28 DARWINISM STATED BY DARWIN HIMSELF. 



move during the whole twenty-four hours ; they move, 

 however, more quickly when going to sleep and when 

 awaking than at other times. 



INFLUENCE OF LIGHT UPON PLANTS. 



The Power The extreme sensitiveness of certain seed- 



in Plan™ en l m gs to light is highly remarkable. The 

 page 565. cotyledons of Phalaris became curved toward 

 a distant lamp, which emitted so little light that a pen- 

 cil held vertically close to the plants did not cast any 

 shadow which the eye could perceive on a white card. 

 These cotyledons, therefore, were affected by a difference 

 in the amount of light on their two sides, which the eye 

 could not distinguish. The degree of their curvature 

 within a given time toward a lateral light did not cor- 

 respond at all strictly with the amount of light which 

 they received ; the light not being at any time in excess. 

 They continued for nearly half an hour to bend toward a 

 lateral light, after it had been extinguished. They bend 

 with remarkable precision toward it, and this depends on 

 the illumination of one whole side, or on the obscuration 

 of the whole opposite side. The difference in the amount 

 of light which plants at any time receive in comparison 

 with what they have shortly before received seems in all 

 cases to be the chief exciting cause of those movements 

 which are influenced by light. Thus seedlings brought 

 out of darkness bend toward a dim lateral light, sooner 

 than others which had previously been exposed to day- 

 light. We have seen several analogous cases with the 

 nycti tropic movements of leaves. A striking instance 

 was observed in the case of the periodic movements of 

 the cotyledons of a cassia : in the morning a pot was 

 placed in an obscure part of a room, and all the cotyle- 

 dons rose up closed ; another pot had stood in the sun- 



