58 FOUNDATIONS OF BOTANY 



reference to gravity,^ 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 region 

 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.-sprouting Peas, on the Disk ^ut another force is introduced 



of a rapidly Whirling oiinostat. in its placc. If a vertical wheel. 



The youngest portions of the roots lik^ a carriage whecl. Were pro- 



all point directly away from the ° j " ^ " 



axis about which they were re- vided With a fcw loOSCly fitting 

 volved. . . , ,, n 



iron rmgs 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 



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

 near its surface. 



