544 PLANT RESPONSE 



rate stimulation of the tip, provided the effect transmitted 

 to the growing region be indirect. 



True perceptive region. — There is a sense, however, in 

 which it is true that the root-tip alone is the perceptive 

 region, but this statement applies exclusively to the stimula- 

 tion caused by gravity ; for, in order that this may take 

 place, the presence of statolithic or other elements through 

 which it can act is necessary. But if such elements be 

 concentrated at the root-tip, it is clear that that region alone 

 can become the seat of stimulation. This is not the case 

 with other forms of stimulus, which act directly, the pre- 

 sence or absence of statoliths being a matter of indifference. 

 This distinction is important, since the outward resemblance 

 of effects at the root-tip, in the two cases of stimulation by 

 light and stimulation by gravity, has sometimes led to the 

 assumption that statolithic bodies formed the medium of 

 excitation in both. That such, however, is not the case is 

 now evident. 



In the geotropic root, then, it is the indirect effect of 

 moderate stimulus, unilaterally transmitted from the distant 

 tip, that causes the responsive curvature. In the case of 

 the apogeotropic stem, on the other hand, it is the direct 

 stimulation of the statolithic elements distributed throughout 

 that region that induces the responsive curvature ; and since 

 I have shown that the curvature caused by direct is opposite 

 to that which is the result of indirect stimulation, it will be seen 

 that the opposite curvatures observed in root and shoot are 

 not due to different sensibilities possessed by the two organs. 



It is to be noted, moreover, that although the statolithic 

 or radial pressure theory affords a very rational explanation 

 of the mode in which stimulation is brought about by 

 gravity, yet the explanation which I have offered, of the 

 occurrence of opposite geotropic effects in root and shoot, 

 does not in any way depend upon the ultimate validity of 

 this or any other particular theory. The aim of my demon- 

 stration has been to show that, through whatever means the 

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



