SENSITIVENESS. 63 



others which are of a periodic character, in that they are 

 influenced by the direction rather than by the intensity of 

 the light. 



The growth of leaves, like growth in general, is retarded 

 by the action of light. Growth, therefore, is carried on 

 independently of and not contemporaneously with nutrition 

 by the leaf, so far as the latter consists in the decomposi- 

 tion of carbonic acid and the fixation of the carbon. Thus 

 it has been shown by Dr. Vines that leaves will grow 1, 

 in darkness, or under the influence of blue light ; 2, in air 

 deprived of carbonic acid ; and, 3, even in the absence of 

 chlorophyll. But although there is thus shown to be no 

 direct relation between nutrition and growth, yet there is, 

 of course, an indirect relation ; growth under the apparently 

 adverse conditions just mentioned being only possible in 

 cases where there is available some store of nourishment 

 previously formed by assimilation. 



Sleep of Leaves. Other movements of leaves are de- 

 pendent chiefly on the amount of light to which they are 

 subjected. Of such nature are the movements popularly 

 supposed to be connected with the sleep of plants, but 

 which have no real analogy with the sleep of animals. 

 Clover and sainfoin leaves show these nocturnal movements 

 very clearly, the leaflets folding up at the approach of night, 

 and unfolding in the morning as the light increases. 

 Plants exposed to the dark end of the solar spectrum 

 manifest similar movements. Some leaves are raised, 

 others depressed, some fold upwards, some downwards, but 

 the object in all cases is probably the same namely, to 

 shield the leaves from the cooling effect of radiation from 

 the surface during the night, a process which produces the 



