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 diffei-ent rays of the spectrum are 

 not equally active ; the paratonic efPect of the more highly refrani 

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

 rays of lower refrangibility (see Fig. 481). 



d. Irritability io 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, Haematococcus, Botrydium, etc.). 



13 C 



Fio. 481.— (A'^ter Wiesnrr). Cnrve illustrating the relative heliotropic effect of rajs of 

 ilifforent refrangibility. The letters A — M in the base-line indicate the position of the 

 more importHnt lines in the solar ppectrnm. The curves f, IJ, III represent the degree of 

 enrvatnre, m der 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 rays 

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



When light falls obliquely npon 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 liglit; 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 photoiactic ; when away from it, negatively phototactic. 



