6. PHOTOTROPISM AND PHOTOTAXIS
271
the reacting protist." Approximate measurements with the blue-green
alga Oscillatoria, which, however, has a gliding motion and is not propelled by flagella, also indicate that the light absorbed by chlorophyll
is phototactically active (80).
A different type of phototaxis is that shown by motile green algae
such as Euglena and Eudorina, or the zoospores of algae or ferns. These
organisms do not merely collect in a light spot, but actually swim in the
direction of the light. As described in Section II, such directional phototaxis (called by some authors "topo-phototaxis") can be positive or
negative; the positive movement commonly takes place at much lower
light intensities than the negative; both are more light-sensitive than the
"shock response" of Rhodospirillum. Indeed Manten (74) concludes that
the phototactic threshold for Rhodospirillum lies at about 10
4 times
higher light intensity than the threshold for the green alga Eudorina.
Positive phototaxis of Chlamydomonas occurs equally well in the absence of C0 2 (80a) and is thus not directly related to photosynthesis.
According to earlier measurements made mostly by Mast (81, 82)
the peak of sensitivity for positive phototaxis of green algae lies always
in the blue or blue-green; 524 m/x for Pandorina and Eudorina, 503 m/x
for Chlamydomonas, and 494 m/x for Volvox — all three in the Volvocales; Mast also found 473-483 m/x for five species of Euglena. However, Luntz (83) found the peak for Eudorina elegans and Volvox minor
to be at 492 m/x, and Laurens and Hooker (84) using a method based
on exposure length rather than light intensity, found the same wavelength optimal for Volvox globator. Oltmanns (85) in a careful study
found that Euglena shows three peaks, the main one at 480-495 m/x, and
secondary peaks at 428 and 405 m/x.
Recent studies in general have agreed well with the above. Biinning
and Schneiderhöhn (86) found Euglena to show four peaks, at about
425, 450, 475, and 495 m/x, two of which agree rather well with Oltmanns' data. However, Halldal very recently (3) has found a single
peak at 493 m/x for five of the Volvocales with a shoulder at about 435
m/x. Gametes of two Ulva species gave a two-peaked or flat-topped curve
with sensitivity almost equal from 440 to 490 m/x, while three members
of the Dinophyceae each gave a single rather sharp peak, which for
Peridinium and Gonyaulax lay at 475 m/x, and for Prorooentrum (an unarmored form) at the surprising wavelength of 570 m/x. Recently
Chlamydomonas has been found to respond like Ulva, showing little
change in sensitivity between 465 and 500 m/x; the sensitivity falls off
sharply on either side of this range, and reaches zero at 440 and 520
m/x (80a). Halldal's action spectra are reproduced in Fig. 7.
Data for negative phototaxis are few; in Halldal's Volvocales the
271
the reacting protist." Approximate measurements with the blue-green
alga Oscillatoria, which, however, has a gliding motion and is not propelled by flagella, also indicate that the light absorbed by chlorophyll
is phototactically active (80).
A different type of phototaxis is that shown by motile green algae
such as Euglena and Eudorina, or the zoospores of algae or ferns. These
organisms do not merely collect in a light spot, but actually swim in the
direction of the light. As described in Section II, such directional phototaxis (called by some authors "topo-phototaxis") can be positive or
negative; the positive movement commonly takes place at much lower
light intensities than the negative; both are more light-sensitive than the
"shock response" of Rhodospirillum. Indeed Manten (74) concludes that
the phototactic threshold for Rhodospirillum lies at about 10
4 times
higher light intensity than the threshold for the green alga Eudorina.
Positive phototaxis of Chlamydomonas occurs equally well in the absence of C0 2 (80a) and is thus not directly related to photosynthesis.
According to earlier measurements made mostly by Mast (81, 82)
the peak of sensitivity for positive phototaxis of green algae lies always
in the blue or blue-green; 524 m/x for Pandorina and Eudorina, 503 m/x
for Chlamydomonas, and 494 m/x for Volvox — all three in the Volvocales; Mast also found 473-483 m/x for five species of Euglena. However, Luntz (83) found the peak for Eudorina elegans and Volvox minor
to be at 492 m/x, and Laurens and Hooker (84) using a method based
on exposure length rather than light intensity, found the same wavelength optimal for Volvox globator. Oltmanns (85) in a careful study
found that Euglena shows three peaks, the main one at 480-495 m/x, and
secondary peaks at 428 and 405 m/x.
Recent studies in general have agreed well with the above. Biinning
and Schneiderhöhn (86) found Euglena to show four peaks, at about
425, 450, 475, and 495 m/x, two of which agree rather well with Oltmanns' data. However, Halldal very recently (3) has found a single
peak at 493 m/x for five of the Volvocales with a shoulder at about 435
m/x. Gametes of two Ulva species gave a two-peaked or flat-topped curve
with sensitivity almost equal from 440 to 490 m/x, while three members
of the Dinophyceae each gave a single rather sharp peak, which for
Peridinium and Gonyaulax lay at 475 m/x, and for Prorooentrum (an unarmored form) at the surprising wavelength of 570 m/x. Recently
Chlamydomonas has been found to respond like Ulva, showing little
change in sensitivity between 465 and 500 m/x; the sensitivity falls off
sharply on either side of this range, and reaches zero at 440 and 520
m/x (80a). Halldal's action spectra are reproduced in Fig. 7.
Data for negative phototaxis are few; in Halldal's Volvocales the