Abstract
Hitherto positively heliotropic Euglenœ have always been found to give the motor reaction when suddenly shaded, and not when suddenly illuminated. Conversely negatively heliotropic individuals were found to react only to sudden illumination and not to sudden shading. Upon this association Jennings has based his theory according to which heliotropic orientation in Euglena is by “trial and error.”
It has been found, however, that it is possible to obtain positively heliotropic Euglenœ which give the motor reaction when suddenly illuminated and not when suddenly shaded. It is also possible to obtain at will negatively heliotropic organisms which give motor reactions when suddenly shaded, and not when suddenly illuminated. Under certain conditions the motor reaction to shading is given by Euglenœ in which no heliotropism can be demonstrated at that light intensity. Under other conditions distinct negative heliotropism is obtained with a light intensity which does not bring about any motor reactions when allowed to shine suddenly on the organisms, or when they are suddenly shaded. These facts show that heliotropic orientation in Euglena does not depend upon the motor reactions, but upon a separate mechanism.
Galvanotropism, which has so far eluded observation in Euglena was obtained by using culture media containing citric acid. With this material it was found that the method of orientation is identical in both galvanotropism and heliotropism. Consequently the orientation to light is as direct as the locomotor mechanism of Euglena permits, and does not take place by “trial and error.”
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