264
KENNETH V. THIMANN AND GEORGE M. CURRY
are summarized in Table V. They bear out the general conclusion of
the work with Avena coleoptiles that curvature towards light is associated with a decreased auxin level on the lighted side and an increased
auxin level on the dark side.
In these experiments the auxin was usually bio-assayed by placing
the tip of the coleoptile or other seedling in contact with a block of
agar, which was so divided that the yields of auxin from the bright side
and from the shaded side were received in separate blocks. Each agar
block was then applied to one side of a standard decapitated Avena
coleoptile and the curvature resulting after 110 minutes measured. It is
of interest, however, that when the auxin was obtained by slitting the
seedling lengthwise and extracting the two halves with solvent (with
subsequent bio-assay on Avena coleoptiles) the same auxin differential
could be established (20, 59) (see Table V).
If the auxin differential, in a positive curvature, genuinely comprises both a decrease on the light side and an increase on the dark side,
it would follow that the auxin must have moved laterally, under the influence of light, from front to back. Recently, however, it has been suggested a number of times that the effect of light in causing curvature is
exerted through the destruction of auxin on the light side. This would
of course mean only a decrease of auxin, and therefore of growth, on
the light side, with no corresponding increase on the dark side. In
Blaauw's terms it would mean a negative light-growth reaction on the
light side only. For a biochemical interpretation it is a crucial question
whether the difference in auxin content on the two sides is due to
destruction on the lighted side or not.
At this point it is therefore worthwhile to review the evidence which
specifically indicates lateral movement of auxin, as opposed to destruction on one side. The first experiment to give such evidence was that of
Boysen-Jensen and Nielsen (60) and Boysen-Jensen (61) in which a
coleoptile was split at the tip by the insertion of a thin sliver of mica.
When the plant was illuminated perpendicular to the plane of the mica,
the curvature was greatly reduced. Controls illuminated parallel to the
plane of the mica curved normally, thus eliminating the effect of slitting injury. These experiments were carried out using prolonged light
exposures, and must be repeated using about 100 ergs/cm.
2 to bring
the curvature down into the first positive range. An opposite indication
comes from simultaneous measurements of growth rate and development
of curvature, from which duBuy and Nuernbergk (62) calculated that
the slowing down of growth on the lighted side was almost complete,
and much greater than the acceleration on the dark side.
The work based on auxin determination was at first not quite con-
KENNETH V. THIMANN AND GEORGE M. CURRY
are summarized in Table V. They bear out the general conclusion of
the work with Avena coleoptiles that curvature towards light is associated with a decreased auxin level on the lighted side and an increased
auxin level on the dark side.
In these experiments the auxin was usually bio-assayed by placing
the tip of the coleoptile or other seedling in contact with a block of
agar, which was so divided that the yields of auxin from the bright side
and from the shaded side were received in separate blocks. Each agar
block was then applied to one side of a standard decapitated Avena
coleoptile and the curvature resulting after 110 minutes measured. It is
of interest, however, that when the auxin was obtained by slitting the
seedling lengthwise and extracting the two halves with solvent (with
subsequent bio-assay on Avena coleoptiles) the same auxin differential
could be established (20, 59) (see Table V).
If the auxin differential, in a positive curvature, genuinely comprises both a decrease on the light side and an increase on the dark side,
it would follow that the auxin must have moved laterally, under the influence of light, from front to back. Recently, however, it has been suggested a number of times that the effect of light in causing curvature is
exerted through the destruction of auxin on the light side. This would
of course mean only a decrease of auxin, and therefore of growth, on
the light side, with no corresponding increase on the dark side. In
Blaauw's terms it would mean a negative light-growth reaction on the
light side only. For a biochemical interpretation it is a crucial question
whether the difference in auxin content on the two sides is due to
destruction on the lighted side or not.
At this point it is therefore worthwhile to review the evidence which
specifically indicates lateral movement of auxin, as opposed to destruction on one side. The first experiment to give such evidence was that of
Boysen-Jensen and Nielsen (60) and Boysen-Jensen (61) in which a
coleoptile was split at the tip by the insertion of a thin sliver of mica.
When the plant was illuminated perpendicular to the plane of the mica,
the curvature was greatly reduced. Controls illuminated parallel to the
plane of the mica curved normally, thus eliminating the effect of slitting injury. These experiments were carried out using prolonged light
exposures, and must be repeated using about 100 ergs/cm.
2 to bring
the curvature down into the first positive range. An opposite indication
comes from simultaneous measurements of growth rate and development
of curvature, from which duBuy and Nuernbergk (62) calculated that
the slowing down of growth on the lighted side was almost complete,
and much greater than the acceleration on the dark side.
The work based on auxin determination was at first not quite con-