110 ADJUSTMENT TO FA'VIROXMENT 



adjustments are not promptly and properly made the 

 plant perishes. 



All animals require food. In a few instances they can 

 remain stationary and have a supply brought to them 

 by currents of water, but in far the majority of cases 

 they must have organs of locomotion to go in search of it 

 and organs of special sense to recognize it when they are 

 in its neighborhood. They need these also to avoid being 

 eaten by other animals. 



Plants and Animals Contrasted. — In conformity 

 with these differences between animals and plants in their 

 relation to the environment there have come in the course 

 of evolution to be great and striking structural and func- 

 tional differences between them. In general animals are 

 highly motile and plants are not. Plants have strong 

 rigid bodies and animals do not. The higher animals 

 have developed highly specialized sense organs by means 

 of which they are acutely aware of many elements of the 

 environment which make little or no impression on the 

 plant. Moreover, they have developed complicated and 

 efficient nervous systems and brains through which the 

 stimuli affecting the sense organs bring about immediate 

 and effective response by the motor organs. 



The Adjustment of Roots to the Soil. — A plant 

 whose roots failed to assume a suitable relation to the soil 

 would very shortly perish. This danger is, however, effec- 

 tively guarded against in several ways. 



Geotropism. — The force of gravity is one of the chief 

 directive stimuli for roots. This does not mean that roots 

 merely hang down on account of their weight. It really 

 means that some roots take a position pointing in the line 

 of gravity, while others take a position at right angles 

 to it. In both cases gravity is the effective stimulus. 

 No matter in what position a germinating seed is placed, 

 its root will turn down. Tw^o things are to be noted in 

 respect to this turning of a root in reference to gravity. 

 First, it is the direction and not the force which is the 



