. An animal may be excited to move by a very small amount of light; and it has been shown that a difference in the illumination of the two sides of the cotyledons of Phalaris, which could not be distinguished by the human eye, sufficed to cause them to bend. It has also been shown that there is no close parallelism between the amount of light which acts on a plant and its degree of curvature; it was indeed hardly possible to perceive any difference in the curvature of some seedlings of Phalaris exposed to a light, which, though dim, was very much brighter than that to which others had been exposed. The retina, after being stimulated by a bright light, feels the effect for some time; and Phalaris continued to bend for nearly half an hour towards the side which had been illuminated. The retina cannot perceive a dim light after it has been exposed to a bright one; and plants which had been kept in the daylight during the previous day and morning, did not move so soon towards an obscure lateral light as did others which had been kept in complete darkness.

 

Even if light does act in such a manner on the growing parts of plants as always to excite in them a tendency to bend towards the more illuminated side—a supposition contradicted by the foregoing experiments on seedlings and by all apheliotropic * Sachs has made some striking remarks to the same effect with respect to the various stimuli¹²⁸ which excite movement in plants. See his paper 'Ueber orthotrope und plagiotrope Pflanzentheile,' 'Arb. des Bot. Inst. in Würzburg,' 1879, B. ii. p. 282. [page 488]

 

organs—yet the tendency differs greatly in different species, and is variable in degree in the individuals of the same species, as may be seen in almost any pot of seedlings of a long cultivated plant.* There is therefore a basis for the modification¹²⁹ of this tendency to almost any beneficial extent. That it has been modified, we see in many cases: thus, it is of more importance for insectivorous plants to place their leaves in the best position for catching¹³⁰ insects than to turn their leaves to the light, and they have no such power. If the stems of twining plants were to bend towards the light, they would often be drawn away from their supports; and as we have seen they do not thus bend. As the stems of most other plants are heliotropic, we may feel almost sure that twining plants, which are distributed throughout the whole vascular series, have lost a power that their non-climbing progenitors¹³¹ possessed. Moreover, with Ipomoea, and probably all other twiners, the stem of the young plant, before it begins to twine¹³², is highly heliotropic, evidently in order to expose the cotyledons or the first true leaves fully to the light. With the Ivy the stems of seedlings are moderately heliotropic, whilst those of the same plants when grown a little older

 

* Strasburger has shown in his interesting work ('Wirkung des Lichtes...auf Schw?rmsporen,' 1878), that the movement of the swarm-spores of various lowly organised plants to a lateral light is influenced by their stage of development, by the temperature to which they are subjected, by the degree of illumination under which they have been raised, and by other unknown causes; so that the swarm-spores of the same species may move across the field of the microscope either to or from the light. Some individuals, moreover, appear to be indifferent to the light; and those of different species behave very differently. The brighter the light, the straighter is their course. They exhibit also for a short time the after-effects of light. In all these respects they resemble the higher plants. See, also, Stahl, 'Ueber den¹ einfluss der Lichts auf die Bewegungs-erscheinungen der Schw?rmsporen' Verh. d. phys.-med. Geselsshalft in Würzburg, B. xii. 1878. [page 489]

 

are apheliotropic. Some tendrils which consist of modified leaves—organs in all ordinary cases strongly diaheliotropic—have been rendered apheliotropic, and their tips crawl into any dark crevice¹³³.

 

Even in the case of ordinary heliotropic movements, it is hardly credible¹³⁴ that they result directly from the action of the light, without any special adaptation. We may illustrate¹³⁵ what we mean by the hygroscopic movements of plants: if the tissues on one side of an organ permit of rapid evaporation¹³⁶, they will dry quickly and contract, causing the part to bend to this side. Now the wonderfully complex movements of the pollinia of Orchis pyramidalis, by which they clasp the proboscis¹³⁷ of a moth¹³⁸ and afterwards change their position for the sake of depositing the pollen-masses on the double stigma—or again the twisting movements, by which certain seeds bury themselves in the ground*—follow from the manner of drying of the parts in question; yet no one will suppose that these results have been gained without special adaptation. Similarly, we are led to believe in adaptation when we see the hypocotyl of a seedling, which contains chlorophyll, bending to the light; for although it thus receives less light, being now shaded by its own cotyledons, it places them—the more important organs—in the best position to be fully illuminated. The hypocotyl may therefore be said to sacrifice itself for the good of the cotyledons, or rather of the whole plant. But if it be prevented from bending, as must sometimes occur with seedlings springing up in an entangled¹³⁹ mass of vegetation, the cotyledons themselves bend so as to face the light; the one farthest off rising

 

* Francis Darwin, 'On the Hygroscopic Mechanism,' etc., 'Transactions Linn. Soc.,' series ii. vol. i. p. 149, 1876. [page 490]

 

up, and that nearest to the light sinking down, or both twisting laterally.* We may, also, suspect that the extreme sensitiveness to light of the upper part of the sheath-like cotyledons of the Gramineae, and their power of transmitting its effects to the lower part, are specialised arrangements for finding the shortest path to the light. With plants growing on a bank, or thrown prostrate by the wind, the manner in which the leaves move, even rotating on their own axes, so that their upper surfaces may be again directed to the light, is a striking phenomenon. Such facts are rendered more striking when we remember that too intense a light injures the chlorophyll, and that the leaflets of several Leguminosae when thus exposed bend upwards and present their edges to the sun, thus escaping injury. On the other hand, the leaflets of Averrhoa and Oxalis, when similarly exposed, bend downwards¹⁴⁰.

 

It was shown in the last chapter that heliotropism is a modified form of circumnutation; and as every growing part of every plant circumnutates more or less, we can understand how it is that the power of bending to the light has been acquired by such a multitude of plants throughout the vegetable kingdom. The manner in which a circumnutating movement—that is, one consisting of a succession of irregular ellipses¹⁴¹ or loops—is gradually converted into a rectilinear course towards the light, has been already explained. First, we have a succession of ellipses with their longer axes directed towards the light, each of which

 

* Wiesner has made remarks to nearly the same effect with respect to leaves: 'Die undulirende Nutation der Internodien,' p. 6, extracted from B. lxxvii. (1878). Sitb. der k. Akad. der Wissensch. Wien. [page 491]

 

is described nearer and nearer to its source; then the loops are drawn out into a strongly pronounced zigzag line, with here and there a small loop still formed. At the same time that the movement towards the light is increased in extent and accelerated, that in the opposite direction is lessened¹⁴² and retarded¹⁴³, and at last stopped. The zigzag movement to either side is likewise gradually lessened, so that finally the course becomes rectilinear. Thus under the stimulus of a fairly bright light there is no useless expenditure¹⁴⁴ of force.

