270 STUDIES IN GENERAL PHYSIOLOGY 



II. ON THE THEORY OF HELIOTROPISM 



Every attempt to formulate a theory of heliotropism is 

 handicapped by our ignorance of the nature of the changes 

 which are produced by the light in the illuminated tissues. 

 If we acknowledge this gap, then the rest of the heliotropic 

 effects of light upon animals may, perhaps, be understood as 

 follows: Let us imagine any number of sections made 

 parallel to the three principal axes of a bilaterally symmetri- 

 cal, heliotropic animal. Of these elements into which the 

 animal has been divided, always two which occupy symmetri- 

 cal positions with reference to the median plane of the 

 animal possess equal irritability. Every other two elements, 

 however, possess unequal irritability, and generally the 

 irritability of the oral end is greater than that of the aboral 

 end. Corresponding elements on the dorsal and ventral 

 sides have unequal irritabilities. I imagine the importance 

 of this distribution of irritability for the orientation of the 

 animals to be as follows : If the light strikes one side of the 

 animal, changes occur in the illuminated tissues, which at 

 present are unknown. In consequence, a change occurs in 

 the tension of the muscles (or the contractile elements which 

 act like muscles), which may be of two kinds: the light 

 either brings about an increased tension of the muscles on 

 that side of the animal which is exposed to the light (or of 

 those muscles which turn the animal toward this side) ; or 

 the opposite occurs, and the light brings about a decrease in 

 the tension of these muscles and a preponderance of the 

 tension of their antagonists. The first takes place, as I 

 assume, in positively heliotropic animals; the second, in 

 negatively heliotropic animals. These assumptions explain 

 the orientation of animals by light. Let SS 1 (Fig. 65) be 

 parallel rays of light ; a the oral, b the aboral end of a helio- 

 tropic animal. At the beginning of the experiment the 

 animals move in a straight line in the direction ba. The 



