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PHYSIOLOGY: LOEB AND WASTENEYS 
ON THE IDENTITY OF HELIOTROPISM IN ANIMALS AND 
PLANTS 
By Jacques Loeb and Hardolph Wasteneys 
ROCKEFELLER INSTITUTE FOR MEDICAL RESEARCH, NEW YORK 
Presented to the Academy, November 12, 1914 
1. Since 1888 Loeb has in a number of publications expressed the 
idea that the apparent attraction of animals by hght is in reahty a 
phenomenon of automatic orientation of the animals by a source of 
light, comparable to or identical with the well-known phenomena of 
heliotropic orientation of plants. Thus he proved that sessile animal 
organisms react to light in the same way as sessile plants, namely, by 
bending or growing towards (or away from) a source of Hght; while motile 
animals move towards (or away from) a source of light as do the motile 
swarm spores of certain algae. 
He based his theory of these reactions in animals on the following 
three assumptions. First, the light acts chemically upon the photo- 
sensitive elements of the surface of the body (eyes or skin). Second, 
symmetrical elements of the surface are identical chemically, so that if 
one source of light is given and symmetrical elements are struck at the 
same angle by the rays emanating from one source of hght, the velocity 
of the photochemical reactions in the symmetrical elements is identical; 
if, however, they are struck at different angles, the velocity of chemical 
reactions is no longer the same in symmetrical elements. 
Third, the velocity of photochemical reactions in the eyes or the skin 
influences (through the nerves or, as the case may be, other protoplasmic 
conductors) the tension of the muscles (or other contractile elements) 
connected with the photosensitive elements at the surface of the body. 
If symmetrical photosensitive elements are struck by the light at the 
same angle (and only one source of light exists), the symmetrical muscles 
of the body are influenced by the light in the same way and no change 
in the direction of the motion of the animal will occur. If, however, 
symmetrical photosensitive elements are struck at different angles the 
velocity of chemical reactions will not remain the same in the symmetri- 
cal elements and hence the tension of symmetrical muscles connected 
with these elements will not be the same; as a consequence, when the 
animal moves it must show a tendency to deviate from the straight line 
until finally its axis or plane of symmetry goes through the source of 
light again. When this happens the symmetrical elements of the photo- 
sensitive surface are again struck at the same angle by the light and 
there is nov/ no more reason for the animal to deviate from this direction. 
