256
KENNETH V. THIMANN AND GEORGE M. CURRY
for curvature, as any y 2 mm. region more than 1.5 mm. down from the
apex. Since the apical y 2 mm. grows very little and the curvature caused
by light reception in this zone develops in more actively growing regions
far below, some kind of stimulus transmission is indicated; alternatively,
then, the "tip response" could be called a "transmitted response." Historically indeed it was the transmission of the response from the illuminated tip to the unilluminated part below that led to the discovery
of the role of auxin in growth and tropisms.
In contrast, prolonged high-intensity visible light or shorter exposure
to ultraviolet causes growth changes and curvatures that are not far
removed from the site of light reception. The curvatures toward ultraviolet light (wavelength less than 320 τημ) are quite similar to the "base
responses" discussed previously, both in their final appearance and in
the time course of their development (Fig. 1, b). With light of wavelength 280 τημ, large base curvatures are produced free from the complicating factor of simultaneous tip curvatures, and thus more amenable
to analysis (see Section VII, B below). The most sensitive zones for the
base response are the actively growing regions just below the tip and
around the node at the base of the coleoptile. The convenient term
"base response" is a little misleading because the response actually occurs all over the coleoptile. However, even the extreme tip can give
such responses (36, 39), though the lower regions are more sensitive.
The main point is that the base response is localized, in the sense that
the curvature takes place at or near the site of light reception.
If the tip is covered during unilateral exposure to white light, curvatures are not observed 90 minutes later unless light doses are given
which fall in the range of the second positive curvature, i.e., above 5000
ergs/cm.
2 given over a period of 4 minutes or more. This suggests that
the first positive and negative curvatures are attributable to the tip response, while the second positive curvatures arise largely from the base
response. This view is confirmed by the different shapes of the curvatures in the two cases (Fig. 1). With continuous, low-intensity exposures
of the whole coleoptile like those of the early experiments discussed
above, it is impossible to decide which reaction accounts for the large
curvatures observed; probably both mechanisms operate under these
conditions (see curve I x in Fig. 3).
To produce the first observable or "threshold" curvatures Blaauw
found that a definite quantity of energy was required (the Reizmengengesetz). Thus, either a 1/1000 second exposure to an intensity of
26,520 meter-candles [26.5 meter-candle-seconds (MCS)] or a 40 hour
exposure to 0.00017 meter-candles (26.3 MCS) caused a slight curvature
toward the light in about half the Avena coleoptiles observed. Blaauw's
KENNETH V. THIMANN AND GEORGE M. CURRY
for curvature, as any y 2 mm. region more than 1.5 mm. down from the
apex. Since the apical y 2 mm. grows very little and the curvature caused
by light reception in this zone develops in more actively growing regions
far below, some kind of stimulus transmission is indicated; alternatively,
then, the "tip response" could be called a "transmitted response." Historically indeed it was the transmission of the response from the illuminated tip to the unilluminated part below that led to the discovery
of the role of auxin in growth and tropisms.
In contrast, prolonged high-intensity visible light or shorter exposure
to ultraviolet causes growth changes and curvatures that are not far
removed from the site of light reception. The curvatures toward ultraviolet light (wavelength less than 320 τημ) are quite similar to the "base
responses" discussed previously, both in their final appearance and in
the time course of their development (Fig. 1, b). With light of wavelength 280 τημ, large base curvatures are produced free from the complicating factor of simultaneous tip curvatures, and thus more amenable
to analysis (see Section VII, B below). The most sensitive zones for the
base response are the actively growing regions just below the tip and
around the node at the base of the coleoptile. The convenient term
"base response" is a little misleading because the response actually occurs all over the coleoptile. However, even the extreme tip can give
such responses (36, 39), though the lower regions are more sensitive.
The main point is that the base response is localized, in the sense that
the curvature takes place at or near the site of light reception.
If the tip is covered during unilateral exposure to white light, curvatures are not observed 90 minutes later unless light doses are given
which fall in the range of the second positive curvature, i.e., above 5000
ergs/cm.
2 given over a period of 4 minutes or more. This suggests that
the first positive and negative curvatures are attributable to the tip response, while the second positive curvatures arise largely from the base
response. This view is confirmed by the different shapes of the curvatures in the two cases (Fig. 1). With continuous, low-intensity exposures
of the whole coleoptile like those of the early experiments discussed
above, it is impossible to decide which reaction accounts for the large
curvatures observed; probably both mechanisms operate under these
conditions (see curve I x in Fig. 3).
To produce the first observable or "threshold" curvatures Blaauw
found that a definite quantity of energy was required (the Reizmengengesetz). Thus, either a 1/1000 second exposure to an intensity of
26,520 meter-candles [26.5 meter-candle-seconds (MCS)] or a 40 hour
exposure to 0.00017 meter-candles (26.3 MCS) caused a slight curvature
toward the light in about half the Avena coleoptiles observed. Blaauw's