6. PHOTOTROPISM AND PHOTOTAXIS
273
respiration, nerve action, etc., of various drugs and poisons, which stimulate in low concentrations and depress in high.
Lastly there must be mentioned Luntz' study (83) of the nonphotosynthetic Chilomonas, which is about 1000 times less sensitive to visible
light than the photosynthetic organisms studied, but had a peak of
sensitivity in the ultraviolet at 366 m^ (or lower).
The question now is, to what photoreceptors do these action spectra
point? It is clear, first of all, that the directional phototaxis of green
algae is not directly mediated by the photosynthetic system; in not a
single case is the red end of the spectrum photokinetically active. It is
virtually certain also that the photoreceptors are different in different
plants, though they may fall into two or three groups. The spectra show
a general resemblance, both in shape and location, to those of carotenoids, but the similarities are not sharp enough to justify identification
with any specific substances. Halldal (3) found that the peak of carotenoid absorption in the spectrum of living Volvocales was at about 440
m/x (see Fig. 7) while the phototactic peak at 493 τημ could correspond
only to a minor shoulder on the absorption spectrum. Indeed in Prorocentrum the phototactic peak actually falls in a trough of the absorption
spectrum. He concludes that "none of the major pigments present in the
algae is the photoreceptor." None of the water-soluble pigments, either,
showed absorption spectra corresponding to the measured action spectra.
It has been suggested that the absorption spectrum of an active carotenoid might well be modified by combination with a protein. The difficulty is that if this is invoked to explain the "flat-topped" action spectra
then some other explanation has to be used for the single sharp peaks.
It seems necessary to conclude that the photoreceptor is a pigment
present only in traces, probably but not necessarily a carotenoid, and
subject to one or more modifications in different species.
There is another possible explanation of the action spectra, namely
that they are actually difference spectra, representing the difference between the light absorption of the photoreceptor proper and that of a
screening pigment. A number of workers, especially Mast (87) have explained the phototactic stimulus as due to shading of a photoreceptor
spot by a screening pigment; the organism moves so as to minimize the
shading. (In negative phototaxis it would maximize the shading.) As a
rule the screening agent has been thought to be the stigma, an orangered spot not far from the base of the flagellum; the anatomy of the
stigma in Euglena has recently been described in detail (87a). The
stigma contains the carotenoid astaxanthin (88). This pigment has a
single absorption peak falling at 470-492 m^, depending upon the solvent. However, it must be noted that the Dinophyceae, which are photo-
273
respiration, nerve action, etc., of various drugs and poisons, which stimulate in low concentrations and depress in high.
Lastly there must be mentioned Luntz' study (83) of the nonphotosynthetic Chilomonas, which is about 1000 times less sensitive to visible
light than the photosynthetic organisms studied, but had a peak of
sensitivity in the ultraviolet at 366 m^ (or lower).
The question now is, to what photoreceptors do these action spectra
point? It is clear, first of all, that the directional phototaxis of green
algae is not directly mediated by the photosynthetic system; in not a
single case is the red end of the spectrum photokinetically active. It is
virtually certain also that the photoreceptors are different in different
plants, though they may fall into two or three groups. The spectra show
a general resemblance, both in shape and location, to those of carotenoids, but the similarities are not sharp enough to justify identification
with any specific substances. Halldal (3) found that the peak of carotenoid absorption in the spectrum of living Volvocales was at about 440
m/x (see Fig. 7) while the phototactic peak at 493 τημ could correspond
only to a minor shoulder on the absorption spectrum. Indeed in Prorocentrum the phototactic peak actually falls in a trough of the absorption
spectrum. He concludes that "none of the major pigments present in the
algae is the photoreceptor." None of the water-soluble pigments, either,
showed absorption spectra corresponding to the measured action spectra.
It has been suggested that the absorption spectrum of an active carotenoid might well be modified by combination with a protein. The difficulty is that if this is invoked to explain the "flat-topped" action spectra
then some other explanation has to be used for the single sharp peaks.
It seems necessary to conclude that the photoreceptor is a pigment
present only in traces, probably but not necessarily a carotenoid, and
subject to one or more modifications in different species.
There is another possible explanation of the action spectra, namely
that they are actually difference spectra, representing the difference between the light absorption of the photoreceptor proper and that of a
screening pigment. A number of workers, especially Mast (87) have explained the phototactic stimulus as due to shading of a photoreceptor
spot by a screening pigment; the organism moves so as to minimize the
shading. (In negative phototaxis it would maximize the shading.) As a
rule the screening agent has been thought to be the stigma, an orangered spot not far from the base of the flagellum; the anatomy of the
stigma in Euglena has recently been described in detail (87a). The
stigma contains the carotenoid astaxanthin (88). This pigment has a
single absorption peak falling at 470-492 m^, depending upon the solvent. However, it must be noted that the Dinophyceae, which are photo-