432 LECTURE XVII. 



The importance of the direction of the incident rays as 

 determining the curvature was first suggested by Sachs, and 

 was subsequently more fully established by Miiller-Thurgau. 

 Miiller found that the heliotropic effect becomes more marked 

 as the angle of incidence increases from o to 90. The curva- 

 ture is, in fact, the expression of an attempt on the part of the 

 growing organ to place its axis of growth parallel to the 

 direction of the incident rays. This statement is equally true 

 of both positively and negatively heliotropic organs, though 

 the apex of the organ is directed in the one case towards the 

 source of light, and in the other away from it. 



With regard to the relation of the intensity of light to the 

 heliotropic effect, it has long been a matter of common obser- 

 vation that when the two sides of a positively heliotropic 

 organ are exposed to light of different intensity, the organ 

 curves towards the stronger light. But it is erroneous to 

 assume, as is commonly done, that the curvature is merely 

 the effect of the difference in the intensity of illumination of 

 the two sides of the organ. In the case of the delicate hyphse 

 of Fungi, for instance, 'the difference in the illumination of 

 the two sides must be very slight, and yet curvature is effected. 

 The curvature is, as we have seen, dependent upon the direc- 

 tion of the incident rays. But the curvature is affected by 

 the intensity of these rays. It is to Wiesner that we owe 

 a detailed investigation of this subject. He has found that 

 variations in the intensity of the light produce distinct effects 

 upon the curvature of the organ exposed to its action, and he 

 has been led, by careful observation of these effects, to the 

 following generalisations. There is for the organs of every 

 plant an optimum intensity of light which induces the maxi- 

 mum of heliotropic effect ; any increase or diminution of this 

 intensity is followed by a diminution of the heliotropic effect. 

 It was difficult to determine the lower limit of the action of 

 light, that is, the intensity at which a heliotropic effect can 

 only just be perceived, at least in the case of very sensitive 

 organs (especially the stem of Vicia sativa\ for they continue 

 to react to light of very low intensity. With regard to the 

 upper limit, Wiesner found that the degree of intensity at 



