176 



A TEXTBOOK OF BOTANY 



[Ch. IV, 9 



It is clear that plants in growing geotropically are not forced 

 into those positions by the gravitational attraction, for even 

 if the young roots were pulled downward by gravitation, 

 this force could obviously not push the young stem upward, 

 or the side roots and branches out horizontally. In fact, 

 geotropism bears the same relation to gravitation that 



Fig. 121. — A Clinostat ; X &. A powerful clockwork in the case on the 

 right turns the central spindle continuously once in fifteen minutes. It 

 can be used in any position. Thus plant parts can be kept revolving in any 

 desired plane, whereby the action of a stimulus can be made equally all- 

 sided instead of one-sided, — the most convenient method of neutralizing 

 a one-sided stimulation. The instrument is much used in the study of 

 irritability. 



photo tropism does to light (page 54) ; just as the light 

 neither pulls the stems towards it, nor pushes the roots 

 away from it, nor forces the leaves across it, but acts simply 

 as a guiding stimulus to the plant's own assumption of those 

 positions, so gravitation neither pulls nor pushes the roots 

 and stems into the positions they take, but acts simply as a 

 guiding stimulus to the plant's own growth. This action 

 of gravitation as a stimulus, and not directly as a force, ex- 

 plains why parts grow as readily and perfectly away from 

 it or across it as towards it. 



