Until recently the cause of all such bending movements was believed to be due to the increased growth of the side which becomes for a time convex; that this side does temporarily grow more quickly than the concave side has been well established; but De Vries has lately shown that such increased growth follows a previously11 increased state of turgescence on the convex side.* In the case of parts provided with a so-called joint12, cushion or pulvinus, which consists of an aggregate13 of small cells that have ceased to increase in size from a very early age, we meet with similar movements; and here, as Pfeffer has shown** and as we shall see in the course of this work, the increased turgescence of the cells on opposite sides is not followed by increased growth. Wiesner denies in certain cases the accuracy of De Vries' conclusion about turgescence, and maintains*** that the increased extensibility of the cell-walls is the more important element. That such extensibility must accompany increased turgescence in order that the part may bend is manifest, and this has been insisted on by several botanists15; but in the case of unicellular plants it can hardly fail to be the more important element. On the whole we may at present conclude that in-
* Sachs first showed ('Lehrbuch,' etc., 4th edit. p. 452) the intimate connection between turgescence and growth. For De Vries' interesting essay, 'Wachsthumskrümmungen mehrzelliger Organe,' see 'Bot. Zeitung,' Dec. 19, 1879, p. 830.
** 'Die Periodischen Bewegungen der Blattorgane,' 1875.
*** 'Untersuchungen über den14 Heliotropismus,' Sitzb. der K. Akad. der Wissenschaft. (Vienna), Jan. 1880.
[page 3] creased10 growth, first on one side and then on another, is a secondary effect, and that the increased turgescence of the cells, together with the extensibility of their walls, is the primary cause of the movement of circumnutation.*
In the course of the present volume it will be shown that apparently16 every growing part of every plant is continually circumnutating, though often on a small scale. Even the stems of seedlings17 before they have broken through the ground, as well as their buried radicles, circumnutate, as far as the pressure of the surrounding earth permits. In this universally present movement we have the basis or groundwork for the acquirement, according to the requirements of the plant, of the most diversified19 movements. Thus, the great sweeps made by the stems of twining plants, and by the tendrils of other climbers, result from a mere20 increase in the amplitude21 of the ordinary movement of circumnutation. The position which young leaves and other organs ultimately assume is acquired by the circumnutating movement being increased in some one direction. the leaves of various plants are said to sleep at night, and it will be seen that their blades then assume a vertical22 position through modified circumnutation, in order to protect their upper surfaces from being chilled through radiation. The movements of various organs to the light, which are so general throughout the vegetable kingdom, and occasionally from the light, or transversely with respect to it, are all modified
* See Mr. Vines' excellent discussion ('Arbeiten des Bot. Instituts in Würzburg,' B. II. pp. 142, 143, 1878) on this intricate subject. Hofmeister's observations ('Jahreschrifte des Vereins für Vaterl. Naturkunde in Würtemberg,' 1874, p. 211) on the curious movements of Spirogyra, a plant consisting of a single row of cells, are valuable in relation to this subject.
[page 4] forms of circumnutation; as again are the equally prevalent movements of stems, etc., towards the zenith, and of roots towards the centre of the earth. In accordance with these conclusions, a considerable difficulty in the way of evolution is in part removed, for it might have been asked, how did all these diversified movements for the most different purposes first arise? As the case stands, we know that there is always movement in progress, and its amplitude, or direction, or both, have only to be modified for the good of the plant in relation with internal or external stimuli23.
Besides describing the several modified forms of circumnutation, some other subjects will be discussed. The two which have interested us most are, firstly, the fact that with some seedling18 plants the uppermost part alone is sensitive to light, and transmits an influence to the lower part, causing it to bend. If therefore the upper part be wholly protected from light, the lower part may be exposed for hours to it, and yet does not become in the least bent, although this would have occurred quickly if the upper part had been excited by light. Secondly24, with the radicles of seedlings, the tip is sensitive to various stimuli, especially to very slight pressure, and when thus excited, transmits an influence to the upper part, causing it to bend from the pressed side. On the other hand, if the tip is subjected to the vapour of water proceeding25 from one side, the upper part of the radicle bends towards this side. Again it is the tip, as stated by Ciesielski, though denied by others, which is sensitive to the attraction of gravity, and by transmission causes the adjoining parts of the radicle to bend towards the centre of the earth. These several cases of the effects of contact, other irritants, vapour, light, and the [page 5] attraction of gravity being transmitted from the excited part for some little distance along the organ in question, have an important bearing on the theory of all such movements.
