5 EXPERIMENTAL PLANT PHYSIOLOGY. 



tropic movements, and one from the red end of the spectrum 

 which causes heat or thermotropic movements, as may be 

 shown by Experiment 72. The purpose of the heliotropic as 

 well as all other movements of plants is doubtless that of plac- 



FIG. 52. 



Diagram of light positions of leaves. The arrows denote the direction of 

 the rays. (Vochting.) 



ing the plant-organ in the position best suited to the perform- 

 ance of its functions. Heliotropic movements place the leaves 

 in a position most favorable for photosynthesis and transpira- 

 tion. 



