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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    responsible for the lieliotropic phenomena which take place so energetically behind this screen, but can be due only to the activity of the more refrangible rays. The other external conditions which must be considered in lieliotropic investigations are so simple that they do not call for any special explanations. Where they are of impor¬ tance they will be self-evident. It is very essential, however, to realize that the helio¬ tropism of an animal often manifests itself clearly only dur¬ ing a definite, often decisive, period of its existence, only to diminish again or to disappear entirely later. It was only by observing for weeks and months the animals described in this treatise, which for the most part I raised myself, that I have been able to establish this fact. The caterpillars of Porthesia chrysorrhoea, for example, are energetically positively heliotropic only during a certain period of their existence, when they have just left the coc- coon in which they have wintered, and have not yet taken food. At this time the entire existence of these animals is a function of the light. Under natural conditions they hatch out on a warm spring day. The light compels them to creep to the tips of the branches, where they find their first nourishment in the young buds. When fed they are still positively lieliotropic, but very much less so than before. If anvone should examine them in this condition, he would scarcely pronounce them heliotropic. It is not, however, a certain date of the year which gov¬ erns this heliotropism; for whenever I forced the animals to leave their nest (by raising the temperature), whether at the beginning of summer or of winter, they were indefatigable in their attempts at creeping toward the source of light. Winged ants are pronouncedly dependent on light only at a definite period of their existence—at the time of their nuptial flight. The same animals which were actively lielio-
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