GEOTROPISM. II 459 



inverted it is the inner flanks of the spirals which exhibit increased growth, 

 hence bringing about a straightening of them. 



If geotropism be so active a factor in the twining phenomenon we are led to 

 conclude that twiners can regularly embrace only such supports as are more or less 

 erect. Indeed the longer the overhanging part of the shoot is, the more will the 

 inclined supports be seized, as one may observe especially in plants like the hop. 



The clasping of the support at right angles is compensated for by a pecu- 

 liarity of the climber, which we have hitherto not drawn attention to. As in 

 the case of many etiolated shoots, twiners have inordinately long internodes with 

 at first very small leaves, and there are many advantages in these leaves not 

 reaching their full size until after the stem which bears them has obtained a firm 

 hold on the support, first, because the weight of the stem would be appreciably 

 increased, and, secondly, because the interruptions in the rotation which would 

 result from the striking of large leaves against the support are thereby avoided. 

 The advantages of possessing long internodes require no special explanation, 

 so that we may at this point close our very fragmentary observations on geo- 

 tropism in general and twining in particular. 



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