 

As with plants every character is more or less variable, there seems to be no great difficulty in believing that their circumnutating movements may have been increased or modified in any beneficial manner by the preservation¹⁴⁵ of varying individuals. The inheritance of habitual¹⁴⁶ movements is a necessary contingent¹⁴⁷ for this process of selection, or the survival of the fittest; and we have seen good reason to believe that habitual movements are inherited by plants. In the case of twining species the circumnutating movements have been increased in amplitude¹⁴⁸ and rendered more circular; the stimulus being here an internal or innate¹⁴⁹ one. With sleeping plants the movements have been increased in amplitude and often changed in direction; and here the stimulus is the alternation of light and darkness, aided, however, by inheritance. In the case of heliotropism, the stimulus is the unequal illumination of the two sides of the plant, and this determines, as in the foregoing cases, the modification of the circumnutating movement in such a manner that the organ bends to the light. A plant which has been rendered heliotropic by the above means, might readily lose this tendency, judging from the cases already given, as soon as it became useless or [page 492] injurious. A species which has ceased to be heliotropic might also be rendered apheliotropic by the preservation of the individuals which tended to circumnutate (though the cause of this and most other variations is unknown) in a direction more or less opposed to that whence the light proceeded. In like manner a plant might be rendered diaheliotropic. [page 493]

 

1 den   

n.兽穴；秘密地方；安静的小房间，私室

参考例句：

• There is a big fox den on the back hill.后山有一个很大的狐狸窝。
• The only way to catch tiger cubs is to go into tiger's den.不入虎穴焉得虎子。

2 exclusion   

n.拒绝，排除，排斥，远足，远途旅行

参考例句：

• Don't revise a few topics to the exclusion of all others.不要修改少数论题以致排除所有其他的。
• He plays golf to the exclusion of all other sports.他专打高尔夫球，其他运动一概不参加。

3 lateral   

adj.侧面的，旁边的

参考例句：

• An airfoil that controls lateral motion.能够控制横向飞行的机翼。
• Mr.Dawson walked into the court from a lateral door.道森先生从一个侧面的门走进法庭。

4 illuminated   

adj.被照明的；受启迪的

参考例句：

• Floodlights illuminated the stadium. 泛光灯照亮了体育场。
• the illuminated city at night 夜幕中万家灯火的城市

5 decompose   

vi.分解；vt.（使）腐败，（使）腐烂

参考例句：

• The eggs began to decompose after a day in the sun.鸡蛋在太阳下放了一天后开始变坏。
• Most animals decompose very quickly after death.大多数动物死后很快腐烂。

6 starch   

n.淀粉；vt.给...上浆

参考例句：

• Corn starch is used as a thickener in stews.玉米淀粉在炖煮菜肴中被用作增稠剂。
• I think there's too much starch in their diet.我看是他们的饮食里淀粉太多了。

7 fissures   

n.狭长裂缝或裂隙( fissure的名词复数 )；裂伤；分歧；分裂v.裂开( fissure的第三人称单数 )

参考例句：

• Rising molten rock flows out on the ocean floor and caps the fissures, trapping the water. 上升熔岩流到海底并堵住了裂隙，结果把海水封在里面。 来自辞典例句
• The French have held two colloquia and an international symposium on rock fissures. 法国已经开了两次岩石裂缝方面的报告会和一个国际会议。 来自辞典例句

8 decomposes   

腐烂( decompose的第三人称单数 )； （使）分解； 分解（某物质、光线等）

参考例句：

• The debris slowly decomposes into compost. 这些垃圾慢慢地分解成了堆肥。
• Plastic is a substance that hardly decomposes in the nature. 塑料是一种在自然中极难降解的物质。

9 seedling   

n.秧苗，树苗

参考例句：

• She cut down the seedling with one chop.她一刀就把小苗砍倒了。
• The seedling are coming up full and green.苗长得茁壮碧绿。

10 seedlings   

n.刚出芽的幼苗( seedling的名词复数 )

参考例句：

• Ninety-five per cent of the new seedlings have survived. 新栽的树苗95%都已成活。 来自《现代汉英综合大词典》
• In such wet weather we must prevent the seedlings from rotting. 这样的阴雨天要防止烂秧。 来自《现代汉英综合大词典》

11 upwards   

adv.向上，在更高处...以上

参考例句：

• The trend of prices is still upwards.物价的趋向是仍在上涨。
• The smoke rose straight upwards.烟一直向上升。

12 fully   

adv.完全地，全部地，彻底地；充分地

参考例句：

• The doctor asked me to breathe in,then to breathe out fully.医生让我先吸气，然后全部呼出。
• They soon became fully integrated into the local community.他们很快就完全融入了当地人的圈子。

13 pitchers   

大水罐( pitcher的名词复数 )

参考例句：

• Over the next five years, he became one of the greatest pitchers in baseball. 在接下来的5年时间里，他成为了最了不起的棒球投手之一。
• Why he probably won't: Pitchers on also-rans can win the award. 为什麽不是他得奖：投手在失败的球队可以赢得赛扬奖。

14 decomposing   

腐烂( decompose的现在分词 )； （使）分解； 分解（某物质、光线等）

参考例句：

• The air was filled with the overpowering stench of decomposing vegetation. 空气中充满了令人难以忍受的腐烂植物的恶臭。
• Heat was obtained from decomposing manures and hot air flues. 靠肥料分解和烟道为植物提供热量。

15 drawn   

v.拖，拉，拔出；adj.憔悴的，紧张的

参考例句：

• All the characters in the story are drawn from life.故事中的所有人物都取材于生活。
• Her gaze was drawn irresistibly to the scene outside.她的目光禁不住被外面的风景所吸引。

