Darwinism stated by Darwin himself · Charles Darwin
INFLUENCE OF LIGHT UPON PLANTS.
We found that if seedlings, kept in a dark place, were laterally illuminated by a small wax-taper for only two or three minutes at intervals of about three quarters of an hour, they all became bowed to the point where the taper had been held. We felt much surprised at this fact, and, until we had read Wiesner’s observations, we attributed it to the after-effects of the light; but he has shown that the same degree of curvature in a plant may be induced in the course of an hour by several interrupted illuminations lasting altogether for twenty minutes as by a continuous illumination of sixty minutes. We believe that this case, as well as our own, may be explained by the excitement from light being due not so much to its actual amount, as to the difference in amount from that previously received; and in our case there were repeated alternations from complete darkness to light. In this and in several of the above-specified respects, light seems to act on the tissues of plants almost in the same manner as it does on the nervous system of animals.
Text from Project Gutenberg, public domain in the United States.