Volume 1

Studies in general physiology / by Jacques Loeb.

  • Jacques Loeb
Date:
1905
Catalogue details

Licence: Public Domain Mark

Credit: Studies in general physiology / by Jacques Loeb. Source: Wellcome Collection.

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    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 Wortmann, Botanische Zei¬ tung, 1887.)
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