GEO-ELECTRIC RESPONSE OF ROOT 475 



reaction which results from the effect of indirect stimu- 

 lus, in contrast to the contractile reaction due to direct 

 stimulation. 



We may now proceed a step further and try to obtain 

 some idea oi the difference in the mechanics of geotropic 

 stimulation of the shoot and of the root, to account for 

 the different responses in the two organs. The reason 

 of this difference lies in the fact that in the shoot the 

 perceptive and responding region is one and the same ; 

 every cut-piece of stem exhibits the characteristic geo- 

 tropic curvature. In the root the case is different ; for 

 the removal of the sensitive root-tip reduces or abolishes 

 the geotropic action ; the region of maximum geotropic 

 perception is thus separated from that of response. It 

 tnusi b3 l)orne in mind th it this holds good only in the 

 case of ffvavitatiotial stimulus, for the decapitated root 

 still continues to respond to other forms of stimulation 

 such as chemical or photic. 



The cause of this difference in ' the reactions to 

 geotropic and other stimuli lies in the fact that in the 

 latter case, energy is supplied from outside. But in 

 geotropism the force of gravity is by itself inoperative ; 

 it is only through the weight of the cell contents that 

 the stimulus becomes effective. Want of recognition of this 

 fundamental difference has led many observers in their 

 far-fetched and sweeping attempt, to establish an identity 

 of reaction of the root to geotropic and photic stimula- 

 tions, in spite of facts which plainly contradict it. Thus 

 the root moves away from the incident vertical line 

 of gravity ; but under light, the root very often moves 

 towards the stimulus. The negative phototropic response 

 of the root of Si}mpis is an exceptional phenomenon for 

 which full explanation has been given in page 376. 



We shall next consider whether the particular distri- 

 bution of the falling starch-grains (which offers a rational 



