270 STUDIES IN GENERAL PHYSIOLOGY 
Il 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. Jn 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, (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 
Digitized by Microsoft® 
