CONCERNING THE THEORY OF TROPISMS 



JACQUES LOEB 



About twenty years ago' I began a physico-chemical analysis 

 of the behavior of lower animals which had heretofore been 

 explained in the anthropomorphic way characteristic of archaic 

 science. My main efforts were directed toward the analysis of the 

 role which light and gravitation play in the reactions of animals. 

 I showed first, that the orientation and the direction of the pro- 

 gressive motion of certain animals can be controlled unequivocally 

 by the direction of the rays emanating from a source of light, and 

 I showed, moreover, that this type of reaction is, as far as we can 

 judge, in every point identical with the heliotropic reaction of 

 plants. A few years later I showed that there exists another group 

 of animal reactions to light, which is not covered by the theory of 

 tropisms, but which depends upon the rapidity of the change of 

 the intensity of the light. ^ This latter type of reaction I designated 

 as Unterschiedsempfindlichkeit. Those who are familiar with the 

 terminology of the physicist will most readily understand the dif- 

 ference between the two types of reaction if I state that helio.trop- 

 ism depends upon the value / (z), where / is the intensity of light, 

 while in Unterschiedsempfindlichkeit the reaction depends upon 



the ^ lue of-— where t is the time. Both forms of reaction may 



dt ^ 



occur in the same animal {e. g., Spirographis), but this is neither 

 necessary nor the rule. 



* My first two papers on animal heliotropism and geotropism appeared in January, i888, and 

 not, as is often stated, in 1890 (Sitzungsber. der Wiirzburger Phys.-med., Gesellschaft, 1888). 



^I refer the reader to the following papers: Pfliiger's Archiv, vol. 54, p. 100, 1893. (Studies 

 in General Physiology, vol. 2, p. 286.) Pfliiger's Archiv, vol. 56, p. 247, 1894. (Studies, vol. 2, p. 

 345.) Pfliiger's Archiv, vol. 66, p. 439, 1897. Comparative Physiology of the Brain. Dynamics 

 of Living Matter, pp. 135-137. 



The Journal of Experimental Zoology, vol. iv, no. i. 



