THE RELATIVE EFFICIENCY OF VARIOUS PARTS 



OF THE SPECTRUM FOR THE HELIOTROPIC 



REACTIONS OF ANIMALS AND PLANTS 



JACQUES LOEB AND HARDOLPH WASTENEYS 



From the Rockefeller Institute for Medical Research, Neiv York 



I. INTRODUCTION 



While the older authors had treated the motile reactions of 

 animals to light as an indication of their love for or antipathy to 

 this kind of energy, one of us in 1888 pointed out that we are 

 dealing in these cases with phenomena of orientation comparable 

 to the orientation of plants to light. "^ In order to indicate the 

 identity of the mechanism in both cases he proposed the same 

 term for both, namely, heliotropism (or phototropism) . Loeb 

 stated in his first full pamphlet (1889)- that (if one source of light 

 be given) the animals orient themselves so that their plane of 

 symmetry falls into the direction of the rays of light, "whereby 

 the symmetrical points of the surface of the body are struck by 

 the light at the same angle. " In 1897 the same writer expressed 

 the idea that the action of light which caused the hehotropic 

 reactions was chemical.'' Since it is reasonable to assume that 

 symmetrical elements of the surface of the body are not only 

 morphologically but also chemically alike, we must suppose 

 that if the symmetrical elements of the surface of the animal are 

 struck by the rays of light at the same angle, the velocity of 

 the photochemical reactions in symmetrical elements of the 

 surface (e.g., the eyes or skin) are the same, since the intensity 

 of the illumination of a surface element varies with the cosine 



^ Loeb, J. Sitzungsb. d. Wi'irzburger physik.-med. Gesellsch., 1888. 



- Der Heliotropismus der Tiere und seine Ubereinstimmung mit dem Helio- 

 tropismus der Pflanzen. Wiirzburg, 1889. 



^ Loeb, J. Zur Theorie der physiologischen Licht- und Sclnvcrkraftwirkungen, 

 Pflliger's Archiv, Bd. 66, p. 439, 1897. 



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