THE PLANT BODY 37 



root tip is the most sensitive part of the root and is the perceptive 

 region for the gravity stimulus which causes curvature. 



We may now profitably turn to a few common garden and field 

 plants for a practical application of the principles established above 

 with reference to the adjustment of the plant parts in response to 

 stimuli and consider briefly the part which these responses play 

 in determining the attitudes and ultimate form of these plants. 



ADJUSTMENTS IN SOME COMMON PLANTS 



Caladium and clover. The power of two quite different plants 

 to adjust their organs to the environment may be seen by con- 

 trasting the ultimate positions assumed by the organs of cala- 

 dium and red clover (Figs. 21 and 22). These two plants have 

 very different forms of roots and leaves, and yet each plant, on 

 account of its ability to respond in its own way to the forces 

 of its environment, has succeeded in placing its roots and leaves 

 in the position most favorable to its own maintenance and 

 growth. As may be seen from Fig. 21, the caladium is what 

 is known as a surface feeder, spreading its roots out horizontally 

 and absorbing foods and moisture from the surface layers of the 

 soil. Many desert plants arrange their roots in this manner in 

 order to avail themselves of the light rains and heavy dews 

 which occur in arid regions at certain times of the year. The 

 leaves of the oaladium are also favorably adjusted to light as a 

 result of their ability to turn in response to the light stimulus. 

 The roots are evidently diageotropic and the leaves diaphoto- 

 tropic in their response. 



The common red clover (Trifolium pratense) (Fig. 22) presents 

 a strong contrast to the caladium not only in the form of its 

 organs but also in its responses to light and gravity. The strong 

 taproot is here progeotropic and bores deeply into the soil, so 

 that the lateral diageotropic roots absorb food and moisture from 

 much deeper areas than the similar roots of the caladium. The 

 leaves of the clover, like those of the bean, are furnished with 

 pulvini (5) and thus adjust themselves rapidly and effectively 

 to changes in the external environment. During the day they 



