6. PHOTOTROPISM AND PHOTOTAXIS
247
zoospores of green algae, as well as the motile algae such as the unicellular Chlamydomonas and the colonial Volvocales, behave in the
same way (see Section VI, A). The ecological result of this behavior
is that small aquatic organisms often assemble near the edge of a region
of shadow, e.g., the small crustacean Caprella (J). The intensity at
which change of sign occurs has seldom been precisely determined;
with Allolobophora in continuous white light it is at about 0.012 metercandles (9). However, there is often marked variation in sensitivity
within a population, and some Balanus nauplii may remain positive at
intensities high enough to make most of the others negative. Furthermore, with Balanus, as also with the alga Volvox and doubtless with
other organisms, the positively phototactic individuals become gradually
negative on continuing exposure, as if the product of the light reaction
were accumulating. This invites comparison with the proposed explanation for negative phototropic curvatures in Avena (see Section VII, C)
based on irreversible bleaching of the photoreceptor. There can also be
adaptive changes, light-adapted organisms becoming less sensitive. Mast
(10) even found that light-adapted colonies of Volvox or Pandorina are
sometimes positive in high intensities and negative in low ones.
(2) Influence of Salts and Temperature. The phototactic reaction,
and particularly the change of sign, is influenced by several factors
other than light. In marine copepods, annelid larvae, etc., dilution of
the sea water makes positive animals become negative, and negative
animals more strongly negative. Correspondingly, concentrating the sea
water (by evaporation) tends to weaken positive reactions and
strengthen negative ones. Individual salts have a host of effects (see
Rose 6). Isotonic NaCl stops the reaction altogether. C0 2 is also very
active; negatively phototactic Gammarus pulex, as well as Volvox, is
made positive by dissolved C0 2 . This effect is probably due to pH
change, since a variety of acids have a similar effect (10). The individual constituents of sea water, especially Ca and Mg, are very effective
in determining reversal of sign (10a). Temperature is also effective;
Loeb has described a number of cases in which, with the same light
intensity, phototaxis is positive at low temperature, negative at high.
These include Daphnia, Limulus, Artemia, Temoria, and Polygordius.
However, the algae Hematococcus, Volvox, and Pandorina change in
the opposite direction. There is some evidence also, with Balanus, that
the rate at which sign-reversal takes place due to increased light intensity is itself temperature-sensitive.
(3) Bunsen-Roscoe Law or Product Law. One of the most frequent
subjects of study has been the relation between the movement and the
total light dosage, i.e., the product of the intensity and the duration of
247
zoospores of green algae, as well as the motile algae such as the unicellular Chlamydomonas and the colonial Volvocales, behave in the
same way (see Section VI, A). The ecological result of this behavior
is that small aquatic organisms often assemble near the edge of a region
of shadow, e.g., the small crustacean Caprella (J). The intensity at
which change of sign occurs has seldom been precisely determined;
with Allolobophora in continuous white light it is at about 0.012 metercandles (9). However, there is often marked variation in sensitivity
within a population, and some Balanus nauplii may remain positive at
intensities high enough to make most of the others negative. Furthermore, with Balanus, as also with the alga Volvox and doubtless with
other organisms, the positively phototactic individuals become gradually
negative on continuing exposure, as if the product of the light reaction
were accumulating. This invites comparison with the proposed explanation for negative phototropic curvatures in Avena (see Section VII, C)
based on irreversible bleaching of the photoreceptor. There can also be
adaptive changes, light-adapted organisms becoming less sensitive. Mast
(10) even found that light-adapted colonies of Volvox or Pandorina are
sometimes positive in high intensities and negative in low ones.
(2) Influence of Salts and Temperature. The phototactic reaction,
and particularly the change of sign, is influenced by several factors
other than light. In marine copepods, annelid larvae, etc., dilution of
the sea water makes positive animals become negative, and negative
animals more strongly negative. Correspondingly, concentrating the sea
water (by evaporation) tends to weaken positive reactions and
strengthen negative ones. Individual salts have a host of effects (see
Rose 6). Isotonic NaCl stops the reaction altogether. C0 2 is also very
active; negatively phototactic Gammarus pulex, as well as Volvox, is
made positive by dissolved C0 2 . This effect is probably due to pH
change, since a variety of acids have a similar effect (10). The individual constituents of sea water, especially Ca and Mg, are very effective
in determining reversal of sign (10a). Temperature is also effective;
Loeb has described a number of cases in which, with the same light
intensity, phototaxis is positive at low temperature, negative at high.
These include Daphnia, Limulus, Artemia, Temoria, and Polygordius.
However, the algae Hematococcus, Volvox, and Pandorina change in
the opposite direction. There is some evidence also, with Balanus, that
the rate at which sign-reversal takes place due to increased light intensity is itself temperature-sensitive.
(3) Bunsen-Roscoe Law or Product Law. One of the most frequent
subjects of study has been the relation between the movement and the
total light dosage, i.e., the product of the intensity and the duration of