ADJUSTMENT TO GRAVITY, CONTACT, AND SHOCK 137 



to maintaining this angle, leaves also tend to keep their surfaces 

 horizontal. Flowers and fruits are sometimes diageotropic also. 

 In certain species they change their relation to gravity, as is 

 regularly the case in anthotropic and carpotropic movements. 

 For example, the bud of Chamcenerium angustifolium is positively 

 geotropic, the flower diageotropic, and the fruit negatively geo- 

 tropic. Branches of the stem and root are usually diageotropic, 

 though they are capable of changing this relation in considerable 

 measure. 



158. Cause and reaction. The exact way in which the stimulus 

 of gravity is perceived by the plant is not known with certainty. 



Fig. 40. A young plant of Fuchsia sp., showing the effect upon loaf posi- 

 tion when the control exerted by gravity is destroyed by growing the 

 plant under the action of centrifugal force. 



It has been suggested that the fall of starch grains or other in- 

 clusions to one side when the position of the plant is change<l 

 sets up a stimulus in the protoplasm, but this does not ex]ilain 

 all cases of geotropism. It seems more probable that protojilasm 



