CHAP. IX. SENSITIVENESS TO LIGHT. 453 



if not accidental, appears to indicate that the shoots 

 retain a trace of an original apheliotropic tendency. 

 With Lonicera Irachypoda the semicircles from and to 

 the light differed considerably in time ; for 5 semi- 

 circles from the light required on a mean 202'4 

 minutes, and 4 to the light, 229*5 minutes ; but the 

 shoot moved very irregularly, and under these circum- 

 stances the observations were much too few. 



It is remarkable that the same part on the same 

 plant may be affected by light in a widely different 

 manner at different ages, and as it appears at different 

 seasons. The hypocotyledonous stems of Ipomcea 

 cserulca and purpurea are extremely heliotropic, whilst 

 the stems of older plants, only about a foot in height, 

 are, as we have just seen, almost wholly insensible to 

 light. Sachs states (and we have observed the same 

 fact) that the hypocotyls of the Ivy (Hedera helix) are 

 slightly heliotropic ; whereas the stems of plants grown 

 to a few inches in height become so strongly aphelio- 

 tropic, that they bend at right angles away from the 

 light. Nevertheless, some young plants which had 

 behaved in this manner early in the summer again 

 became distinctly heliotropic in the beginning of 

 September ; and the zigzag courses of their stems, as 

 they slowly curved towards a north-east window, were 

 traced during 10 days. The stems of very young 

 plants of Tropneolum majus are highly heliotropic, whilst 

 those of older plants, according to Sachs, are slightly 

 apheliotropic. In all these cases the heliotropism of 

 the very young stems serves to expose the cotyledons, 

 or when the cotyledons are hypogean the first true 

 leaves, fully to the light ; and the loss of this power 

 by the older stems, or their becoming apheliotropic, 

 is connected with their habit of climbing. 



Most seedling plants are strongly heliotropic, and 



