474 TRANSFORMATION OF ENERGY 



however, as to whether and how far the latent period is dependent on the 

 intensity of the hght. 



While in the case of gravity in nature we have to consider only one variable, 

 i. e. the direction in which the force acts, in the case of light as a stimulus we 

 have several variables, viz. the direction, the intensity, and yet another, which 

 we have not as yet considered, viz. the quality of the light. It has long been 

 recognized that rays of different wave length do not act in the same way. The 

 more highly refrangible rays which are more especially concerned in the forma- 

 tive activity of light (p. 310) have been found to be also those especially con- 

 cerned in heliotropism. Wiesner (1878) found that the rays at the limits 

 of the violet and ultra-violet regions were the most active, and that the activity 

 decreased from that point so that, in yellow light, practically no heliotropic 

 curvature took place at all. [Compare Dandeno, 1903.] The movements 

 begin again, however, in red light and increase towards the ultra-red, although 

 this is not true of every plant examined. Negatively heliotropic organs, 

 according to Wiesner, behave like positively heliotropic ones. 



Examining more closely the action of light, we have to recognize in the 

 first instance in all cases a purely physical or chemical action which may lead 

 to a heliotropic stimulus if the light falls unequally on different sides ; wherein 

 the primary effect lies, however, we are quite ignorant. Possibly light may 

 first of all induce certain chemical changes ; but an action like that which 

 takes place in silver salts is inconceivable, because the red rays, which perform 

 no function in photography, were found to be active in Wiesner's experi- 

 ments. It is certainly possible that heliotropic curvature in red rays is a 

 phenomenon sui generis, which may be compared with thermotropic processes, 

 which we shall consider later on (Lecture XXXVII), and if that be so, 

 heliotropism might be, in the restricted sense, included amongst those light 

 effects which, for the sake of brevity, may be termed photographic. On the 

 other hand it is also conceivable that definite chemical reactions are rendered 

 possible owing to the action of those kinds of rays which Wiesner found to act 

 heliotropically. 



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 geotropism 

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

 weight associated with gravity that is perceived. Still the difference between 

 geotropism and heliotropism is sufficiently great to cause us to treat these twO' 

 phenomena separately ; the likeness lies in the nature of the reaction, in the 

 character of the curvature ; the differences lie in the nature of the perception. 



In addition to the chemical effect of light we have also to consider a mechani- 

 cal influence. According to Maxwell, a pressure amounting to about 0-5 mg. 

 per sqm. is induced in any medium through which a light wave is propagated, 

 in the direction of the path of propagation. The existence of this pressure 

 has been more recently confirmed experimentally by Lebedew. Apart alto- 

 gether from its limited amount there are other reasons for doubting whether 

 heliotropic perception is at all dependent on this pressure. [Haberlandt 

 (1905) has recently advanced a unique hypothesis as to light perception in plants. 

 On the upper epidermis of many plants papillae are to be found which, according 

 to Haberlandt, act as converging lenses. The papilla concentrates a spot 

 of light on the protoplasm lining the inner wall of the epidermal cell. The 

 protoplasm of the inner wall is sensitive to this light and is able so to orientate 

 itself in response, that, according to the position of the leaf in regard to the 

 source of light, the light is concentrated at different places on the inner wall 



