144 MODIFIED CIECUMNUTATION. Chap. VIII, 



the action of apogeotropism was ^uite eliminated. 

 Nevertheless, they did not rise nearly so much at 

 night, as when subjected to apogeotropism. Is it 

 not possible, or even probable, that leaves and coty- 

 ledons, which have moved upwards in the evening 

 through the action of apogeotropism during countless 

 generations, may inherit a tendency to this movement ? 

 We have seen that the hypocotyls of several Legu- 

 minous plants have from a remote period inherited a 

 tendency to arch themselves ; and we know that the 

 sleep-movements of leaves are to a certain extent 

 inherited, independently of the alternations of light 

 and darkness. 



In our observations on the circumnutation of those 

 cotyledons and leaves which do not sleep at night, we 

 met with hardly any distinct cases of their sinking 

 a little in the evening, and rising again in the morn- 

 ing, — that is, of movements the reverse of those just 

 discussed. We have no doiibt that such cases occur, 

 inasmuch as the leaves of many plants sleep by 

 sinking vertically downwards. How to account for the 

 few cases which were observed must be left doubtful. 

 The young leaves of Cannabis sativa sink at night 

 between 30° and 40° beneath the horizon ; and Kraus 

 attributes this to epinasty in conjunction with the 

 absorption of water. Whenever epinastio growth is 

 vigorous, it might conquer diaheliotropism in the 

 evening, at which time it would be of no import- 

 ance to the plant to keep its leaves horizontal. 

 The cotyledons of Anoda Wriglitii, of one variety of 

 Gossypium, and of several species of Ipomoea, remain 

 horizontal in the evening whilst they are very young ; 

 as they grow a little older they curve a little down- 

 wards, and when large and heavy sink so much that 

 they come imder our definition of sleep. In the case of 