[Terminology.—A brief explanation of some terms which will be used, must here be given. With seedlings, the stem which supports the cotyledons (i.e. the organs which represent the first leaves) has been called by many botanists the hypocotyledonous stem, but for brevity sake we will speak of it merely as the hypocotyl: the stem immediately above the cotyledons will be called the epicotyl or plumule. The radicle can be distinguished26 from the hypocotyl only by the presence of root-hairs and the nature of its covering. The meaning of the word circumnutation has already been explained. Authors speak of positive and negative heliotropism,*—that is, the bending of an organ to or from the light; but it is much more convenient to confine the word heliotropism to bending towards the light, and to designate as apheliotropism bending from the light. There is another reason for this change, for writers, as we have observed, occasionally drop the adjectives positive and negative, and thus introduce confusion into their discussions. Diaheliotropism may express a position more or less transverse to the light and induced by it. In like manner positive geotropism, or bending towards the centre of the earth, will be called by us geotropism; apogeotropism will mean bending in opposition27 to gravity or from the centre of the earth; and diageotropism, a position more or less transverse to the radius28 of the earth. The words heliotropism and geotropism properly mean the act of moving in relation to the light or the earth; but in the same manner as gravitation, though defined as "the act of tending to the centre," is often used to express the cause of a body falling, so it will be found convenient occasionally to employ heliotropism and geotropism, etc., as the cause of the movements in question.
The term epinasty is now often used in Germany, and implies that the upper surface of an organ grows more quickly than the
* The highly useful terms of Heliotropism and Geotropism were first used by Dr. A. B. Frank: see his remarkable29 'Beitr?ge zur Pflanzenphysiologie,' 1868. [page 6] lower surface, and thus causes it to bend downwards30. Hyponasty is the reverse, and implies increased growth along the lower surface, causing the part to bend upwards.*
Methods of Observation.—The movements, sometimes very small and sometimes considerable in extent, of the various organs observed by us, were traced in the manner which after many trials we found to be best, and which must be described. Plants growing in pots were protected wholly from the light, or had light admitted from above, or on one side as the case might require, and were covered above by a large horizontal sheet of glass, and with another vertical sheet on one side. A glass filament31, not thicker than a horsehair, and from a quarter to three-quarters of an inch in length, was affixed32 to the part to be observed by means of shellac dissolved in alcohol. The solution was allowed to evaporate, until it became so thick that it set hard in two or three seconds, and it never injured the tissues, even the tips of tender radicles, to which it was applied34. To the end of the glass filament an excessively minute bead35 of black sealing-wax was cemented, below or behind which a bit of card with a black dot was fixed33 to a stick driven into the ground. The weight of the filament was so slight that even small leaves were not perceptibly pressed down. another method of observation, when much magnification of the movement was not required, will presently be described. The bead and the dot on the card were viewed through the horizontal or vertical glass-plate (according to the position of the object), and when one exactly covered the other, a dot was made on the glass-plate with a sharply pointed36 stick dipped in thick Indian-ink. Other dots were made at short intervals37 of time and these were afterwards joined by straight lines. The figures thus traced were therefore angular; but if dots had been made every 1 or 2 minutes, the lines would have been more curvilinear, as occurred when radicles were allowed to trace their own courses on smoked glass-plates. To make the dots accurately38 was the sole difficulty, and required some practice. Nor could this be done quite accurately, when the movement was much magnified, such as 30 times and upwards; yet even in this case the general course may be trusted. To test the accuracy of the above method of observation, a filament was fixed to an
* These terms are used in the sense given them by De Vries, 'Würzburg Arbeiten,' Heft ii 1872, p. 252.
