American journal of physiology . othesis would explain these facts somewhat as fol-lows. Let there be two sources of light, and let an individual, supra-optimally stimulated, move by the first type of orientation away fromone of the lights. Light from the other source falls on one side,augmenting the contractile phase, and so hastening the progress of ^ Davenport: Loc. at., p. 20S. - Towle: This journal, 1900, iii, p. 365. The Theory of Phototactic Response. 471 that side. Thus the organism turns also away from the second Hght,until, when equally stimulated on its two sides it progresses equal
American journal of physiology . othesis would explain these facts somewhat as fol-lows. Let there be two sources of light, and let an individual, supra-optimally stimulated, move by the first type of orientation away fromone of the lights. Light from the other source falls on one side,augmenting the contractile phase, and so hastening the progress of ^ Davenport: Loc. at., p. 20S. - Towle: This journal, 1900, iii, p. 365. The Theory of Phototactic Response. 471 that side. Thus the organism turns also away from the second Hght,until, when equally stimulated on its two sides it progresses equallyrapidly from both lights. That is, its course, if projected behind theanimal, falls midway between the two sources of light. The explana-tion is analogous when the movement is toward the lights, or whenthere are more lights than two. As Miss Towle has shown, further-more, if the lights are of unequal intensity, the resultant course ofthe animal is a function of both the position and the intensity of thetwo lights. ?\r \ ^ Ar. Stentor 1111 The third case ofheliotropic phenom-ena to be consideredis one that was firstclearly observed byOltmanns ^ in experi-ments with Volvox(globator and minor).It may essentially bestated as follows: Ifan organism is free tomove at right anglesto a parallel series oflight rays artificiallyso modified that theillumination at oneend of the course isbright and graduallyshades off to darknessat the other end, the organism again seeks its region of optimal illumination. Oltmannsrealized these conditions practically as follows (Fig. 5). Parallelrays of light fell horizontally at right angles to the long axis ofa narrow trough of water destined to receive the organisms. Be-tween the trough and the incoming light there intervened a prismaticscreen. Thin at one end, it there let almost all the light through;but becoming gradually thicker it gradually diminished the intensityof the rays till at the opposite end the screen was opaque and inter-cepted all th
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