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 by 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 by 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 bave 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 
