770 GENERAL CONDITIONS OF PLANT-LIFE. 



the chlorophyll-granules which are carried along by the very watery protoplasm accu- 

 mulate at various spots. A current from thirty elements causes permanent cessation of 

 the movement even if the connection is only momentary. Induced currents act like 

 constant ones ; but the number of induction-shocks which pass through the cells in a 

 unit of time appears to have no considerable influence on the action. 



The changes of form of protoplasm under the influence of a sufficiently strong elec- 

 tric current are, according to the observations of Heidenhain, Briicke, Max Schultze, 

 and Kiihne, similar to those caused by a high temperature near the extreme limit or 

 beyond it. From those of Kiihne it appears to result that protoplasm is a very bad 

 conductor of electricity, and that the excitement caused by a current at particular 

 spots in the protoplasm is not easily transferred to other spots. 



Cohn, Kabsch, and others, state that weak induction-currents act on the sensitive 

 parts of the leaves of Mimosa, the stamens of Beriberis, Mahonia, and Centaurea Scabiosa, 

 and the gynostemium of Stylidium graminifolium like concussion or contact, the parts 

 moving as if under the influence of these agencies. According to Kabsch, stronger 

 induction-currents, which permeate the whole plant, destroy the sensitiveness of the 

 gynostemium of Stylidium even for mechanical excitation ; but after half an hour the 

 sensitiveness again returns. The statement of Kabsch is noteworthy that the move- 

 ment of the leaflets of Desmodium gyrans are permanently prevented by stronger in- 

 duction-currents, which however do not kill them. 



Sect. io. — Action of G-ravitation on the Processes of Vegetation^ 



Since the attraction of the earth acts uninterruptedly on all parts of the plant, the 

 entire vegetable organisation must be so contrived that the weight of the separate 

 parts of the plant is serviceable, or at least not injurious, to the various purposes of 

 the life of the plant. 



In observing these relationships the first thing Is to distinguish between those 

 contrivances which have for their object to bring the weight of the parts of the plant 

 into harmony with the purposes of its life — gravitation itself not taking any direct 

 recognisable part in the attainment of these objects — and those phenomena of 

 vegetation on the other hand which are brought into existence by the direct influence 

 of gravitation on the mechanism of growth. 



To the first of these groups belongs the fact that the branches and foliage of 

 upright stems are distributed nearly equally on all sides, and that in larger plants 

 the firmness and elasticity of the masses of tissue in the stem is promoted by the 

 formation of wood, or is brought about by other means, as for instance in the trunk 

 of Musa. But since it is very common in the organic world for the same purpose 

 to be attained by very diflferent means, slender delicate stems with but little wood 

 can protect themselves from sinking down and can expose- their foliage to the light 

 by twining round firm supports, or by climbing with the help of tendrils, hooks, 

 spines, &c. The same purpose is evidently served by the various floating con- 

 trivances of water-plants and those of fruits and seeds; in all these cases the structure 

 is obviously adapted to make the weight of the part of the plant serviceable or at 

 least not injurious to its life ; although it cannot be maintained that gravitation takes 

 any part in the formation of wood, in the sensitiveness of tendrils, or in the produc- 

 tion of a floating apparatus. The only explanation of these arrangements lies in 



^ These statements are intended in the first place to draw the attention of students to the pro- 

 cesses of vegetation which are especially influenced by gravitation. Its action on the mechanism of 

 growth will be fully described in Chap, IV, where also the literature is quoted. 



