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 tlie 

 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 gravitropie, or 

 pro-gravi tropic, and that of the root as fiegative phototropic 

 and negative gravitropie or a nti-gravi tropic. 



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

 tropic and geotropic action. Such differences arise from the 

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

 tion of gravitational stimulus separated from that of response ; 



