Page:The Effect of External Influences upon Development.djvu/60

56 Beitrag zur Pflanzenbiologie.' Extract du Jardin Botanique de Buitenzorg, vol xi., 1893; and Haberlandt, Eine botanische Tropenreise, Leipzig, 1893, pp. 112 and 113.

NOTE II (p. 11).

The matter is, if possible, still clearer in the case of geotropism, for there is no doubt that the habit of the plant to react in some particular way to the force of gravity cannot be a general and primary peculiarity of plants. A Volvox colony, rotating freely in water, is in a sense a living klinostat, uninfluenced by the attraction of the earth. The geotropic sensitiveness of plants can only have arisen when they became attached to the ground, and may therefore, even in its most general form, be regarded as an adaptation. This becomes much more evident when we consider that the various parts of existing plants have totally different susceptibilities to the stimulus of gravity: not only may the root be positively geotropic, while the stem is negatively geotropic, but each of the secondary roots and of the branches passes off at a perfectly definite angle from the primary root and stem, and this angle is due, at least to a certain extent, to a reaction to the stimulus of gravity. Thus the geotropic sensitiveness of plants, just like the sensitiveness of their different parts to light, may manifest itself in opposite manners; and—what is still more noteworthy—the sensitiveness is specially regulated for every part in such a way that the plant is enabled to adapt itself most advantageously to its different conditions. But a mode of reaction which responds very differently to the same stimulus cannot be originally or primarily characteristic of the plant: it cannot always have been possessed by all plants, but is due to an adaptation in connexion with the fixed condition of these organisms. The delicate 'molecular' structure, to which this wonderful sensitiveness is to be referred, cannot depend upon the direct action of external stimuli, but only on their indirect influence, i.e. on processes of selection.