16 possessed   

adj.疯狂的；拥有的，占有的

参考例句：

• He flew out of the room like a man possessed.他像着了魔似地猛然冲出房门。
• He behaved like someone possessed.他行为举止像是魔怔了。

17 ivy   

n.常青藤，常春藤

参考例句：

• Her wedding bouquet consisted of roses and ivy.她的婚礼花篮包括玫瑰和长春藤。
• The wall is covered all over with ivy.墙上爬满了常春藤。

18 vascular   

adj.血管的，脉管的

参考例句：

• The mechanism of this anomalous vascular response is unknown.此种不规则的血管反应的机制尚不清楚。
• The vascular changes interfere with diffusion of nutrients from plasma into adjacent perivascular tissue and cells.这些血管变化干扰了营养物质从血浆中向血管周围邻接的组织和细胞扩散。

19 remarkable   

adj.显著的，异常的，非凡的，值得注意的

参考例句：

• She has made remarkable headway in her writing skills.她在写作技巧方面有了长足进步。
• These cars are remarkable for the quietness of their engines.这些汽车因发动机没有噪音而不同凡响。

20 investigation   

n.调查，调查研究

参考例句：

• In an investigation,a new fact became known, which told against him.在调查中新发现了一件对他不利的事实。
• He drew the conclusion by building on his own investigation.他根据自己的调查研究作出结论。

21 revolving   

adj.旋转的，轮转式的；循环的v.（使）旋转( revolve的现在分词 )；细想

参考例句：

• The theatre has a revolving stage. 剧院有一个旋转舞台。
• The company became a revolving-door workplace. 这家公司成了工作的中转站。

22 pointed   

adj.尖的，直截了当的

参考例句：

• He gave me a very sharp pointed pencil.他给我一支削得非常尖的铅笔。
• She wished to show Mrs.John Dashwood by this pointed invitation to her brother.她想通过对达茨伍德夫人提出直截了当的邀请向她的哥哥表示出来。

23 insufficient   

adj.(for，of)不足的，不够的

参考例句：

• There was insufficient evidence to convict him.没有足够证据给他定罪。
• In their day scientific knowledge was insufficient to settle the matter.在他们的时代，科学知识还不能足以解决这些问题。

24 varied   

adj.多样的，多变化的

参考例句：

• The forms of art are many and varied.艺术的形式是多种多样的。
• The hotel has a varied programme of nightly entertainment.宾馆有各种晚间娱乐活动。

25 deflected   

偏离的

参考例句：

• The ball deflected off Reid's body into the goal. 球打在里德身上反弹进球门。
• Most of its particles are deflected. 此物质的料子大多是偏斜的。

26 vestige   

n.痕迹，遗迹，残余

参考例句：

• Some upright stones in wild places are the vestige of ancient religions.荒原上一些直立的石块是古老宗教的遗迹。
• Every vestige has been swept away.一切痕迹都被一扫而光。

27 considerably   

adv.极大地；相当大地；在很大程度上

参考例句：

• The economic situation has changed considerably.经济形势已发生了相当大的变化。
• The gap has narrowed considerably.分歧大大缩小了。

28 bent   

n.爱好，癖好；adj.弯的；决心的，一心的

参考例句：

• He was fully bent upon the project.他一心扑在这项计划上。
• We bent over backward to help them.我们尽了最大努力帮助他们。

29 affected   

adj.不自然的，假装的

参考例句：

• She showed an affected interest in our subject.她假装对我们的课题感到兴趣。
• His manners are affected.他的态度不自然。

30 zigzag   

n.曲折，之字形；adj.曲折的，锯齿形的；adv.曲折地，成锯齿形地；vt.使曲折；vi.曲折前行

参考例句：

• The lightning made a zigzag in the sky.闪电在天空划出一道Z字形。
• The path runs zigzag up the hill.小径向山顶蜿蜒盘旋。

31 tempted   

v.怂恿（某人）干不正当的事；冒…的险（tempt的过去分词）

参考例句：

• I was sorely tempted to complain, but I didn't. 我极想发牢骚，但还是没开口。
• I was tempted by the dessert menu. 甜食菜单馋得我垂涎欲滴。

32 diffused   

散布的，普及的，扩散的

参考例句：

• A drop of milk diffused in the water. 一滴牛奶在水中扩散开来。
• Gases and liquids diffused. 气体和液体慢慢混合了。

33 fig   

n.无花果(树)

参考例句：

• The doctor finished the fig he had been eating and selected another.这位医生吃完了嘴里的无花果，又挑了一个。
• You can't find a person who doesn't know fig in the United States.你找不到任何一个在美国的人不知道无花果的。

34 fissured   

adj.裂缝的v.裂开( fissure的过去式和过去分词 )

参考例句：

• South African vine having a massive rootstock covered with deeply fissured bark. 南非藤蔓植物，有很大的根状茎，皮上有很深的裂纹。 来自互联网
• The concentrated leakage passage in fissured rock is studied with dummy heat source method. 利用虚拟热源法研究坝基裂隙岩体中存在的集中渗漏通道。 来自互联网

35 encumbered   

v.妨碍，阻碍，拖累( encumber的过去式和过去分词 )

参考例句：

• The police operation was encumbered by crowds of reporters. 警方的行动被成群的记者所妨碍。
• The narrow quay was encumbered by hundreds of carts. 狭窄的码头被数百辆手推车堵得水泄不通。 来自辞典例句

36 spores   

n.（细菌、苔藓、蕨类植物）孢子( spore的名词复数 )v.（细菌、苔藓、蕨类植物）孢子( spore的第三人称单数 )

参考例句：

• Ferns, mosses and fungi spread by means of spores. 蕨类植物、苔藓和真菌通过孢子传播蔓生。
• Spores form a lipid membrane during the process of reproducing. 孢于在生殖过程中形成类脂膜。 来自英汉非文学 - 生命科学 - 预防生物武器

37 varnish   

n.清漆；v.上清漆；粉饰

参考例句：

• He tried to varnish over the facts,but it was useless.他想粉饰事实，但那是徒劳的。
• He applied varnish to the table.他给那张桌子涂上清漆。

