CHAP. XII CONCLUDING REMARKS. 565 



advantageous for the leaf to be fixed by growth in its 

 inclined position. For it has to assume its former 

 horizontal position, as soon as possible after the sun 

 has ceased shining too brightly on it. 



The extreme sensitiveness of certain seedlings to 

 light, as shown in our ninth chapter, is highly remark- 

 able. The cotyledons of Phalaris became curved 

 towards a distant lamp, which emitted so little light, 

 that a pencil held vertically close to the plants, did 

 not cast any shadow which the eye oould perceive 

 on a white card. These cotyledons, therefore, were 

 affected by a difference in he amount of light on their 

 two sides, which the eye could not distinguish. The 

 degree of their curvature within a given time towards 

 a lateral light did not correspond 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 towards a lateral light, 

 after it had been extinguished. They bend with 

 remarkable precision towards it, and this depends on 

 the illumination of one whole side, or on the obscura- 

 tion 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 re- 

 ceived, 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 towards 

 a dim lateral light, sooner than others which had pre-^ 

 viously been exposed to daylight. We have seen 

 several analogous cases with the nyctitropic move- 

 ments 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 cotyledons rose up 

 closed ; another pot had stood in the sunlight, and 



