HELIOTROPISM 471 



consist of at least four distinct processes, i. e. perception, excitation, transmission 

 of the excitation, and curving, let us examine these processes separately, as 

 far as the present state of our knowledge will permit. Let us look at perception 

 first. In the case of gravity, we could at least afhrm with certainty that the 

 first purely physical effect on the plant must be the influence of weight on the 

 sensitive protoplasm, and we arrived at that conclusion because the effect of 

 gravity could be replaced by centrifugal force. We cannot, however, so far 

 replace sunlight by any other agent, and so we are compelled to seek some other 

 solution of the problem. It may certainly be supposed that light operates in 

 this case just as gravity does ; in this case, as in that, we have to consider in the 

 orthotropic organ a change in the direction of the active agent concerned, 

 here as there we have to deal with phenomena due to unequal growth. Sachs 

 laid special stress on the fact (compare Muller-Thurgau, 1876, and Sachs, 

 1882) that the heliotropic rest position of an organ lies in the direction of the 

 incident rays, just as the geotropic rest position lies in the direction in which 

 gravity acts. A stimulus is administered whenever the long axis of the organ 

 forms an angle with the line of operation of the stimulus. We do not know 

 whether, from this correspondence between geotropism and heliotropism, Sachs 

 himself drew the conclusion it suggests that gravity and light were com- 

 parable from a purely physical point of view ; at all events it might be possible 

 to conclude so and to advance the hypothesis that gravity like light depends 

 on undulatory motions of ether ; one might do so if indeed the likeness between 

 geotropism and heliotropism, established by Sachs, were proved to be one of 

 complete conformity of the two stimulation phenomena ; but this is hy no means 

 the case. 



It is possible, indeed it is highly probable, that the reaction or curvature 

 is the same in both cases, at present, at least, we know of no difference. The 

 heliotropic excitation may indeed be the same as the geotropic, but there is 

 a great difference in the perception in the two cases, as is shown quite clearly 

 by certain observations made by Correns (1892). When Correns studied 

 the effect of oxygen on tropistic movements, he was able to establish the fact 

 that the seedlings of Helianthus in the presence of the slightest traces of oxygen, 

 were still capable of exhibiting geotropic curvatures, while a far higher per- 

 centage was needed (i per cent, of the normal amount) for the performance 

 of heliotropic movements. Geotropic curvature ceased when growth came to 

 an end, hence it is possible that in this case perception was quite independent 

 of the presence of oxygen, while heliotropic perception appeared only when 

 relatively large amounts of oxygen were present. 



Although it follows of necessity from this fact that geotropic and helio- 

 tropic perception are essentially different, still Sachs's view might yet be 

 correct, according to which it is the direction of the incident ray that is perceived. 

 In order to estimate the truth of this theory more accurately we must compare 

 it with the earlier conceptions of heliotropism. According to these the essential 

 condition for heliotropic curving is an unequal intensity of light on either 

 side of an orthotropic organ. The rest position would thus be in a line 

 parallel with the path of the light, because when so placed all sides of the 

 organ would be equally brightly illuminated ; when unilaterally illuminated, 

 the fact that the light rays stream through the plant in an oblique direction, 

 would not lead to perception, but that result would be attained by the unequal 

 illumination of the two sides. 



If we compare this conception with Sachs's views, we are driven to the con- 

 clusion that it is impossible to accept the explanation previously offered by De 

 Candolle, that the result is due to the direct action of light of different intensity 

 on the rate of growth. We have already (p. 461) refuted this theory, and the 

 behaviour of the Gramineae, studied above, is sufficient in itself to disprove it. 



