NEGATIVE HELIOTROPISM 607 



the excitation is longitudinally transmitted to the growing 

 region lower down, and induces a concavity there which 

 increases with the duration of the stimulus ; but if the 

 stimulus of light be applied directly on the growing region 

 itself, instead of on the upper part of the specimen, then, by 

 reason of the transverse transmission of excitation to the 

 distal side, we obtain a state of things in which there is no 

 resultant curvature at all. In this case, then, the direct effect 

 of stimulus on the proximal side of the growing region is 

 balanced or neutralised by the transmitted effect on the distal 

 side ; but this condition of balance will be upset if the uni- 

 lateral stimulus of light, hitherto acting on the growing 

 region alone, be allowed to act simultaneously on the upper 

 part of the specimen also. The longitudinally transmitted 

 stimulus from the upper part being now added to the direct 

 excitation of the proximal side of the growing region, causes 

 an over-balance of responsive effect on that side, resulting in 

 a positive heliotropic curvature. The fact that there is no 

 heliotropic movement, when only the lower part of the 

 seedling is unilaterally acted on by stimulus, is thus not due 

 to any absence in that region of heliotropic sensibility, but to 

 the neutralisation of the proximal effect by the equal excita- 

 tion of the distal. Such transmission of stimulation along 

 the length of the organ is observed to take place in a specially 

 marked manner in the cotyledon of graminaceous plants. 

 We may account for this by the fact that such organs are 

 parallel-veined — that is to say, the fibro-vascular elements, 

 which we already know as good conductors of excitation, run 

 along their length. This is no doubt the reason of their 

 ready transmission of stimulus to a distance. 



Negative heliotropism of a radial organ. — We have 

 seen that when moderate stimulus acts unilaterally on a 

 growing organ a positive curvature is induced, and that, with 

 stronger or long-continued stimulation, this reaches the distal 

 side, producing neutralisation. We shall now proceed tc 

 trace out the continuity of responsive heliotropic effects, from 

 the positive curvature to the negative, through the inter- 



