i90 CONCLUDING EEMAEKS AND Chap. IX. 



up, and that nearest to the light sinking down, oi 

 both twisting laterally.* We may, also, suspect IHat 

 the extreme sensitiveness to light of the upper part 

 of the sheath-like cotyledons of the Grraminese, and 

 their power of transmitting its effects to the lower 

 part, are specialised arrangements for finding the 

 shortest path to the light. With plants growing on 

 a bank, or thrown prostrate by the wind, the manner 

 in which the leaves move, even rotating on their own 

 axes, so that their upper surfaces may be again directed 

 to the light, is a striking phenomenon. Such facts 

 are rendered more striking when we remember that 

 too intense a light injures the chlorophyll, and that 

 the leaflets of several Leguminosso when thus exposed 

 bend upwards and present their edges to the sun, thus 

 escaping injury. On the other hand, the leaflets of 

 Averrhoa and Oxalis, when similarly exposed, bend 

 downwards. 



It was shown in the last chapter that heliotropism 

 is a modified form of circumnutation ; and as every 

 growing part of every plant circumnutates more or less, 

 we can understand how it is that the power of bending 

 to the light has been acquired by such a multitude 

 of plants throughout the vegetable kingdom. The 

 manner in which a circumnutating movement— that 

 is, one consisting of a succession of irregular ellipses 

 or loops — is gradually converted into a rectilinear 

 course towards the light, has been already explained. 

 First, we have a succession of ellipses with their 

 longer axes directed towards the light, each of whicli 



* Wiesner has made remarks to ^ tracteil from B. Ixxvii. ClSli) 



nearly the same effect with respect Sitb. der k. Akad, der Wisseusoh. 



to leaves : ' Die undulirtnde Nu- Wieii. 

 tation der luternodieu,' p. (J, ex- 



