294
KENNETH V. THIMANN AND GEORGE M. CURRY
along the plant, and wavelength sensitivity indicate two phototropic
mechanisms in Avena. With short exposures the reactions depend, over
a range of several log units, on the amount of energy applied (I X t),
suggesting that the response is initiated by a simple photochemical
change in some light-absorbing substance in the plant. With supraoptimal light doses the response decreases again (Fig. 2), which may be
due to a light-saturation phenomenon; i.e., if the photochemical action
on the exposed side of the plant is maximal at a certain dose, then increased amounts of light will only serve to push the reaction to completion on the shaded side, thus decreasing the gradient of photochemical action and its resultant growth rate redistribution. This can
explain the falling off of the response curve, but not the production of
negative or second positive curvatures.
In Phycomyces there is a very similar light-saturation effect, as
described by Castle (41), the result of which is that virtually no curvature takes place at intensities above 1800 MC, if the exposures are
longer than 0.6 second. However, this saturation does not account for
the large positive curvatures which can take place in continuous light.
The discrepancy between these and the light-growth reaction was
pointed out on pp. 259^261. The phenomena certainly suggest that
there may be a second mechanism accounting for phototropism at high
light dosages and long exposures in Phycomyces. It was pointed out
earlier, however (see Section IV, B), that in the light-growth reactions
of Phycomyces, Delbrück and Reichart (45) have shown the existence
of a rapid adaptation phenomenon which, if it occurs in phototropism
also, may complicate the interpretation considerably, particularly in
regard to curvatures in continuous light (32, 136).
Blaauw (32) believed that two opposing light reactions operate in
the phototropic system, both in Avena and in Phycomyces. The first
would begin to operate at low doses, the counteraction at high doses.
The failure of the reciprocity law at high doses could be attributed to
interactions between the two. This idea can account for negative curvatures, but a third mechanism must then be invoked to explain the second
positive curvatures. This concept is of historical interest as one of the
first interpretations of phototropism invoking a dual mechanism.
B. BASE RESPONSE
The important features of the base response are: (a) the growing
regions are the most sensitive, (b) the reaction appears near the site
of light reception, (c) the reaction is relatively quick, and (d) a recovery process occurs, i.e., the curvatures (and the light-growth reactions) are transient. The response to short wavelength ultraviolet pre-
KENNETH V. THIMANN AND GEORGE M. CURRY
along the plant, and wavelength sensitivity indicate two phototropic
mechanisms in Avena. With short exposures the reactions depend, over
a range of several log units, on the amount of energy applied (I X t),
suggesting that the response is initiated by a simple photochemical
change in some light-absorbing substance in the plant. With supraoptimal light doses the response decreases again (Fig. 2), which may be
due to a light-saturation phenomenon; i.e., if the photochemical action
on the exposed side of the plant is maximal at a certain dose, then increased amounts of light will only serve to push the reaction to completion on the shaded side, thus decreasing the gradient of photochemical action and its resultant growth rate redistribution. This can
explain the falling off of the response curve, but not the production of
negative or second positive curvatures.
In Phycomyces there is a very similar light-saturation effect, as
described by Castle (41), the result of which is that virtually no curvature takes place at intensities above 1800 MC, if the exposures are
longer than 0.6 second. However, this saturation does not account for
the large positive curvatures which can take place in continuous light.
The discrepancy between these and the light-growth reaction was
pointed out on pp. 259^261. The phenomena certainly suggest that
there may be a second mechanism accounting for phototropism at high
light dosages and long exposures in Phycomyces. It was pointed out
earlier, however (see Section IV, B), that in the light-growth reactions
of Phycomyces, Delbrück and Reichart (45) have shown the existence
of a rapid adaptation phenomenon which, if it occurs in phototropism
also, may complicate the interpretation considerably, particularly in
regard to curvatures in continuous light (32, 136).
Blaauw (32) believed that two opposing light reactions operate in
the phototropic system, both in Avena and in Phycomyces. The first
would begin to operate at low doses, the counteraction at high doses.
The failure of the reciprocity law at high doses could be attributed to
interactions between the two. This idea can account for negative curvatures, but a third mechanism must then be invoked to explain the second
positive curvatures. This concept is of historical interest as one of the
first interpretations of phototropism invoking a dual mechanism.
B. BASE RESPONSE
The important features of the base response are: (a) the growing
regions are the most sensitive, (b) the reaction appears near the site
of light reception, (c) the reaction is relatively quick, and (d) a recovery process occurs, i.e., the curvatures (and the light-growth reactions) are transient. The response to short wavelength ultraviolet pre-