434 



LECTURE XVII. 



heliotropic effect of the different rays is the same whether the 

 curvature be positive or negative. 



B C 



FIG. 50 (after Wiesner). Curve illustrating heliotropic effect of rays of different 

 refrangibility. The letters A H on the base line indicate the positions of the 

 most conspicuous lines of the solar spectrum. The curves I, II, III represent 

 the curvatures, under the influence of the different rays, of the Vetch, the 

 Cress, and the Willow respectively. The curve x y represents the relative 

 effect of the different rays in retarding growth ; it is greatest at y and least 

 at x. 



The effect of the dark heat-rays in affecting the direction 

 of growth has been made the subject of special investigation 

 by Wortmann. He has found, in agreement with the state- 

 ments made above, that they are capable of exerting a very 

 considerable influence in producing curvature. The phe- 

 nomena thus produced may be conveniently designated by 

 the term Thermotropism, suggested by van Tieghem. It 

 appears, from Wortmann's experiments, as might be expected, 

 that the reaction of organs to the action of the dark rays 

 exhibits the same varieties as those with which we have be- 

 come acquainted with in the case of the luminous rays ; some 

 curve away from the source of radiant heat, others towards it. 

 For instance, when sporangiferous hyphae of Phycomyces 

 and shoots of Cress-seedlings were exposed to radiant heat, 

 by placing them, either in the normal position or rotating on 

 a clinostat, in an appropriate position with regard to a hot 

 smoked tin-plate, they in all cases curved away from the 

 source of heat, that is they shewed themselves to be negatively 



