PHYSIOLOGY 



491 



factors, such as light, may not also be concerned in the assumption 

 of the horizontal position. 



In the geotropic and rheotropic movements of roots, it has been 

 shown that it is the root-tip which is the sensitive portion of the 

 root, and the position in which this is placed regulates the movement 

 in the older parts of the root. So long as the root-tip is prevented 

 from assuming its normal position the movements in the other parts 

 of the root will continue (Darwin, Pfeffer, Czapek). On the other 

 hand, if a root is placed horizontally until the tip bends down, and the 

 latter is then held in this position while the rest of the root remains 

 horizontal, no change will take place in the position of the r&ot. 



Twining. The movements of twining stems are caused by 

 increased growth on one side, due to geotropic stimulus ; this causes 

 the revolving movement of the apex, and when the stem comes in 

 contact with a support it twines about it. 



FIG. 462. Leaves of Oxalis Bowei, showing day and night positions. 



BAILEY.) 



(After 



Light 



Light is a powerful stimulus both to the metabolic activity of the 

 protoplasm and to its movements. The effect of light upon the 

 movements of zoospores and plasmodia has already been referred to, 

 and light is an equally powerful stimulus in governing the move- 

 ments of the organs of the higher plants. 



The growth of green organs toward the light, the opening of 

 flowers, and the sleep-movements of leaves, are familiar phenomena. 

 These movements often depend upon the temperature, and even in 

 bright sunshine some flowers will remain closed if the temperature 

 is low. 