[page 7] inanimate object which was made to slide along a straight edge and dots were repeatedly made on a glass-plate; when these were joined, the result ought to have been a perfectly39 straight line, and the line was very nearly straight. It may be added that when the dot on the card was placed half-an-inch below or behind the bead of sealing-wax, and when the glass-plate (supposing it to have been properly curved) stood at a distance of 7 inches in front (a common distance), then the tracing represented the movement of the bead magnified 15 times.
Whenever a great increase of the movement was not required, another, and in some respects better, method of observation was followed. This consisted in fixing two minute triangles of thin paper, about 1/20 inch in height, to the two ends of the attached glass filament; and when their tips were brought into a line so that they covered one another, dots were made as before on the glass-plate. If we suppose the glass-plate to stand at a distance of seven inches from the end of the shoot bearing the filament, the dots when joined, will give nearly the same figure as if a filament seven inches long, dipped in ink, had been fixed to the moving shoot, and had inscribed40 its own course on the plate. The movement is thus considerably41 magnified; for instance, if a shoot one inch in length were bending, and the glass-plate stood at the distance of seven inches, the movement would be magnified eight times. It would, however, have been very difficult to have ascertained42 in each case how great a length of the shoot was bending; and this is indispensable for ascertaining43 the degree to which the movement is magnified.
After dots had been made on the glass-plates by either of the above methods, they were copied on tracing paper and joined by ruled lines, with arrows showing the direction of the movement. The nocturnal courses are represented by straight broken lines. the first dot is always made larger than the others, so as to catch the eye, as may be seen in the diagrams. The figures on the glass-plates were often drawn44 on too large a scale to be reproduced on the pages of this volume, and the proportion in which they have been reduced is always given.* Whenever it could be approximately told how much the movement had been magnified, this is stated. We have perhaps
* We are much indebted to Mr. Cooper for the care with which he has reduced and engraved45 our diagrams.
[page 8] introduced a superfluous46 number of diagrams; but they take up less space than a full description of the movements. Almost all the sketches47 of plants asleep, etc., were carefully drawn for us by Mr. George Darwin.
As shoots, leaves, etc., in circumnutating bend more and more, first in one direction and then in another, they were necessarily viewed at different times more or less obliquely48; and as the dots were made on a flat surface, the apparent amount of movement is exaggerated according to the degree of obliquity49 of the point of view. It would, therefore, have been a much better plan to have used hemispherical glasses, if we had possessed50 them of all sizes, and if the bending part of the shoot had been distinctly hinged and could have been placed so as to have formed one of the radii51 of the sphere. But even in this case it would have been necessary afterwards to have projected the figures on paper; so that complete accuracy could not have been attained52. From the distortion of our figures, owing to the above causes, they are of no use to any one who wishes to know the exact amount of movement, or the exact course pursued; but they serve excellently for ascertaining whether or not the part moved at all, as well as the general character of the movement.]
In the following chapters, the movements of a considerable number of plants are described; and the species have been arranged according to the system adopted by Hooker in Le Maout and Decaisne's 'Descriptive Botany.' No one who is not investigating the present subject need read all the details, which, however, we have thought it advisable to give. To save the reader trouble, the conclusions and most of the more important parts have been printed in larger type than the other parts. He may, if he thinks fit, read the last chapter first, as it includes a summary of the whole volume; and he will thus see what points interest him, and on which he requires the full evidence.
Finally, we must have the pleasure of returning our [page 9] sincere thanks to Sir Joseph Hooker and to Mr. W. Thiselton Dyer for their great kindness, in not only sending us plants from Kew, but in procuring53 others from several sources when they were required for our observations; also, for naming many species, and giving us information on various points.
