HELIOTROPISM OF ANIMALS 17 



nature of gravitation, men determined accurately the details 

 of the movement of falling stones, of pendulums, etc., and 

 described them in the most simple and definite terms. In 

 biology, especially in regard to the mechanical effects of 

 light which concern us here, the task of the investigator can 

 only be to determine and describe the circumstances upon 

 which depend the movements of animals under the influ- 

 ence of light. 



IV. REMARKS ON THE METHOD OF EXPERIMENTATION. THE 



HELIOTROPISM OF AN ANIMAL USUALLY BECOMES EVIDENT 

 ONLY AT A DEFINITE EPOCH IN ITS EXISTENCE. THE 

 HELIOTROPISM OF AN ANIMAL CAN EASILY BE OBSCURED 

 BY A SPECIAL FORM OF CONTACT-IRRITABILITY 



The facts which I have to prove are so simple that almost 

 all technical apparatus can be dispensed with. If one 

 attempts to demonstrate that the orientation of the animals 

 is controlled by the direction of the rays of light, care must 

 be taken that light falls upon the animals from only one side. 

 To accomplish this it is sufficient to carry on the experiments 

 in a room which is lighted from one side only. Since the 

 animals with which we are dealing in this discussion are 

 dorsiventral and place their median planes in the direction 

 of the rays of light, progressive movements are possible in 

 only two directions either toward the source of light 

 (when they will be called positively heliotropic), or away 

 from the source of light (in which case they will be called 

 negatively heliotropic). 1 



Diffuse daylight was used as the source of light, and only 

 where specially mentioned was sunlight employed. 



i Some botanists designate the movements of motile plant organisms toward a 

 source of light as " phototactic," in contrast to the "heliotropic" movements of 

 sessile plants. Since the observations of Sachs, Stahl, and Wortmann, however, 

 leave no room for doubt that the processes are identical in both cases, it seems to me 

 that this separation is not justified. Otherwise a " phototactic " animal ought to 

 become "heliotropic" when its progressive movements are prevented. For this 

 reason I use the same term for similar processes. (See WOKTMANN, Botanische Zei- 