38 germinated   

v.（使）发芽( germinate的过去式和过去分词 )

参考例句：

• First, the researchers germinated the seeds. 研究人员首先让种子发芽。 来自辞典例句
• In spring they are germinated and grown for a year in beds. 春季里，他们在苗床发芽并生长一年。 来自辞典例句

39 previously   

adv.以前，先前（地）

参考例句：

• The bicycle tyre blew out at a previously damaged point.自行车胎在以前损坏过的地方又爆开了。
• Let me digress for a moment and explain what had happened previously.让我岔开一会儿，解释原先发生了什么。

40 transparent   

adj.明显的，无疑的；透明的

参考例句：

• The water is so transparent that we can see the fishes swimming.水清澈透明，可以看到鱼儿游来游去。
• The window glass is transparent.窗玻璃是透明的。

41 taper   

n.小蜡烛，尖细，渐弱；adj.尖细的；v.逐渐变小

参考例句：

• You'd better taper off the amount of time given to rest.你最好逐渐地减少休息时间。
• Pulmonary arteries taper towards periphery.肺动脉向周围逐渐变细。

42 vertical   

adj.垂直的，顶点的，纵向的；n.垂直物，垂直的位置

参考例句：

• The northern side of the mountain is almost vertical.这座山的北坡几乎是垂直的。
• Vertical air motions are not measured by this system.垂直气流的运动不用这种系统来测量。

43 interval   

n.间隔，间距；幕间休息，中场休息

参考例句：

• The interval between the two trees measures 40 feet.这两棵树的间隔是40英尺。
• There was a long interval before he anwsered the telephone.隔了好久他才回了电话。

44 intervals   

n.[军事]间隔( interval的名词复数 )；间隔时间；[数学]区间；（戏剧、电影或音乐会的）幕间休息

参考例句：

• The forecast said there would be sunny intervals and showers. 预报间晴，有阵雨。
• Meetings take place at fortnightly intervals. 每两周开一次会。

45 specified   

adj.特定的

参考例句：

• The architect specified oak for the wood trim. 那位建筑师指定用橡木做木饰条。
• It is generated by some specified means. 这是由某些未加说明的方法产生的。

46 analogous   

adj.相似的；类似的

参考例句：

• The two situations are roughly analogous.两种情況大致相似。
• The company is in a position closely analogous to that of its main rival.该公司与主要竞争对手的处境极为相似。

47 intermittent   

adj.间歇的，断断续续的

参考例句：

• Did you hear the intermittent sound outside?你听见外面时断时续的声音了吗？
• In the daytime intermittent rains freshened all the earth.白天里，时断时续地下着雨，使整个大地都生气勃勃了。

48 specially   

adv.特定地；特殊地；明确地

参考例句：

• They are specially packaged so that they stack easily.它们经过特别包装以便于堆放。
• The machine was designed specially for demolishing old buildings.这种机器是专为拆毁旧楼房而设计的。

49 perpendicular   

adj.垂直的，直立的；n.垂直线，垂直的位置

参考例句：

• The two lines of bones are set perpendicular to one another.这两排骨头相互垂直。
• The wall is out of the perpendicular.这墙有些倾斜。

50 conclusively   

adv.令人信服地，确凿地

参考例句：

• All this proves conclusively that she couldn't have known the truth. 这一切无可置疑地证明她不可能知道真相。 来自《简明英汉词典》
• From the facts,he was able to determine conclusively that the death was not a suicide. 根据这些事实他断定这起死亡事件并非自杀。 来自《简明英汉词典》

51 conclusive   

adj.最后的，结论的；确凿的，消除怀疑的

参考例句：

• They produced some fairly conclusive evidence.他们提供了一些相当确凿的证据。
• Franklin did not believe that the French tests were conclusive.富兰克林不相信这个法国人的实验是结论性的。

52 noted   

adj.著名的，知名的

参考例句：

• The local hotel is noted for its good table.当地的那家酒店以餐食精美而著称。
• Jim is noted for arriving late for work.吉姆上班迟到出了名。

53 superfluous   

adj.过多的，过剩的，多余的

参考例句：

• She fined away superfluous matter in the design. 她删去了这图案中多余的东西。
• That request seemed superfluous when I wrote it.我这样写的时候觉得这个请求似乎是多此一举。

54 mere   

adj.纯粹的；仅仅，只不过

参考例句：

• That is a mere repetition of what you said before.那不过是重复了你以前讲的话。
• It's a mere waste of time waiting any longer.再等下去纯粹是浪费时间。

55 linen   

n.亚麻布，亚麻线，亚麻制品；adj.亚麻布制的，亚麻的

参考例句：

• The worker is starching the linen.这名工人正在给亚麻布上浆。
• Fine linen and cotton fabrics were known as well as wool.精细的亚麻织品和棉织品像羊毛一样闻名遐迩。

56 stimulus   

n.刺激，刺激物，促进因素，引起兴奋的事物

参考例句：

• Regard each failure as a stimulus to further efforts.把每次失利看成对进一步努力的激励。
• Light is a stimulus to growth in plants.光是促进植物生长的一个因素。

57 contraction   

n.缩略词，缩写式，害病

参考例句：

• The contraction of this muscle raises the lower arm.肌肉的收缩使前臂抬起。
• The forces of expansion are balanced by forces of contraction.扩张力和收缩力相互平衡。

58 ascertain   

vt.发现，确定，查明，弄清

参考例句：

• It's difficult to ascertain the coal deposits.煤储量很难探明。
• We must ascertain the responsibility in light of different situtations.我们必须根据不同情况判定责任。

59 uncertainty   

n.易变，靠不住，不确知，不确定的事物

参考例句：

• Her comments will add to the uncertainty of the situation.她的批评将会使局势更加不稳定。
• After six weeks of uncertainty,the strain was beginning to take its toll.6个星期的忐忑不安后，压力开始产生影响了。

60 apex   

n.顶点，最高点

参考例句：

• He reached the apex of power in the early 1930s.他在三十年代初达到了权力的顶峰。
• His election to the presidency was the apex of his career.当选总统是他一生事业的顶峰。

