OCR Text |
Show 4 INTRODUCTION. forms of circumnutation; as again are the equ~lly prevalent movements of stems, &c., towards the zenith, and of roots towards the centre of the earth .. In accordance with these conclusions, a considerable drfficulty in the way of evolution is. in part ~em_oved? for it might have been asked, how did all then divers~fied movements for the most different purposes first anse? As the case stands, we know that there is ah_vays movement in progress, and its amplitude, or duection, 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 li~ht, ~nd transmits an influence to the lower part, causmg 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 seedlino ·s the tip is sensitive to various stimuli, espe- B' . d cially to very slight pressure, and, when thus .exc~te , 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 INTRODUCTION. 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 r·adicle can be distinguished from the hypocotyl only by the presence of root-hairs and the nature of its covering. The meaning of the word circttmmttatiun 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 apheliotTopism bending from the light. There iR 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 ; apogeotr·opism will mean bending in opposition to gravity 01· from the centre of the earth; and diageotropism, a position more or less transverse to the radius of the earth. The words heliotropism and geotropism propm·ly 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, &c., 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 remarkable 'Beitrage zur Pflanzenphysiologie,' 1868. |