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- The Power of Movement in Plants - 2/98 -
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 exclusion of light from their tips--Effects transmitted beneath the surface of the ground--Lateral 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...Page 449-492
MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY GRAVITATION.
Means of observation--Apogeotropism--Cytisus--Verbena--Beta--Gradual conversion of the movement of circumnutation into apogeotropism in Rubus, Lilium, Phalaris, Avena, and Brassica--Apogeotropism retarded by heliotropism--Effected by the aid of joints or pulvini--Movements of flower-peduncles of Oxalis--General remarks on apogeotropism--Geotropism-- Movements of radicles--Burying of seed-capsules--Use of process--Trifolium subterraneum--Arachis--Amphicarpaea--Diageotropism--Conclusion...493-522
LOCALISED SENSITIVENESS TO GRAVITATION, AND ITS TRANSMITTED EFFECTS.
General considerations--Vicia faba, effects of amputating the tips of the radicles--Regeneration of the tips--Effects of a short exposure of the tips to geotropic action and their subsequent amputation--Effects of amputating the tips obliquely--Effects of cauterising the tips--Effects of grease on the tips--Pisum [page x.] sativum, tips of radicles cauterised transversely, and on their upper and lower sides--Phaseolus, cauterisation and grease on the tips--Gossypium-- Cucurbita, tips cauterised transversely, and on their upper and lower sides--Zea, tips cauterised--Concluding remarks and summary of chapter-- Advantages of the sensibility to geotropism being localised in the tips of the radicles...Page 523-545
SUMMARY AND CONCLUDING REMARKS.
Nature of the circumnutating movement--History of a germinating seed--The radicle first protrudes and circumnutates--Its tip highly sensitive-- Emergence of the hypocotyl or of the epicotyl from the ground under the form of an arch--Its circumnutation and that of the cotyledons--The seedling throws up a leaf-bearing stem--The circumnutation of all the parts or organs--Modified circumnutation--Epinasty and hyponasty--Movements of climbing plants--Nyctitropic movements--Movements excited by light and gravitation--Localised sensitiveness--Resemblance between the movements of plants and animals--The tip of the radicle acts like a brain...546-573
THE MOVEMENTS OF PLANTS.
THE chief object of the present work is to describe and connect together several large classes of movement, common to almost all plants. The most widely prevalent movement is essentially of the same nature as that of the stem of a climbing plant, which bends successively to all points of the compass, so that the tip revolves. This movement has been called by Sachs "revolving nutation;" but we have found it much more convenient to use the terms circumnutation and circumnutate. As we shall have to say much about this movement, it will be useful here briefly to describe its nature. If we observe a circumnutating stem, which happens at the time to be bent, we will say towards the north, it will be found gradually to bend more and more easterly, until it faces the east; and so onwards to the south, then to the west, and back again to the north. If the movement had been quite regular, the apex would have described a circle, or rather, as the stem is always growing upwards, a circular spiral. But it generally describes irregular elliptical or oval figures; for the apex, after pointing in any one direction, commonly moves back to the opposite side, not, however, returning along the same line. Afterwards other irregular ellipses or ovals are successively described, with their longer [page 2] axes directed to different points of the compass. Whilst describing such figures, the apex often travels in a zigzag line, or makes small subordinate loops or triangles. In the case of leaves the ellipses are generally narrow.
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 previously increased state of turgescence on the convex side.* In the case of parts provided with a so-called joint, cushion or pulvinus, which consists of an aggregate 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 botanists; 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 den Heliotropismus,' Sitzb. der K. Akad. der Wissenschaft. (Vienna), Jan. 1880.
[page 3] creased 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 apparently every growing part of every plant is continually circumnutating, though often on a small scale. Even the stems of seedlings 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 diversified movements. Thus, the great sweeps made by the stems of twining plants, and by the tendrils of other climbers, result from a mere increase in the amplitude 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 vertical 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 stimuli.
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 seedling 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. Secondly, 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 proceeding 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 distinguished 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,
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