68 



FOUNDATIONS OF BOTANY 



reference to gravity, 1 is called geotropism. When, as in 

 the case of the primary root, the effect of gravity is to 

 make the part if unobstructed turn or move downward, 

 we say that the geotropism is positive. If the tendency is 

 to produce upward movement, we say that the geotropism 

 is negative; if horizontal movement, that it is lateral. It 

 was stated in the preceding section that the direct cause 

 of the downward extension of roots is unequal growth. 

 We might easily suppose that this unequal growth is not 

 due to gravity, but to some other cause. To test this sup- 

 position, the simplest plan (if it could be carried out) would 

 be to remove the plants studied to some distant re*gion 

 where gravity does not exist. This of course cannot be 



done, but we can easily turn a 

 young seedling over and over 

 so that gravity will act on it 

 now in one direction, now in 

 another, and so leave no more 

 impression than if it did not act 

 at all (Exp. XX). Or we can 

 whirl a plant so fast that not 

 only is gravity done away with, 



FiG.27.-spro U tingpeas,ontheDisk but another force is introduced 

 of a rapidly whirling ciinostat. i n its place. If a vertical wheel, 



like a carriage wheel, were pro- 

 vided with a few loosely fitting 

 iron rings strung on the spokes, 

 when the wheel was revolved rapidly the rings would all 

 fly out to the rim of the wheel. So in Fig. 27 it will be 



The youngest portions of the roots 

 all point directly away from the 

 axis about which they were re- 

 volved. 



1 Gravity means the pull which the earth exerts upon all objects on or 

 near its surface. 



