202 THE LIFE OF THE PLANT 



perties of the organs or upon the complexity of the 

 apparently simple factor. Here we have, it appears, 

 an example of the latter case. 



The way in which botanists conceived the dependence 

 of growth upon external influences became of necessity 

 considerably more complex after the brilliant and 

 always original investigations of Darwin. He showed 

 that the point affected by the external influence may 

 not coincide with the place where its effect is mani- 

 fested. Thus, for instance, the force of gravity appears 

 to act chiefly on the root-tip, although its effect is 

 manifested by geotropic curvature in the region of 

 most vigorous growth, lying some distance from the 

 tip. We come to this conclusion because rootlets, the 

 tips of which have been cut off, hardly ever curve until 

 a new root- tip is produced. Stems appear to behave 

 differently ; but on the other hand some shoots show a 

 similar behaviour in relation to light, a phenomenon 

 which is not observed in roots. Thus, for instance, the 

 tip of an oat seedling, and, especially, of the seedling 

 of canary grass, are remarkably sensitive to light. If 

 we cover these with caps made of tinfoil, the helio- 

 tropic curvature generally observed in the lower parts 

 is considerably weakened. 



These facts were sufficient to make some botanists 

 think that in the root-tip and at the tip of the seedlings 

 of cereals there existed special sense-organs, which 

 communicated their impressions to the growing parts 

 in some unknown way and caused them to curve. We 

 shall see presently that there is no reason whatever 

 for presupposing any such sense-organs or nerves in 

 plants ; here we need only say in passing that there 

 is no reason whatever for such an explanation of the 

 facts just described until other simpler explanations 

 have been exhausted ; and this condition, for all scientific 

 explanation, is as yet far from being fulfilled in the 

 sphere we are now engaged in studying. 



