214 PART III. PHYSIOLOGY. [ 48 



An interesting demonstration of the relation of the rate of 

 growth to light is afforded by the observation of the growth of 

 any member at given intervals every hour, or every two or three 

 hours daring an entire day of twenty-four hours. By this means 

 it has been ascertained that a growing member exhibits a regular 

 daily periodicity in the variations in its rate of growth, which 

 has a direct relation to the alternation of day and night. 



The paratonic action of light varies with its intensity : the more 

 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. 



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, Hsematococcus, Botrydium, etc.). 

 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. 

 Another important case is the change of position of the chlorophyll- 

 corpuscles in the cells (see p. 172). 



Motile cellular members, whether mature or growing, are, as a 

 rule, sensitive to the directive influence of the incident rays of 

 light. Among mature motile members, foliage-leaves are those 

 which most markedly respond to the directive or heliotropic influ- 

 ence of light ; among growing members, it is more especially stems 

 and leaves which are sensitive, but roots have in many cases been 

 found to be so. All these irritable members take up a definite 