61 stationary   

adj.固定的，静止不动的

参考例句：

• A stationary object is easy to be aimed at.一个静止不动的物体是容易瞄准的。
• Wait until the bus is stationary before you get off.你要等公共汽车停稳了再下车。

62 backwards   

adv.往回地，向原处，倒，相反，前后倒置地

参考例句：

• He turned on the light and began to pace backwards and forwards.他打开电灯并开始走来走去。
• All the girls fell over backwards to get the party ready.姑娘们迫不及待地为聚会做准备。

63 waned   

v.衰落( wane的过去式和过去分词 )；（月）亏；变小；变暗淡

参考例句：

• However,my enthusiasm waned.The time I spent at exercises gradually diminished. 然而，我的热情减退了。我在做操上花的时间逐渐减少了。 来自《用法词典》
• The bicycle craze has waned. 自行车热已冷下去了。 来自《现代汉英综合大词典》

64 vertically   

adv.垂直地

参考例句：

• Line the pages for the graph both horizontally and vertically.在这几页上同时画上横线和竖线，以便制作图表。
• The human brain is divided vertically down the middle into two hemispheres.人脑从中央垂直地分为两半球。

65 opposition   

n.反对，敌对

参考例句：

• The party leader is facing opposition in his own backyard.该党领袖在自己的党內遇到了反对。
• The police tried to break down the prisoner's opposition.警察设法制住了那个囚犯的反抗。

66 cylindrical   

adj.圆筒形的

参考例句：

• huge cylindrical gas tanks 巨大的圆柱形贮气罐
• Beer cans are cylindrical. 啤酒罐子是圆筒形的。

67 deviation   

n.背离，偏离；偏差，偏向；离题

参考例句：

• Deviation from this rule are very rare.很少有违反这条规则的。
• Any deviation from the party's faith is seen as betrayal.任何对党的信仰的偏离被视作背叛。

68 circumference   

n.圆周，周长，圆周线

参考例句：

• It's a mile round the circumference of the field.运动场周长一英里。
• The diameter and the circumference of a circle correlate.圆的直径与圆周有相互关系。

69 cylinder   

n.圆筒，柱(面)，汽缸

参考例句：

• What's the volume of this cylinder?这个圆筒的体积有多少？
• The cylinder is getting too much gas and not enough air.汽缸里汽油太多而空气不足。

70 axis   

n.轴，轴线，中心线；坐标轴，基准线

参考例句：

• The earth's axis is the line between the North and South Poles.地轴是南北极之间的线。
• The axis of a circle is its diameter.圆的轴线是其直径。

71 obliquely   

adv.斜； 倾斜； 间接； 不光明正大

参考例句：

• From the gateway two paths led obliquely across the court. 从门口那儿，有两条小路斜越过院子。 来自辞典例句
• He was receding obliquely with a curious hurrying gait. 他歪着身子，古怪而急促地迈着步子，往后退去。 来自辞典例句

72 accurately   

adv.准确地，精确地

参考例句：

• It is hard to hit the ball accurately.准确地击中球很难。
• Now scientists can forecast the weather accurately.现在科学家们能准确地预报天气。

73 relatively   

adv.比较...地，相对地

参考例句：

• The rabbit is a relatively recent introduction in Australia.兔子是相对较新引入澳大利亚的物种。
• The operation was relatively painless.手术相对来说不痛。

74 laterally   

ad.横向地；侧面地；旁边地

参考例句：

• Shafts were sunk, with tunnels dug laterally. 竖井已经打下，并且挖有横向矿道。
• When the plate becomes unstable, it buckles laterally. 当板失去稳定时，就发生横向屈曲。

75 intelligible   

adj.可理解的，明白易懂的，清楚的

参考例句：

• This report would be intelligible only to an expert in computing.只有计算机运算专家才能看懂这份报告。
• His argument was barely intelligible.他的论点不易理解。

76 respiration   

n.呼吸作用；一次呼吸；植物光合作用

参考例句：

• They tried artificial respiration but it was of no avail.他们试做人工呼吸，可是无效。
• They made frequent checks on his respiration,pulse and blood.他们经常检查他的呼吸、脉搏和血液。

77 abrupt   

adj.突然的，意外的；唐突的，鲁莽的

参考例句：

• The river takes an abrupt bend to the west.这河突然向西转弯。
• His abrupt reply hurt our feelings.他粗鲁的回答伤了我们的感情。

78 determined   

adj.坚定的；有决心的

参考例句：

• I have determined on going to Tibet after graduation.我已决定毕业后去西藏。
• He determined to view the rooms behind the office.他决定查看一下办公室后面的房间。

79 standing   

n.持续，地位；adj.永久的，不动的，直立的，不流动的

参考例句：

• After the earthquake only a few houses were left standing.地震过后只有几幢房屋还立着。
• They're standing out against any change in the law.他们坚决反对对法律做任何修改。

80 strictly   

adv.严厉地，严格地；严密地

参考例句：

• His doctor is dieting him strictly.他的医生严格规定他的饮食。
• The guests were seated strictly in order of precedence.客人严格按照地位高低就座。

81 insignificant   

adj.无关紧要的，可忽略的，无意义的

参考例句：

• In winter the effect was found to be insignificant.在冬季，这种作用是不明显的。
• This problem was insignificant compared to others she faced.这一问题与她面临的其他问题比较起来算不得什么。

82 stimulated   

a.刺激的

参考例句：

• The exhibition has stimulated interest in her work. 展览增进了人们对她作品的兴趣。
• The award has stimulated her into working still harder. 奖金促使她更加努力地工作。

83 rigid   

adj.严格的，死板的；刚硬的，僵硬的

参考例句：

• She became as rigid as adamant.她变得如顽石般的固执。
• The examination was so rigid that nearly all aspirants were ruled out.考试很严，几乎所有的考生都被淘汰了。

84 smeared   

弄脏； 玷污； 涂抹； 擦上

参考例句：

• The children had smeared mud on the walls. 那几个孩子往墙上抹了泥巴。
• A few words were smeared. 有写字被涂模糊了。

