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. 
tat sh enn 
