260
KENNETH V. THIMANN AND GEORGE M. CURRY
distributed over the whole growth zone, i.e., from about 0.2 to 3 mm.
below the sporangium. The growth rate of each part of the growth
zone was increased by the same factor, which in one case was 2.43.
The light exposure used was 180 ft.-candles white light for 20 seconds,
or about 30,000 MCS. A comparable, though still more transient, increase in growth rate occurs when the plants are briefly exposed to
ultraviolet (43).
Thus, the negative curvatures which are caused by ultraviolet can
be ascribed to an acceleration of growth (positive light-growth reaction)
on the front side. With visible light, however, the paradox is that the
curvatures are positive although the growth rate is accelerated. To account for this, Blaauw (46), Buder (47), Castle (48), and others have
assumed that one-sided light somehow exerts a greater net growthaccelerating effect on the back side of the plant than on the front, and
have explained this as due to the lens-like action of the clear cylindrical
cell, which produces a bright stripe of high light intensity in the center
of the back wall and thus accelerates growth there more than anywhere
else. Buder indeed showed that when the plants are immersed in mineral
oil, whose refractive index is high enough to prevent formation of the
bright stripe, continuous white light now causes a negative curvature.
An interesting parallel case was studied by Dassek (49) in the
rhizoids of the liverworts Marchantia and Lunularia. These clear, unicellular, hair-like outgrowths are negatively phototropic and exhibit a
negative light-growth reaction. Since they are so transparent the explanation given for Phycomyces was invoked. Furthermore, immersion in
liquid paraffin caused the curvature to become positive. An exactly
similar situation holds in the germinating mycelia of Puccinia and
Uromyces (50) which also show negative phototropism, a negative
light-growth reaction, and reversal of the tropism in paraffin oil. Unfortunately the support to the lens interpretation offered by the experiments in paraffin oil is weakened by Ziegler's finding (51) that a similar
reversal of sign of curvature takes place when Avena is immersed in oil.
The coleoptile, of course, is multicellular and relatively opaque.
The intensity requirements, time courses, and magnitudes of the
light-growth reactions of Phycomyces match those of the short exposure
(transient) curvatures, in accord with Blaauw's theory. The same is
true for exposure to ultraviolet. However, in continuous light we encounter another and more serious paradox. The light-growth reaction
in symmetrically illuminated sporangiophores comes to an end after
about ten minutes, a fact which is ascribed by Delbriick and Reichardt
(45) to the onset of "adaptation" (see A in Fig. 5). The level of adaptation, A, is defined as the intensity with which the specimen will find
KENNETH V. THIMANN AND GEORGE M. CURRY
distributed over the whole growth zone, i.e., from about 0.2 to 3 mm.
below the sporangium. The growth rate of each part of the growth
zone was increased by the same factor, which in one case was 2.43.
The light exposure used was 180 ft.-candles white light for 20 seconds,
or about 30,000 MCS. A comparable, though still more transient, increase in growth rate occurs when the plants are briefly exposed to
ultraviolet (43).
Thus, the negative curvatures which are caused by ultraviolet can
be ascribed to an acceleration of growth (positive light-growth reaction)
on the front side. With visible light, however, the paradox is that the
curvatures are positive although the growth rate is accelerated. To account for this, Blaauw (46), Buder (47), Castle (48), and others have
assumed that one-sided light somehow exerts a greater net growthaccelerating effect on the back side of the plant than on the front, and
have explained this as due to the lens-like action of the clear cylindrical
cell, which produces a bright stripe of high light intensity in the center
of the back wall and thus accelerates growth there more than anywhere
else. Buder indeed showed that when the plants are immersed in mineral
oil, whose refractive index is high enough to prevent formation of the
bright stripe, continuous white light now causes a negative curvature.
An interesting parallel case was studied by Dassek (49) in the
rhizoids of the liverworts Marchantia and Lunularia. These clear, unicellular, hair-like outgrowths are negatively phototropic and exhibit a
negative light-growth reaction. Since they are so transparent the explanation given for Phycomyces was invoked. Furthermore, immersion in
liquid paraffin caused the curvature to become positive. An exactly
similar situation holds in the germinating mycelia of Puccinia and
Uromyces (50) which also show negative phototropism, a negative
light-growth reaction, and reversal of the tropism in paraffin oil. Unfortunately the support to the lens interpretation offered by the experiments in paraffin oil is weakened by Ziegler's finding (51) that a similar
reversal of sign of curvature takes place when Avena is immersed in oil.
The coleoptile, of course, is multicellular and relatively opaque.
The intensity requirements, time courses, and magnitudes of the
light-growth reactions of Phycomyces match those of the short exposure
(transient) curvatures, in accord with Blaauw's theory. The same is
true for exposure to ultraviolet. However, in continuous light we encounter another and more serious paradox. The light-growth reaction
in symmetrically illuminated sporangiophores comes to an end after
about ten minutes, a fact which is ascribed by Delbriick and Reichardt
(45) to the onset of "adaptation" (see A in Fig. 5). The level of adaptation, A, is defined as the intensity with which the specimen will find