248 CAUSES OF VARIATION 



this particular orientation with reference to the rays of light, 

 whether parallel to their direction or transverse, has received 

 the special name of " phototaxis." l 



Effective rays. All experiments indicate that the blue rays 

 are the effective ones in producing heliotropic effects. 2 Some 

 organisms seem sensitive to the yellow, but both red and ultra- 

 violet are alike inoperative. Heliotropism is therefore an effect 

 decidedly due to the luminous rays. 



Contributary conditions. 3 Heliotropism is dependent upon a 

 variety of conditions both external and internal to the organism. 



1 There is great uncertainty as to terms. The one just quoted ("phototaxis ") 

 is in its root a protest against the old term " heliotropism " in that it recognizes 

 light as the active agent rather than the sun (helios), which is a source not only of 

 light but of heat and chemical energy as well; and it recognizes, too, that these 

 influences are exerted as light, quite independent of the sun or any other particu- 

 lar source. 



Of late there has been a strong disposition to substitute this root for the older 

 term, giving us phototropism in place of " heliotropism." Those disposed to this 

 view would go farther and discriminate between those movements that appear to 

 occur with reference to the direction of the rays, which is " phototaxis," and those 

 which are made with reference to the intensity of illumination, which is " photop- 

 athy." These students also recognize the fact that irritability and the consequent 

 movements, whether positive or negative, depend very much upon the intensity, 

 so that organisms that are negatively heliotropic at high intensity are positively 

 heliotropic at low intensity, suggesting a middle point at which the organism will 

 be Reid, as with deep-sea animals that come near the surface at night but sink to 

 considerable depths in the bright light of day, the water acting as a screen. This 

 neutral point or satisfied condition is described as " phototonus." 



It may be necessary to recognize all these distinctions when the subject is 

 better understood, but it is more than likely that these different behaviors are 

 only different manifestations of the same natural irritability to light on the part of 

 protoplasm in general, and that the so-called " phototaxis " or even "phototonus " 

 is only the condition of the organism after it has brought itself into such position 

 that one irritability is balanced by another (which is always easy with bilateral 

 symmetry), or that it is a kind of acclimatization acquired by the protoplasm to 

 the particular intensity at hand. 



The theoretical objection to direct reference to the sun is certainly sound. The 

 attraction (or repulsion) is with reference to light as light, and not to the sun as 

 its source. But the sun is preeminently the source of light, and for this reason, 

 and with a proper understanding of the facts, the author prefers for our purposes 

 to adhere to the single old term, at least under the present state of kriowledge, 

 for if properly understood it seems substantially correct and serves all practical 

 purposes fairly well. 



2 C. B. Davenport, Experimental Morphology, Part I, pp. 201-203; Loeb, 

 Studies in General Physiology, pp. 29-31. 



8 Ibid. pp. 196-201. 



