TPIE PLANT BODY 31 



The gravity sense. It will be of special interest to the stu- 

 dent at this point to learn something of the method by which 

 some of the great botanists of the past laid the foundation for 

 our present understanding of the nature of plant movements 

 and of the relation which exists between external stimuli and 

 the response of the plant to them. For this purpose the classi- 

 cal experiments of Thomas Andrew Knight, Julius von Sachs, 

 and Charles Darwin on the gravity sense of plants and its 

 relation to plant movements may well serve as an illustration. 



Gravity is the most universal and constant external force 

 acting upon the living plant world, and it is not surprising, 

 therefore, that of all the outside forces this is found to be the 

 most potent in directing the adjustments of plant organs to 

 their environment by movements or tropisms. 



The general nature of the response of plants to gravity is 

 suggested by the fact that the stems and roots of all plants 

 take up the same position with reference to the earth's center 

 at all points on the earth's surface. Thus, plants on opposite 

 sides of the earth are found to have the main root growing 

 toward the earth's center and the main stem away from it. 

 Likewise, growing plants which have been prostrated by storms, 

 or which happen to grow on steep hillsides, always tend to 

 place their stems in a vertical position with reference to the 

 earth's center. This suggests the general law that roots re- 

 spond to gravity by growing toward the earth's center, while 

 stems tend to grow in the opposite direction in response to the 

 same stimulus. 



This general law was first tested out in 1806 by Thomas 

 Andrew Knight, an English botanist, who conceived the idea of 

 substituting centrifugal force for gravity, to see how roots and 

 stems would respond to other forces than gravity. Knight 

 attached boxes, in which young plants were growing, to the cir- 

 cumference of rapidly rotating wheels. When the wheels were 

 rotated rapidly enough, he observed that the roots grew toward 

 the circumference of the wheels, with the acting centrifugal force, 

 while the stems grew toward the center of the wheels, or against 

 the acting force. If the wheels were rotated less rapidly, the 



