292
KENNETH V. THIMANN AND GEORGE M. CURRY
For Fhycomyces, Blaauw (32) found a single peak near 495 m/x,
but his filter combinations only permitted examination of a very few
limited spectral regions. Castle (52), also using only a few spectral
regions, found high sensitivity around 425 m/x and almost none at
525 m/x. A carotenoid is present in the sporangiophores (Castle, 52,
and subsequent workers) and indeed is best detected in the growing
zone a few mm. below the sporangium (135a). A detailed action spectrum for the positive phototropic curvature in visible light, and a less
detailed one for the negative curvature in the ultraviolet have just been
determined in the authors' laboratory and are shown in Fig. 13 (135b).
Since the relation between curvature and light dosage in Fhycomyces
is not a simple one (cf. Sections IV, B and IV, C) the action spectra
were determined with continuous light, using a balance method against
a standard blue light of low intensity. The correspondence between the
spectrum in the visible and that for the tip reaction of Avena coleoptiles is striking: the peaks at 445, 470, and 370 m/x compare very closely
with 445, 474, and 370 listed in Table VI, and the small shoulder
around 430 is probably present too. It may be tentatively concluded not
only that the photoreceptor in Fhycomyces is a carotenoid but that it is
probably the same carotenoid as in Avena. Since the two plants are so
widely separated systematically, this hints at a limitation in the number
of effective photoreceptors in plants.
In the blue-green alga Tolypothrix, the erect aerial trichomes curve
towards the light, though insensitive to gravity (74). The curvature is
very slow, 72 hours being required for an experiment. However, by
using continuous illumination at relatively low intensities with the active
light, and simultaneous illumination from above with phototropically
inactive orange light, the culture was kept healthy and an action
spectrum could be determined (74). It has a peak at about 450 m/x and
a shoulder at 470-480 m/x. At 500 m/x and above there is only very slight
response. The organism contained (besides chlorophyll a and phycocyanin) a carotenoid believed to be myxoxanthin, with a single broad
band at 470 m/x, and ^-carotene, which was purified. The peaks of the
latter at 450 and about 480 m/x, with a faint shoulder at 420 m/x, were
considered (74) to identify it as the photoreceptor for this alga.
VII. Mechanisms
A. VARIATION OF CURVATURE WITH DOSE
We have seen that analysis of curvature and light-growth reactions
with respect to the kind of response, time course, sensitivity distribution
KENNETH V. THIMANN AND GEORGE M. CURRY
For Fhycomyces, Blaauw (32) found a single peak near 495 m/x,
but his filter combinations only permitted examination of a very few
limited spectral regions. Castle (52), also using only a few spectral
regions, found high sensitivity around 425 m/x and almost none at
525 m/x. A carotenoid is present in the sporangiophores (Castle, 52,
and subsequent workers) and indeed is best detected in the growing
zone a few mm. below the sporangium (135a). A detailed action spectrum for the positive phototropic curvature in visible light, and a less
detailed one for the negative curvature in the ultraviolet have just been
determined in the authors' laboratory and are shown in Fig. 13 (135b).
Since the relation between curvature and light dosage in Fhycomyces
is not a simple one (cf. Sections IV, B and IV, C) the action spectra
were determined with continuous light, using a balance method against
a standard blue light of low intensity. The correspondence between the
spectrum in the visible and that for the tip reaction of Avena coleoptiles is striking: the peaks at 445, 470, and 370 m/x compare very closely
with 445, 474, and 370 listed in Table VI, and the small shoulder
around 430 is probably present too. It may be tentatively concluded not
only that the photoreceptor in Fhycomyces is a carotenoid but that it is
probably the same carotenoid as in Avena. Since the two plants are so
widely separated systematically, this hints at a limitation in the number
of effective photoreceptors in plants.
In the blue-green alga Tolypothrix, the erect aerial trichomes curve
towards the light, though insensitive to gravity (74). The curvature is
very slow, 72 hours being required for an experiment. However, by
using continuous illumination at relatively low intensities with the active
light, and simultaneous illumination from above with phototropically
inactive orange light, the culture was kept healthy and an action
spectrum could be determined (74). It has a peak at about 450 m/x and
a shoulder at 470-480 m/x. At 500 m/x and above there is only very slight
response. The organism contained (besides chlorophyll a and phycocyanin) a carotenoid believed to be myxoxanthin, with a single broad
band at 470 m/x, and ^-carotene, which was purified. The peaks of the
latter at 450 and about 480 m/x, with a faint shoulder at 420 m/x, were
considered (74) to identify it as the photoreceptor for this alga.
VII. Mechanisms
A. VARIATION OF CURVATURE WITH DOSE
We have seen that analysis of curvature and light-growth reactions
with respect to the kind of response, time course, sensitivity distribution