196 BOTANY. 



the concave side, and it is evident that this is the immediate 

 cause of the bending. We do not, however, know how grav- 

 itation causes this inequality in the growth of the cells, 

 and the problem is the more difficult from the fact that the 

 more rapid elongation of the cells is in one case upon the 

 upper and in the other upon the under side of the organ. 

 Moreover, in " weeping trees " the branches are positively, in- 

 stead of negatively, geotropic, although we know of no struc- 

 tural difference between these and the branches of ordinary 

 trees. 



V. CERTAIN" MOVEMENTS OF PLANTS. 



257. Under this head are to be considered a few only of 

 the more important movements in plants. It must be remem- 

 bered that living protoplasm has everywhere, under proper 

 conditions, the power of spontaneous movement. In the 

 lower forms of vegetation this results in visible movements, 

 which are of common occurrence ; but in the greater part 

 of the vegetable kingdom, while the protoplasm is doubt- 

 less as active, the cell-walls which enclose it are so rigid that 

 its physical activity is incapable of producing external move- 

 ment. Thus most parts of ordinary plants do not perform 

 movements which are the direct results of the physical activ- 

 ity of the protoplasm ; but this is not because of a want of 

 activity in the protoplasm, but mainly from the rigidity of 

 the walls surrounding it. In a comparatively small number 

 of instances,- however, the structure of the organs of even 

 the higher plants is such that movements directly due to pro- 

 toplasmic activity are performed. Such are the so-called 

 spontaneous movements of the leaves of some plants, and 

 those dependent upon external stimuli, as light, heat, me- 

 chanical irritation, etc., which have been called paratonic 

 movements. 



258. Spontaneous Movements. The most remarkable 

 case of movements apparently not dependent upon external 

 agents is that of the leaves of Desmodium gyrans, an Indian 

 plant. The small lateral leaflets of the trifoliate leaf bend 

 upon their slender stalks (petiolules) in such a way that their 