85 specifying   

v.指定( specify的现在分词 )；详述；提出…的条件；使具有特性

参考例句：

• When we describe what the action will affect, we are specifying the noun of the sentence. 当描述动作会影响到什么时，我们指定组成句子的名词。 来自About Face 3交互设计精髓
• Procurement section only lists opportunistic infection drugs without specifying which drugs. 采购部分只说明有治疗机会性感染的药物，但并没有说明是什么药物。 来自互联网

86 amputation   

n.截肢

参考例句：

• In ancient India,adultery was punished by amputation of the nose.在古代印度，通奸要受到剖鼻的处罚。
• He lived only hours after the amputation.截肢后，他只活了几个小时。

87 interfere   

v.(in)干涉，干预；(with)妨碍，打扰

参考例句：

• If we interfere, it may do more harm than good.如果我们干预的话，可能弊多利少。
• When others interfere in the affair,it always makes troubles. 别人一卷入这一事件，棘手的事情就来了。

88 specimens   

n.样品( specimen的名词复数 )；范例；（化验的）抽样；某种类型的人

参考例句：

• Astronauts have brought back specimens of rock from the moon. 宇航员从月球带回了岩石标本。
• The traveler brought back some specimens of the rocks from the mountains. 那位旅行者从山上带回了一些岩石标本。 来自《简明英汉词典》

89 impermeable   

adj.不能透过的，不渗透的

参考例句：

• The canoe is made from an impermeable wood.独木舟由防水木头制成。
• The external layer of the skin is relatively impermeable to water.皮肤的外层不透水。

90 opaque   

adj.不透光的；不反光的，不传导的；晦涩的

参考例句：

• The windows are of opaque glass.这些窗户装着不透明玻璃。
• Their intentions remained opaque.他们的意图仍然令人费解。

91 interfered   

v.干预( interfere的过去式和过去分词 )；调停；妨碍；干涉

参考例句：

• Complete absorption in sports interfered with his studies. 专注于运动妨碍了他的学业。 来自《简明英汉词典》
• I am not going to be interfered with. 我不想别人干扰我的事情。 来自《简明英汉词典》

92 hazy   

adj.有薄雾的，朦胧的；不肯定的，模糊的

参考例句：

• We couldn't see far because it was so hazy.雾气蒙蒙妨碍了我们的视线。
• I have a hazy memory of those early years.对那些早先的岁月我有着朦胧的记忆。

93 oxide   

n.氧化物

参考例句：

• Oxide is usually seen in our daily life.在我们的日常生活中氧化物很常见。
• How can you get rid of this oxide coating?你们该怎样除去这些氧化皮？

94 furrows   

n.犁沟( furrow的名词复数 )；（脸上的）皱纹v.犁田，开沟( furrow的第三人称单数 )

参考例句：

• I could tell from the deep furrows in her forehead that she was very disturbed by the news. 从她额头深深的皱纹上，我可以看出她听了这个消息非常不安。 来自《简明英汉词典》
• Dirt bike trails crisscrossed the grassy furrows. 越野摩托车的轮迹纵横交错地布满条条草沟。 来自辞典例句

95 furrow   

n.沟；垄沟；轨迹；车辙；皱纹

参考例句：

• The tractor has make deep furrow in the loose sand.拖拉机在松软的沙土上留下了深深的车辙。
• Mei did not weep.She only bit her lips,and the furrow in her brow deepened.梅埋下头，她咬了咬嘴唇皮，额上的皱纹显得更深了。

96 protruded   

v.（使某物）伸出，（使某物）突出( protrude的过去式和过去分词 )

参考例句：

• The child protruded his tongue. 那小孩伸出舌头。 来自《简明英汉词典》
• The creature's face seemed to be protruded, because of its bent carriage. 那人的脑袋似乎向前突出，那是因为身子佝偻的缘故。 来自英汉文学

97 impermeability   

n.不能渗透的性质或状态,不渗透性,不透过性

参考例句：

• Air-entraining agent increases impermeability of hybrid fiber concrete. 引气剂有助于提高混杂纤维混凝土的抗渗性。
• Foam has a robust and independent foam connection, water absorption in less than 25%. impermeability. 发泡有强韧而独立的发泡连接，吸水率在25%以内。具有防潮、抗渗性能。

98 counteract   

vt.对…起反作用，对抗，抵消

参考例句：

• The doctor gave him some medicine to counteract the effect of the poison.医生给他些药解毒。
• Our work calls for mutual support.We shouldn't counteract each other's efforts.工作要互相支持，不要互相拆台。

99 alluded   

提及，暗指( allude的过去式和过去分词 )

参考例句：

• In your remarks you alluded to a certain sinister design. 在你的谈话中，你提到了某个阴谋。
• She also alluded to her rival's past marital troubles. 她还影射了对手过去的婚姻问题。

100 slit   

n.狭长的切口；裂缝；vt.切开，撕裂

参考例句：

• The coat has been slit in two places.这件外衣有两处裂开了。
• He began to slit open each envelope.他开始裁开每个信封。

101 slits   

n.狭长的口子，裂缝( slit的名词复数 )v.切开，撕开( slit的第三人称单数 )；在…上开狭长口子

参考例句：

• He appears to have two slits for eyes. 他眯着两眼。
• "You go to--Halifax,'she said tensely, her green eyes slits of rage. "你给我滚----滚到远远的地方去！" 她恶狠狠地说，那双绿眼睛冒出了怒火。

102 potent   

adj.强有力的，有权势的；有效力的

参考例句：

• The medicine had a potent effect on your disease.这药物对你的病疗效很大。
• We must account of his potent influence.我们必须考虑他的强有力的影响。

103 confirmation   

n.证实，确认，批准

参考例句：

• We are waiting for confirmation of the news.我们正在等待证实那个消息。
• We need confirmation in writing before we can send your order out.给你们发送订购的货物之前，我们需要书面确认。

104 stimulation   

n.刺激，激励，鼓舞

参考例句：

• The playgroup provides plenty of stimulation for the children.幼儿游戏组给孩子很多启发。
• You don't get any intellectual stimulation in this job.你不能从这份工作中获得任何智力启发。