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essentially
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| adv.本质上，实质上，基本上 | |
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revolves
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| v.（使）旋转( revolve的第三人称单数 )；细想 | |
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revolving
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| adj.旋转的，轮转式的；循环的v.（使）旋转( revolve的现在分词 )；细想 | |
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briefly
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| adv.简单地，简短地 | |
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bent
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| n.爱好，癖好；adj.弯的；决心的，一心的 | |
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apex
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| n.顶点，最高点 | |
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upwards
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| adv.向上，在更高处...以上 | |
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ellipses
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| 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 ) | |
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zigzag
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| 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 | |
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creased
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| （使…）起折痕，弄皱( crease的过去式和过去分词 )； （皮肤）皱起，使起皱纹； 皱皱巴巴 | |
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previously
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| adv.以前，先前（地） | |
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joint
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| adj.联合的，共同的；n.关节，接合处；v.连接，贴合 | |
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aggregate
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| adj.总计的，集合的；n.总数；v.合计；集合 | |
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den
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| n.兽穴；秘密地方；安静的小房间，私室 | |
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botanists
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| n.植物学家，研究植物的人( botanist的名词复数 ) | |
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apparently
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| adv.显然地；表面上，似乎 | |
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seedlings
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| n.刚出芽的幼苗( seedling的名词复数 ) | |
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seedling
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| n.秧苗，树苗 | |
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diversified
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| adj.多样化的，多种经营的v.使多样化，多样化( diversify的过去式和过去分词 )；进入新的商业领域 | |
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mere
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| adj.纯粹的；仅仅，只不过 | |
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amplitude
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| n.广大；充足；振幅 | |
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vertical
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| adj.垂直的，顶点的，纵向的；n.垂直物，垂直的位置 | |
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stimuli
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secondly
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proceeding
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| n.行动，进行，(pl.)会议录，学报 | |
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distinguished
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| adj.卓越的，杰出的，著名的 | |
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opposition
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| n.反对，敌对 | |
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radius
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| n.半径，半径范围；有效航程，范围，界限 | |
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remarkable
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| adj.显著的，异常的，非凡的，值得注意的 | |
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downwards
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| adj./adv.向下的（地），下行的（地） | |
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filament
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| n.细丝；长丝；灯丝 | |
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affixed
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| adj.[医]附着的，附着的v.附加( affix的过去式和过去分词 )；粘贴；加以；盖（印章） | |
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fixed
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| adj.固定的，不变的，准备好的；(计算机)固定的 | |
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applied
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| adj.应用的；v.应用，适用 | |
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bead
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| n.念珠；(pl.)珠子项链；水珠 | |
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pointed
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| adj.尖的，直截了当的 | |
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intervals
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| 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 | |
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accurately
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| adv.准确地，精确地 | |
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perfectly
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| adv.完美地，无可非议地，彻底地 | |
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inscribed
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| v.写，刻( inscribe的过去式和过去分词 )；内接 | |
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considerably
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| adv.极大地；相当大地；在很大程度上 | |
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ascertained
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| v.弄清，确定，查明( ascertain的过去式和过去分词 ) | |
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ascertaining
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| v.弄清，确定，查明( ascertain的现在分词 ) | |
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drawn
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| v.拖，拉，拔出；adj.憔悴的，紧张的 | |
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engraved
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| v.在（硬物）上雕刻（字，画等）( engrave的过去式和过去分词 )；将某事物深深印在（记忆或头脑中） | |
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superfluous
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| adj.过多的，过剩的，多余的 | |
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sketches
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| n.草图( sketch的名词复数 )；素描；速写；梗概 | |
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obliquely
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| adv.斜； 倾斜； 间接； 不光明正大 | |
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obliquity
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| n.倾斜度 | |
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possessed
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| adj.疯狂的；拥有的，占有的 | |
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radii
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| n.半径；半径（距离）( radius的名词复数 )；用半径度量的圆形面积；半径范围；桡骨 | |
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attained
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| （通常经过努力）实现( attain的过去式和过去分词 )； 达到； 获得； 达到（某年龄、水平、状况） | |
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procuring
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| v.（努力）取得， （设法）获得( procure的现在分词 )；拉皮条 | |
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