476 LIFE MOVEMENTS IN PLANTS 



explanatiow of geotropic stimulation) in the shoot and in 

 the root, is capable of furnishing an explanation of the 

 different geotropic responses in the two organs. In this 

 connection, the results of investigation of Haberlandt and 

 Nemec are highly suggestive. Haberlandt finds statoliths 

 present in the responding region of the stem ; the 

 geotropic stimulation of the stem is therefore direct. 

 Nemec's investigation on the distribution of statoliths in 

 the root show, on the other hand, that it is the central 

 portion of the root cap that contains the falling starch 

 grains, and this would account for the indirect geotropic 

 stimulation of the root. 



The theory of statoliths is, however, not essential for 

 the explanation of the opposite geotropic effects in the 

 shoot and in the root. The observed fact, that the percep- 

 tive region in the root is separated from the responding 

 region, is sufficient to explain the difference of geotropic 

 action in the two organs. Through whatever means the 

 stimulus of gravity may act, it is inevitable, from the 

 fact that the stimulation of the shoot is direct and of the 

 root indirect, that an identical stimuhis should in two 

 cases induce responsive reactions of opposite signs. 



It will thus be seen that the postulation of two different 

 irritabilities in the shoot and in the root is wholly 

 unnecessary and unwarranted by facts. For the irrita- 

 bility of the root has been shown to be in no way 

 dift"erent from that of other organs; an uniformity is thus' 

 found to exist in the reaction of all vegetable tissues. 



SUMMAKY. 



On subjection of the tip of the root to the stimulus 

 of gravity, the upper side exhibits excitatory reaction of 

 galvanometric negativity. This shows that the root-tip 

 undergoes direct stimulation. 



