PHYSIOLOGY: LOEB AND WASTENEYS 
45 
If this theory of heliotropism were correct, it was necessary to show 
that the law of photochemical action should hold for the heliotropic 
reactions of plants as well as of animals. This law (which was first 
established by Hankel for a narrow range of light intensity and extended 
by Bunsen and Roscoe over a wider range) says, that within certain 
Hmits the photochemical effect of Kght is equal to the product of inten- 
sity into the duration of illumination. Five years ago it was shown 
independently by Blaauw^ and by Froschl that the heliotropic reactions 
of plants obey the law of Bunsen and Roscoe, i.e., that the time required 
to bring about the heliotropic curvature of plants changes inversely 
with the intensity of illumination. A year ago Ewald and Loeb^ showed 
that the same law holds also for the heliotropic curvature of an animal, 
namely, that of the polyps of Eudendrium. 
2. Although the question of wave length does not enter into Loeb's 
theory of heliotropic orientation, it seemed of interest to compare the 
relative efficiency of the various parts of the spectrum upon the pro- 
duction of heliotropic curvatures in Eudendrium with that found for 
the heliotropic curvatures in plants. The most exact measurements on 
plants are those by Blaauw on the seedlings of Avena. Blaauw used a 
carbon arc spectrum, and found that the most efficient part of the 
spectrum was a region in the blue between 466 and 478 i^x. In this 
region an exposure of only 4 seconds sufhced to call forth heliotropic 
curvatures in 50 percent of the stems of the seedlings. For longer waves 
a longer exposure was required, thus for a wave length of 499 txfi an 
exposure of 120 seconds, and for a wave length of 534 ixn an exposure 
of 6300 seconds. The yellow and red parts of the spectrum seemed 
absolutely ineffective. 
We undertook last summer a series of experiments to ascertain the 
relative efficiency of the various parts of a carbon arc spectrum on the 
newdy regenerated polyps of Eudendrium. The spectrum was thrown 
on a glass trough with parallel walls which contained a row of Euden- 
drium stems with a number of newly regenerated polyps. The position 
of each polyp was noted at the beginning of the experiment. Loeb 
and Ewald had found that the minimal time of exposure to produce 
heliotropic curvatures in more than 50 percent of the polyps is, for the 
same intensity of light, considerably greater than in the case of the seed- 
lings of Avena. It required an exposure of at least 5 minutes in order 
to call forth heKo tropic curvatures of the polyps of Eudendrium in the 
spectrum. The experiments were rendered difficult by the fact that 
the young polyps are very delicate and suffer easily. If a stem with 
sickly polyps is included in an experiment it is liable to disturb the 
