Ch. Ill, 7] ADJUSTMENTS TO LIGHT 57 



mechanism is concerned. It is easy to recognize in the reflex 

 actions of our own bodies the corresponding factors and 

 mechanisms. 



Since stems and leaves turn usually towards the stronger 

 light, one may well ask why the vegetation of the northern 

 hemisphere does not all bend towards the south. The reason 

 seems connected with a fact already mentioned, that leaves 

 cannot use all of the energy in full summer sunlight, while 

 a strong diffused light is enough for their needs. Apparently 

 their full power of response is aroused by such diffused light, 

 which comes about equally from all parts of the sky. 



Where many leaf blades grow closely together, they tend 

 to move out from under one another's shade, their petioles 

 bending or elongating in ways which effect this result. Thus 

 the blades on a horizontal branch of a tree are commonly 

 brought into one flat plane. The effect is particularly strik- 

 ing in Ivies, where the leaf blades become often so evenly 

 distributed as to suggest the name of leaf-mosaic (Fig. 26). 



A familiar light adjustment is involved in the so-called 

 "sleep movements, " where the leaflets of compound leaves, 

 as of Clover, Oxalis, Beans, Acacias, Sensitive Plants, droop 

 or close together in darkness and spread widely apart in 

 light (Fig. 27). The response to the light stimulus is plain, 

 but the significance of the 

 movement in the plant's 

 economy is still uncertain. 

 The leaflets of other plants 

 exhibit an analogous move- 

 ment under very intense Fig. 27. -Leaf of a Clover, in "awake" 

 light, in which they Close and "asleep" positions. (From Darwin, 

 ., , . , Power of Movement in Plants.) 



together or assume vertical 



positions, returning to the horizontal position when the light 

 is less intense ; and this movement has been interpreted as 

 protective to the Leaf tissues against too intense insolation. 

 A permanent condition of this protective light adjustment, 

 which, at its perfection, involves a Betting of the leal' edges 



