SUPPLEMENT 141 



owing to the action of those kinds of rays which WIESNER found to act helio- 

 tropically. 



Meanwhile we need not consider further the so-called chemical effect of 

 light. If it can be proved that the plant reacts, as we may say, not to light 

 but to an effect produced by light, then heliotropism may be regarded as 

 a special instance of chemotropism (Lecture XXXVII), and would, for that 

 reason, be still further removed from the category of phenomena that geo- 

 tropism belongs to, where, as we have seen, it is not gravity itself as such, but 

 the actual weight associated with gravity that is perceived. That geotropic 

 and heliotropic perception are not identical is shown by the quite different 

 relationship in which they stand to external conditions, especially to oxygen. 

 Thus CORRENS (1892) has shown 



1. 39 P. 477, 1. 37, for Although it follows . . . summary only, read On the 

 other hand, RICHTER (1906) has shown that so-called ' laboratory air ' inhibits 

 geoperception, while photoperception is at least not affected, possibly even 

 increased. 



In order that a heliotropic excitation should follow on a light impact, 

 the light, as we have seen, must affect the plant unilaterally. We speak of 

 light in heliotropism as the stimulus-medium, and its unilateral incidence as 

 the stimulus-impact. Just as we were able to prevent geotropic curvature by 

 continuously rotating the plant on a klinostat, so we may also prevent helio- 

 tropic response by rotating the plant or the source of light. Again, geopercep- 

 tion takes place on the klinostat, and similarly photoperception occurs in 

 a plant illuminated on all sides. Perception is followed by excitation, and 

 excitation by a reaction, but since in ortho tropic organs this will occur equally 

 on all sides, no curvature can result. When we inquire more closely as to the 

 significance of unilateral light incidence, we find that it has been differently 

 interpreted by different authors. SACHS laid special stress on the fact (compare 

 MULLER-THURGAU, 1876, and SACHS'S Lectures) that the light rays penetrate 

 the plant body obliquely. He assumed that an orthotropic organ is in a helio- 

 tropic rest position when the light rays impinge on it in the direction of its 

 axis, whilst plagiotropic organs take up a definite angle to the line of incidence 

 of light. On the other hand (DARWIN, 1881 ; OLTMANNS, 1892), it was held 

 that the reaction depended on unequal light intensity in different parts of the 

 plant. Neither view has as yet been exactly established, and it is questionable 

 whether it is possible to place the plant under such conditions that different 

 parts may be illuminated unequally without passing from the more to the less 

 luminous light rays. Although, therefore, it is not at present possible and 

 perhaps may never be possible to come to a decision as to these two views, 

 still, in consequence of more recent experiments carried out by FITTING (1907), 

 the idea we previously brought forward may be set aside, viz. that the stimulus 

 which induced heliotropic curvature lay in the inequality in illumination on 

 different flanks of the perceptive organ. We fancied, more especially, that 

 a seedling Avena, one half of which, say the left, is shaded, would exhibit 

 curvature to the right if the right half were illuminated equally brightly from 

 before and behind. One of FITTING'S experiments renders that view improb- 

 able. If the apical region of the cotyledon of Avena be split longitudinally, 

 and if one half be darkened while the other half is illuminated on both sides, 

 no curvature follows, neither in the apex itself nor in the darkened base. On 

 the contrary, curvature always takes place in the direction of the light when 

 the light impinges unilaterally on the half cotyledon, and it is perfectly 

 immaterial whether the other half be darkened or entirely removed. The 

 darkened base also in this case curves in the direction of the light impinging on 

 the apex, whether this be on the outside, the inside, or a flank of the segment 

 of the cotyledon. Further still, FITTING was able to show that the transference 