105 briefly   

adv.简单地，简短地

参考例句：

• I want to touch briefly on another aspect of the problem.我想简单地谈一下这个问题的另一方面。
• He was kidnapped and briefly detained by a terrorist group.他被一个恐怖组织绑架并短暂拘禁。

106 quills   

n.（刺猬或豪猪的）刺( quill的名词复数 )；羽毛管；翮；纡管

参考例句：

• Quills were the chief writing implement from the 6th century AD until the advent of steel pens in the mid 19th century. 从公元6世纪到19世纪中期钢笔出现以前，羽毛笔是主要的书写工具。 来自《简明英汉词典》
• Defensive quills dot the backs of these troublesome creatures. 防御性的刺长在这些讨人厌的生物背上。 来自互联网

107 secondly   

adv.第二，其次

参考例句：

• Secondly,use your own head and present your point of view.第二，动脑筋提出自己的见解。
• Secondly it is necessary to define the applied load.其次，需要确定所作用的载荷。

108 inexplicable   

adj.无法解释的，难理解的

参考例句：

• It is now inexplicable how that development was misinterpreted.当时对这一事态发展的错误理解究竟是怎么产生的，现在已经无法说清楚了。
• There are many things which are inexplicable by science.有很多事科学还无法解释。

109 prodigy   

n.惊人的事物，奇迹，神童，天才，预兆

参考例句：

• She was a child prodigy on the violin.她是神童小提琴手。
• He was always a Negro prodigy who played barbarously and wonderfully.他始终是一个黑人的奇才，这种奇才弹奏起来粗野而惊人。

110 penetrate   

v.透(渗)入；刺入，刺穿；洞察，了解

参考例句：

• Western ideas penetrate slowly through the East.西方观念逐渐传入东方。
• The sunshine could not penetrate where the trees were thickest.阳光不能透入树木最浓密的地方。

111 penetrated   

adj. 击穿的，鞭辟入里的 动词penetrate的过去式和过去分词形式

参考例句：

• The knife had penetrated his chest. 刀子刺入了他的胸膛。
• They penetrated into territory where no man had ever gone before. 他们已进入先前没人去过的地区。

112 halfway   

adj.中途的，不彻底的，部分的；adv.半路地，在中途，在半途

参考例句：

• We had got only halfway when it began to get dark.走到半路，天就黑了。
• In study the worst danger is give up halfway.在学习上，最忌讳的是有始无终。

113 prostrate   

v.拜倒，平卧，衰竭；adj.拜倒的，平卧的，衰竭的

参考例句：

• She was prostrate on the floor.她俯卧在地板上。
• The Yankees had the South prostrate and they intended to keep It'so.北方佬已经使南方屈服了,他们还打算继续下去。

114 erect   

n./v.树立，建立，使竖立；adj.直立的，垂直的

参考例句：

• She held her head erect and her back straight.她昂着头，把背挺得笔直。
• Soldiers are trained to stand erect.士兵们训练站得笔直。

115 inclination   

n.倾斜；点头；弯腰；斜坡；倾度；倾向；爱好

参考例句：

• She greeted us with a slight inclination of the head.她微微点头向我们致意。
• I did not feel the slightest inclination to hurry.我没有丝毫着急的意思。

116 distinguished   

adj.卓越的，杰出的，著名的

参考例句：

• Elephants are distinguished from other animals by their long noses.大象以其长长的鼻子显示出与其他动物的不同。
• A banquet was given in honor of the distinguished guests.宴会是为了向贵宾们致敬而举行的。

117 remains   

n.剩余物，残留物；遗体，遗迹

参考例句：

• He ate the remains of food hungrily.他狼吞虎咽地吃剩余的食物。
• The remains of the meal were fed to the dog.残羹剩饭喂狗了。

118 subterranean   

adj.地下的，地表下的

参考例句：

• London has 9 miles of such subterranean passages.伦敦像这样的地下通道有9英里长。
• We wandered through subterranean passages.我们漫游地下通道。

119 aggregate   

adj.总计的，集合的；n.总数；v.合计；集合

参考例句：

• The football team had a low goal aggregate last season.这支足球队上个赛季的进球总数很少。
• The money collected will aggregate a thousand dollars.进帐总额将达一千美元。

120 caustic   

adj.刻薄的，腐蚀性的

参考例句：

• He opened his mouth to make a caustic retort.他张嘴开始进行刻薄的反击。
• He enjoys making caustic remarks about other people.他喜欢挖苦别人。

121 inevitable   

adj.不可避免的，必然发生的

参考例句：

• Mary was wearing her inevitable large hat.玛丽戴着她总是戴的那顶大帽子。
• The defeat had inevitable consequences for British policy.战败对英国政策不可避免地产生了影响。

122 anterior   

adj.较早的；在前的

参考例句：

• We've already finished the work anterior to the schedule.我们已经提前完成了工作。
• The anterior part of a fish contains the head and gills.鱼的前部包括头和鳃。

123 physiological   

adj.生理学的，生理学上的

参考例句：

• He bought a physiological book.他买了一本生理学方面的书。
• Every individual has a physiological requirement for each nutrient.每个人对每种营养成分都有一种生理上的需要。

124 apparently   

adv.显然地；表面上，似乎

参考例句：

• An apparently blind alley leads suddenly into an open space.山穷水尽，豁然开朗。
• He was apparently much surprised at the news.他对那个消息显然感到十分惊异。

125 gland   

n.腺体，(机)密封压盖，填料盖

参考例句：

• This is a snake's poison gland.这就是蛇的毒腺。
• Her mother has an underactive adrenal gland.她的母亲肾上腺机能不全。

126 tentacle   

n.触角，触须，触手

参考例句：

• Each tentacle is about two millimeters long.每一个触手大约两毫米长。
• It looked like a big eyeball with a long tentacle thing.它看上去像一个有着长触角的巨大眼球。

127 filament   

n.细丝；长丝；灯丝

参考例句：

• The source of electrons in an electron microscope is a heated filament.电子显微镜中的电子源，是一加热的灯丝。
• The lack of air in the bulb prevents the filament from burning up.灯泡内缺乏空气就使灯丝不致烧掉。

