274
KENNETH V. THIMANN AND GEORGE M. CURRY
kinetically sensitive, contain no visible stigma, and also that the stigma
may be located in quite different positions (relative to the flagellar base)
in different algal groups. A mutant of Chfomydomonas without stigma
was still phototactic, though less rapidly and less regularly so than the
wild type, and Hartshorne (88a) concluded that "the perception of
light in Chlamyfdomonas is a property not restricted to the eyespot."
For these reasons the connection of the stigma with phototactic
movement is very uncertain (cf. Halldal, 3). Gössel's finding (88b)
that for negative phototaxis of chlorophyll-free Euglena the action spectrum is the same in strains with and without a stigma shows indeed that
for the negative response the stigma is not necessary. Regardless of
whether or not the screening function is performed by the stigma, some
kind of screening is a likely mechanism for phototaxis. In this case it
stands to reason that if the photoreceptor and the screening pigment
absorb comparable fractions of the light, the action spectrum will not
represent the absorption spectrum of either. (This point is taken up
further in Section VI, C, 2.) In general, therefore, the possibility cannot
be overlooked that the action spectrum of phototaxis does not represent
a simple absorption spectrum but might be a complex of two, perhaps
both belonging to carotenoids.
The light-directed movements of chloroplasts in the leaves of mosses
and flowering plants have something in common with phototaxis. In
darkness the chloroplasts tend to be randomly distributed (apostrophe),
in weak light they concentrate on the upper surface (epistrophe) and
in intense light they withdraw to the side walls, or in the case of the
alga Mougeotia turn into the profile position, (both considered as
parastrophe). These movements, which have been known since the work
of Böhm in 1856, are quite slow but very temperature sensitive; for
example in Lemna trisulca the maximal profile position required 70
minutes at 10° and only 5 minutes at 30° (89). The effects of wavelength of light on these movements have been studied a number of times,
but have given contradictory results in the hands of different workers
and with different plant material. The most recent study, by Zurzycka,
(90) leads to the conclusion that both red and blue lights are effective
but in different ways; epistrophe occurs only in blue light while parastrophe (the profile position) is caused by both blue and red. It is concluded that the movement into or out of the profile position probably
depends upon photosynthesis, but the evidence is quite slender. In the
first place, NH 2 OH, which inhibits photosynthesis without affecting
respiration rate, stopped entirely the movements which started from the
profile arrangement (91). Secondly, there is a parallelism between the
light intensities causing the movement and those active in photosyn-
KENNETH V. THIMANN AND GEORGE M. CURRY
kinetically sensitive, contain no visible stigma, and also that the stigma
may be located in quite different positions (relative to the flagellar base)
in different algal groups. A mutant of Chfomydomonas without stigma
was still phototactic, though less rapidly and less regularly so than the
wild type, and Hartshorne (88a) concluded that "the perception of
light in Chlamyfdomonas is a property not restricted to the eyespot."
For these reasons the connection of the stigma with phototactic
movement is very uncertain (cf. Halldal, 3). Gössel's finding (88b)
that for negative phototaxis of chlorophyll-free Euglena the action spectrum is the same in strains with and without a stigma shows indeed that
for the negative response the stigma is not necessary. Regardless of
whether or not the screening function is performed by the stigma, some
kind of screening is a likely mechanism for phototaxis. In this case it
stands to reason that if the photoreceptor and the screening pigment
absorb comparable fractions of the light, the action spectrum will not
represent the absorption spectrum of either. (This point is taken up
further in Section VI, C, 2.) In general, therefore, the possibility cannot
be overlooked that the action spectrum of phototaxis does not represent
a simple absorption spectrum but might be a complex of two, perhaps
both belonging to carotenoids.
The light-directed movements of chloroplasts in the leaves of mosses
and flowering plants have something in common with phototaxis. In
darkness the chloroplasts tend to be randomly distributed (apostrophe),
in weak light they concentrate on the upper surface (epistrophe) and
in intense light they withdraw to the side walls, or in the case of the
alga Mougeotia turn into the profile position, (both considered as
parastrophe). These movements, which have been known since the work
of Böhm in 1856, are quite slow but very temperature sensitive; for
example in Lemna trisulca the maximal profile position required 70
minutes at 10° and only 5 minutes at 30° (89). The effects of wavelength of light on these movements have been studied a number of times,
but have given contradictory results in the hands of different workers
and with different plant material. The most recent study, by Zurzycka,
(90) leads to the conclusion that both red and blue lights are effective
but in different ways; epistrophe occurs only in blue light while parastrophe (the profile position) is caused by both blue and red. It is concluded that the movement into or out of the profile position probably
depends upon photosynthesis, but the evidence is quite slender. In the
first place, NH 2 OH, which inhibits photosynthesis without affecting
respiration rate, stopped entirely the movements which started from the
profile arrangement (91). Secondly, there is a parallelism between the
light intensities causing the movement and those active in photosyn-