6. PHOTOTROPISM AND PHOTOTAXIS
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thesis; at that intensity where the linear relation between intensity and
photosynthetic rate begins to fall off, the chloroplasts begin to move into
the profile position, while at the intensity which saturates photosynthesis
the maximum profile position is reached (92). This was true both in
Lemna and in Mougeotia, i.e., a flowering plant and an alga. Further
support is derived from the observations of Babushkin (93) on tobacco
leaves, in which also the profile position appears to be adopted in light
intensities causing saturation of photosynthesis; both movement and
photosynthesis are inhibited by phenylurethane. On the other hand,
strong evidence that the movement cannot be caused simply by photosynthesis is provided by Haupt's recent observation (93a) with Mougeotia that far infrared light of 733 m/x reverses the orientation of chloroplasts caused by red light. The far infrared alone has no effect, so that
its action is simply to reverse that of the red. Thus the photoreceptor
for chloroplast movement may be the same as that for the far infrared
reversible reactions in higher plants, which control stem elongation,
photoperiodism, and the germination of light-sensitive seeds. C0 2 participates in the reaction (93b).
What the significance of the movement caused only by blue light
may be, can only be guessed at. The mechanism of the movements is
also obscure, since the plastids are not themselves motile and are evidently carried passively, though directionally, by the circulating cytoplasm. Blue light does change the viscosity of cytoplasm, with a maximum effectiveness between 460 and 490 m/x (94) and the streaming
rate is highly temperature-sensitive, but the connection of these facts
with directed chloroplast movement is obscure.
B. PHOTORECEPTOR FOR THE "TIP RESPONSE" OF Avena
The evidence in Section V indicates that the tip response of the
Avena coleoptile, and perhaps the corresponding reaction in many other
plants, is due to a modification in the system producing and transporting
auxin, but gives no indication as to what this system is. The auxin of
most plants is 3-indoleacetic acid (IAA), which has no absorption in
the visible spectrum, yet phototropic curvature occurs in visible light.
From the earliest studies (32, 95) it has been generally agreed that, as
with the animals discussed in Section II, blue light is most effective in
phototropism, red light is least effective. It follows that a yellow pigment must exist which, upon absorption of light, can interact with the
colorless IAA system to cause auxin redistribution. Numerous comparisons have therefore been made of the relative phototropic effectiveness
of different wavelengths of light (i.e., action spectra) in order to indicate the nature of the primary photoreceptor.
275
thesis; at that intensity where the linear relation between intensity and
photosynthetic rate begins to fall off, the chloroplasts begin to move into
the profile position, while at the intensity which saturates photosynthesis
the maximum profile position is reached (92). This was true both in
Lemna and in Mougeotia, i.e., a flowering plant and an alga. Further
support is derived from the observations of Babushkin (93) on tobacco
leaves, in which also the profile position appears to be adopted in light
intensities causing saturation of photosynthesis; both movement and
photosynthesis are inhibited by phenylurethane. On the other hand,
strong evidence that the movement cannot be caused simply by photosynthesis is provided by Haupt's recent observation (93a) with Mougeotia that far infrared light of 733 m/x reverses the orientation of chloroplasts caused by red light. The far infrared alone has no effect, so that
its action is simply to reverse that of the red. Thus the photoreceptor
for chloroplast movement may be the same as that for the far infrared
reversible reactions in higher plants, which control stem elongation,
photoperiodism, and the germination of light-sensitive seeds. C0 2 participates in the reaction (93b).
What the significance of the movement caused only by blue light
may be, can only be guessed at. The mechanism of the movements is
also obscure, since the plastids are not themselves motile and are evidently carried passively, though directionally, by the circulating cytoplasm. Blue light does change the viscosity of cytoplasm, with a maximum effectiveness between 460 and 490 m/x (94) and the streaming
rate is highly temperature-sensitive, but the connection of these facts
with directed chloroplast movement is obscure.
B. PHOTORECEPTOR FOR THE "TIP RESPONSE" OF Avena
The evidence in Section V indicates that the tip response of the
Avena coleoptile, and perhaps the corresponding reaction in many other
plants, is due to a modification in the system producing and transporting
auxin, but gives no indication as to what this system is. The auxin of
most plants is 3-indoleacetic acid (IAA), which has no absorption in
the visible spectrum, yet phototropic curvature occurs in visible light.
From the earliest studies (32, 95) it has been generally agreed that, as
with the animals discussed in Section II, blue light is most effective in
phototropism, red light is least effective. It follows that a yellow pigment must exist which, upon absorption of light, can interact with the
colorless IAA system to cause auxin redistribution. Numerous comparisons have therefore been made of the relative phototropic effectiveness
of different wavelengths of light (i.e., action spectra) in order to indicate the nature of the primary photoreceptor.