696 LECTURE XXXIX. 



pronounced the less the light falling on one side penetrates into the tissues of the 

 shaded side. 



That in geotropic curvatures the important point is only as to the direction 

 in which gravitation acts on the part of the plant, and that it is not in any way 

 a matter of a stronger effect on the lower side and a feebler effect on the 

 upper side, requires no proof; and these considerations led me to the conclu- 

 sion that in the case of heliotropic curvatures also, it might not depend upon 

 a difference in the intcnsily of the acting force on opposite sides of the organ, but 

 that, on the contrary, the heliotropic influence is due to the fact that the rays 

 of light enter the tissue of the plant, or even only single cells, in a definite 

 direction. 



This view would be rendered highly probable if it were possible to detect 

 the same relations between the rays of light and the heliotropic curvature, as I 

 had established with respect to gravity for the geotropic curvatures. This re- 

 quired demonstration was afforded by Hermann Müller point by point. But 

 unfortunately the space here at disposal does not admit of my going more 

 particularly into the details of his demonstration : I must therefore refer to JMiiller's 

 excellent treatise mentioned in the notes. 



For nearly eight years I have been able to find no fact which contradicts 

 the theory I have proposed, whereas numerous facts only recently established 

 are to be pointed to as in its favour. Even in the case of the influence of light 

 on the movement of swarm-spores, the important point can only be as to the direc- 

 tion of the rays of light, not as to whether the given swarm-spore is illuminated more 

 strongly in front or behind. The same applies to the movements of protoplasm, 

 in consequence of which the chlorophyll-corpuscles travel in the cells. With these and 

 other movements which are produced in plants by light, moreover, the heliotropic 

 curvatures agree also in that they are due chiefly to the highly refrangible rays 

 of light. If seedlings like that in Fig. 358 are placed in a box which only receives 

 such Kght as has passed through a solution of potassium bichromate, no heliotropic 

 action whatever takes place ; now this light contains only the red, orange, yellow 

 and part of the green rays, and appears very bright to the eye. If the plant is 

 placed in an exactly similar box, and receives the light through a dark blue 

 solution of ammoniacal oxide of copper, the heliotropic curvatures occur with the 

 same energy as if the plants had been exposed to full daylight ; but this blue light 

 only contains the blue, violet, and ultra-violet rays of the solar light. I obtained 

 exactly the same result when the light fell on the plant through sheets of coloured 

 glass. Behind a pane of very dark blue cobalt glass, which transmits red rays as 

 well as the whole of the blue half of the spectrum, the heliotropic curvatures follow 

 as in ordinary daj-light. Behind a pane of dark ruby-red glass, which permits the 

 passage of very little besides the red rays, no curvature whatever resulted. 



Guillemain examined seedlings in the various parts of the spectrum itself, and 

 came to the conclusion that the heliotropic curvature takes place under the influence 

 of all the rays, with the exception of the least refrangible heat-rays. According to 

 him a maximum effect is produced by the ultra-red and ultra-violet rays. Wiesner, 

 on the contrary, finds that in the objective solar spectrum all kinds of rays, from 

 the ultra-red to the ultra-violet, with the exception of the yellow only, exert helio- 



