450 SENSITIVENESS TO LIGHT. CHAP. IX. 



is strengthened by the fact that with Phalaris and 

 Avena the first true leaf, which is bright green and no 

 doubt decomposes carbonic acid, exhibits hardly a 

 trace of heliotropism. The heliotropic movements of 

 many other seedlings probably aid them in like 

 manner in emerging from the ground ; for apogeo- 

 tropism by itself would blindly guide them upwards, 

 against any overlying obstacle. 



Heliotropism prevails so extensively among the 

 higher plants, that there are extremely few, of which 

 some part, either the stem, flower-peduncle, petiole, 

 or leaf, does not bend towards a lateral light. 

 Drosera rotundifolia is one of the few plants the 

 leaves of which exhibit no trace of heliotropism. Nor 

 could we see any in Dionaea, though the plants were 

 not so carefully observed. Sir J. Hooker exposed the 

 pitchers of Sarracenia for some time to a lateral light, 

 but they did not bend towards it.* We can understand 

 the reason why these insectivorous plants should not 

 be heliotropic, as they do not live chiefly by decom- 

 posing carbonic acid ; and it is much more important 

 to them that their leaves should occupy the best 

 position for capturing insects, than that they should 

 be fully exposed to the light. 



Tendrils, which consist of leaves or of other organs 

 modified, and the stems of twining plants, are, as 

 Mohl long ago remarked, rarely heliotropic ; and here 

 again we can see the reason why, for if they had 

 moved towards a lateral light they would have been 

 drawn away from their supports. But some tendrils are 

 apheliotropic, for instance those of Bignonia capreolata 



* According to F. Kurlz (' Ver- tonia Californira are strongly 



handl. des Bot. Vereins der Pro- apheliotropic. We failed to detect 



vinz Brandenburg/ Bd. xx. 1878) this movement in a plant which 



the leaves or pitchers of Darling- we possessed for a short time. 