128 stimuli   

n.刺激（物）

参考例句：

• It is necessary to curtail or alter normally coexisting stimuli.必需消除或改变正常时并存的刺激。
• My sweat glands also respond to emotional stimuli.我的汗腺对情绪刺激也能产生反应。

129 modification   

n.修改，改进，缓和，减轻

参考例句：

• The law,in its present form,is unjust;it needs modification.现行的法律是不公正的，它需要修改。
• The design requires considerable modification.这个设计需要作大的修改。

130 catching   

adj.易传染的，有魅力的，迷人的，接住

参考例句：

• There are those who think eczema is catching.有人就是认为湿疹会传染。
• Enthusiasm is very catching.热情非常富有感染力。

131 progenitors   

n.祖先( progenitor的名词复数 )；先驱；前辈；原本

参考例句：

• The researchers also showed that the progenitors mature into neurons in Petri dishes. 研究人员还表示，在佩特里培养皿中的脑细胞前体可以发育成神经元。 来自英汉非文学 - 生命科学 - 大脑与疾病
• Though I am poor and wretched now, my progenitors were famously wealthy. 别看我现在穷困潦倒，我家上世可是有名的富翁。 来自互联网

132 twine   

v.搓，织，编饰；（使）缠绕

参考例句：

• He tied the parcel with twine.他用细绳捆包裹。
• Their cardboard boxes were wrapped and tied neatly with waxed twine.他们的纸板盒用蜡线扎得整整齐齐。

133 crevice   

n.(岩石、墙等)裂缝；缺口

参考例句：

• I saw a plant growing out of a crevice in the wall.我看到墙缝里长出一棵草来。
• He edged the tool into the crevice.他把刀具插进裂缝里。

134 credible   

adj.可信任的，可靠的

参考例句：

• The news report is hardly credible.这则新闻报道令人难以置信。
• Is there a credible alternative to the nuclear deterrent?是否有可以取代核威慑力量的可靠办法？

135 illustrate   

v.举例说明，阐明；图解，加插图

参考例句：

• The company's bank statements illustrate its success.这家公司的银行报表说明了它的成功。
• This diagram will illustrate what I mean.这个图表可说明我的意思。

136 evaporation   

n.蒸发，消失

参考例句：

• Be careful not to lose too much liquid by evaporation.小心不要因蒸发失去太多水分。
• Our bodies can sweat,thereby losing heat by evaporation.我们的身体能出汗，由此可以蒸发散热。

137 proboscis   

n.（象的）长鼻

参考例句：

• Its proboscis has got stuck to a lot of pollen.它的喙上粘了很多花粉。
• It hovers in front of the flower,using its proboscis to look for nectar. 它在兰花前面飞来飞去, 用喙寻找花蜜.

138 moth   

n.蛾，蛀虫

参考例句：

• A moth was fluttering round the lamp.有一只蛾子扑打着翅膀绕着灯飞。
• The sweater is moth-eaten.毛衣让蛀虫咬坏了。

139 entangled   

adj.卷入的；陷入的；被缠住的；缠在一起的v.使某人（某物/自己）缠绕，纠缠于（某物中），使某人（自己）陷入（困难或复杂的环境中）( entangle的过去式和过去分词 )

参考例句：

• The bird had become entangled in the wire netting. 那只小鸟被铁丝网缠住了。
• Some military observers fear the US could get entangled in another war. 一些军事观察家担心美国会卷入另一场战争。 来自《简明英汉词典》

140 downwards   

adj./adv.向下的（地），下行的（地）

参考例句：

• He lay face downwards on his bed.他脸向下伏在床上。
• As the river flows downwards,it widens.这条河愈到下游愈宽。

141 ellipses   

n.椭园，省略号；椭圆( ellipse的名词复数 )；（语法结构上的）省略( ellipsis的名词复数 )

参考例句：

• The planets move around the sun in ellipses. 各行星围绕太阳按椭圆形运转。 来自《简明英汉词典》
• Summations are almost invariably indicated ellipses instead of the more prevalent sigma notation. 在表示“连加”的式子中，几乎一成不变地使用省略号来代替更为流行的“∑”符号。 来自辞典例句

142 lessened   

减少的，减弱的

参考例句：

• Listening to the speech through an interpreter lessened its impact somewhat. 演讲辞通过翻译的嘴说出来，多少削弱了演讲的力量。
• The flight to suburbia lessened the number of middle-class families living within the city. 随着迁往郊外的风行，住在城内的中产家庭减少了。

143 retarded   

a.智力迟钝的，智力发育迟缓的

参考例句：

• The progression of the disease can be retarded by early surgery. 早期手术可以抑制病情的发展。
• He was so slow that many thought him mentally retarded. 他迟钝得很，许多人以为他智力低下。

144 expenditure   

n.(时间、劳力、金钱等)支出；使用，消耗

参考例句：

• The entry of all expenditure is necessary.有必要把一切开支入账。
• The monthly expenditure of our family is four hundred dollars altogether.我们一家的开销每月共计四百元。

145 preservation   

n.保护，维护，保存，保留，保持

参考例句：

• The police are responsible for the preservation of law and order.警察负责维持法律与秩序。
• The picture is in an excellent state of preservation.这幅画保存得极为完好。

146 habitual   

adj.习惯性的；通常的，惯常的

参考例句：

• He is a habitual criminal.他是一个惯犯。
• They are habitual visitors to our house.他们是我家的常客。

147 contingent   

adj.视条件而定的；n.一组，代表团，分遣队

参考例句：

• The contingent marched in the direction of the Western Hills.队伍朝西山的方向前进。
• Whether or not we arrive on time is contingent on the weather.我们是否按时到达要视天气情况而定。

148 amplitude   

n.广大；充足；振幅

参考例句：

• The amplitude of the vibration determines the loudness of the sound.振动幅度的大小决定声音的大小。
• The amplitude at the driven end is fixed by the driving mechanism.由于驱动机构的作用,使驱动端的振幅保持不变。

149 innate   

adj.天生的，固有的，天赋的

参考例句：

• You obviously have an innate talent for music.你显然有天生的音乐才能。
• Correct ideas are not innate in the mind.人的正确思想不是自己头脑中固有的。