124 PLANT PHYSIOLOGY 



seen from above, but a few plants rotate in the opposite 

 direction. Some botanists regard the whole rotary 

 movement of such plants not as a form of nutation but 

 as a special form of geotropic response. 



183. Those growth movements due to the response 

 to recognized stimuli are often divided into tropic 

 movements where the organ affected is brought to lie 

 with its axis in some definite relation to the direction of 

 the stimulus, and nastic movements where one or the 

 other face of a bifacial organ is placed in some relation to 

 the direction of the stimulus. However, in view of the 

 fact that the general phenomena concerned are the same, 

 they need not be sharply separated here. 



184. The chief tropic movements of plants are 

 phototropism, geotropism, thigmotropism, chemotropism, 

 being responses respectively to the stimulus of light, 

 gravity, contact and chemical substances. Other tro- 

 pisms have been distinguished but will not be discussed 

 here. For all tropisms the point of curvature is the 

 region where the most rapid growth usually occurs. As 

 the result of the stimulus the growth is increased above 

 the normal rate on one side and sometimes even retarded 

 below the normal on the other with the result that a 

 curvature is produced. The perceptive region for the 

 stimulus may be distant some millimeters or even 

 centimeters from the zone of growth. 



185. Phototropism may be illustrated by the action of 

 a plant illuminated on one side only. Usually the 

 stem of such a plant curves toward the source of light 

 (positive phototropism) while the leaves place themselves 

 so as to stand with their surfaces at right angles to the 

 source of the light (photonasty). Sometimes the cur- 

 vature is away from the light as is the case with most 

 roots and with the stems of some climbing plants, e.g. 



