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).’ 
Diffuse daylight was used as the source of light, and only 
where specially mentioned was sunlight employed. 
1 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 tome 
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 WORTMANN, Botanische Zei- 
tung, 1887.) 
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