440 



EFFECT OF LIGHT 



[Ch. xvn 



This list shows that different plants have diverse photo- 

 tropic sensitiveness. Since in this list species preceded by an 

 asterisk (*) live in the sunlight, that preceded by a dagger (f ) 

 is a shade-loving plant, while the others live in an intermediate 

 habitat, Ave may conclude that, in general, sun-loving plants 

 are less sensitive to light than those not markedly sun loving. 

 The former exhibit a sort of acclimatization to light.* 



The effective rays have been determined by Wiesner ('79, 

 p. 191) for seedlings of several species as a result 'of experi- 

 ments in which colored solutions were employed. His results 

 are summarized in Fig. 126, Avhich shows that the phototropic 

 effect is greatest at the violet end of the spectrum, and that as 

 we pass towards the D line, lying between the yellow and the 

 orange, the effect diminishes, becoming null at D. In the red, 

 again, there is a considerable effect. Beyond the visible red. 



8 C 



EiG. 126. — A-H, positions of Fkauenhofer's lines in the spectrum. 



, curves of phototropic effect of the various rays ; the ordinates have only 



relative values. J, /, curve for the seedling of the vetch, Vicia ; //, //, for cress 

 seedlings ; III, for etiolated -svillow shoots, upon which latter the more strongly 

 refractive rays only act phototropically. 



, curve of retardation of growth in length of Helianthus seedlings reared 



in the various rays. The ordinates give the increment in length of the seedling 

 suhjeeted to the ray under consideration ; thus the retardation is least at z, and 

 is greatest at y. (From Wiesner, '81.) 



* Certain plants are so sensitive to differences of illumination on their two 

 sides as to make very delicate photometers. Thus Wiesnek determined as 

 nearly as possible hy Bunsen's photometer the point of equal illumination 

 between two flames, but a seedling still detected a difference between their 

 intensities. Massaet ('88) has made use of this method to demonstrate for 

 phototropism the validity of Weber's law. He found that in Phycomyces a 

 difference of intensity of 18% between two sources of light could be detected; 

 and this held true for all intensities of light. 



