SENSITIVENESS. 63 



and of unequal growth, Mr. Francis Darwin has shown 

 that the power which leaves have of placing themselves 

 at right angles to incident light is due to a special sensi- 

 tiveness. This sensitiveness is capable of regulating the 

 action of other forces, whether external to the plant, as 

 that of gravitation, or internal, such as that controlling 

 the direction and amount of growth. The movements 

 of the leaves towards the light are different from others 

 which are of a periodic character, in that they are influ- 

 enced hy the direction rather than by the intensity of 

 the light. 



The growth of leaves, like growth in general, is re- 

 tarded 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 decomposition of carbonic acid and the fixation of 

 the carbon. Thus it has been shown hy Dr. Vines that 

 leaves will grow in darkness, or under the influence of 

 blue light ; in air deprived of carbonic acid ; and 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 move- 

 ments very clearly, the leaflets folding up at the approach 

 of night, and unfolding in the morning as the light in- 

 creases. Plants exposed to the dark end of the solar 



