RESPONSIVE GROWTH-CURVATURES IN PLANTS 733 



could take place while the effect induced by radiation as 

 such was being observed. Under these crucial conditions it 

 was demonstrated that the effect of radiation is to induce 

 responsive contraction. 



Under moderate unilateral stimulus of light, as in the 



case of gravity, two definite and distinct effects were observed, 



according as stimulus was applied directly on the responding 



region or on the distant tip. In the former case we obtained 



a positive, and in the latter a negative, responsive movement. 



Up to this point, then, the actions of light and of gravitation 



are parallel in their effects— that is to say, the positive 



heliotropic movement of the stem corresponds to the so-called 



negative geotropic movement of the same organ ; and the 



negative heliotropic movement of the root to its so-called 



positive geotropic. Looked at in relation to the direction of 



stimulus, however, it may be said that the response which is 



commonly known as ' negative geotropic ' is actually positive, 



and vice versa ; for, accepting the theory of statolithic or 



hydrostatic pressure as to the effective cause of stimulation, 



the direction of the excitatory pressure is in the direction of 



the lines of force of gravity. In a stem laid horizontally, 



then, and acted on by vertical lines of gravitational force, or by 



vertical rays of light, we obtain the same directive response 



to these similar directive stimuli, by the bending upwards of 



the organ to meet the rays, or the lines of force. Some 



confusion is therefore inevitable when one of these responses 



is designated as positive, and the other as negative, for the 



essential similarity of the two is here masked by the use 



of directly opposite terms. This difficulty might perhaps be 



overcome by naming the normal responsive movement of 



the stem as positive phototropic and positive gravitropic, or 



pro-gravitropic, and that of the root as negative phototropic 



and negative gravitropic or anti-gravitropic. 



We next turn to the differences between the effects of helio- 

 tropic and geotropic action. Such differences arise from the 

 two facts that : ( 1 ) only in the root is the region of the percep- 

 tion of gravitational stimulus separated from that of response ; 



