CHAPTER III. SPECIAL PHYSIOLOGY OF MOVEMENT. 



747 



intense the light the more marked the paratonic action. Exposure 

 to very intense light may entirely arrest growth for the time 

 being. 



It has been found that the different rays of the spectrum are 

 not equally active ; the paratonic effect of the more highly refran- 

 gible rays (violet, indigo, blue) is far greater than that of the 

 rays of lower refrangibility (see Fig. 481). 



d. Irritability to the Direction of Incidence of the rays of Light 

 (Heliotropism) . This kind of irritability is extremely common, 

 and generally manifests itself in the most striking manner. The 

 most active rays of light are those of high refrangibility (violet, 

 indigo, blue). 



A remarkable example of this is afforded by the zoospores of 

 various plants (e.g. Ulothrix, Hsematococcas, Botrydium, etc.). 



n 



J3 C 



G 



Fio. 481 . (After Wiesner). Carve illustrating the relative heliotropic effect of rays of 

 different refrangibility. The letters A H in the base-line indicate the position of the 

 more important lines in the solar spectrum. The curves I, II, HI represent the degree of 

 curvature, uuder the influence of the different rays, presented by the Vetch, the Cress, and 

 the Willow respectively. The curve xy represents the relative effect of the different ray 8 

 in retarding growth ; it is greatest at y, and least at x. 



When light falls obliquely upon them, these zoospores arrange 

 themselves in the water so that their long axes are parallel to the 

 direction of incidence of the rays ; this phenomenon is termed Photo- 

 taxis. Moreover, the direction of their movement is also determined 

 by the direction of incidence of the light. They move in the line 

 of incidence, but they may move either towards or away from the 

 source of light ; the direction depending partly on the intensity of 

 the light, and partly on the degree of irritability of the zoospore. 

 When a zoospore moves towards a source of light, it is said to be 

 positively phototactic ; when away from it, negatively phototactic. 



