302
KENNETH V. THIMANN AND GEORGE M. CURRY
redistribution of the processes consuming normal metabolic energy in
growth.
In connection with the idea of a light-induced lateral transport of
auxin, Went (160) calculated that 0.001 erg is required for a lateral
transport of auxin in the tip sufficient to give curvature (10~
13 moles of
auxin displaced 1 mm.). He says (22) "the minimal amount of light
energy required for such a curvature is about 1 erg. This fact makes it
possible to view the induction of phototropic curvature as a normal
physicochemical reaction, for which the energy is supplied by the exciting light" (italics ours). There can be no doubt that light initiates
the process by a normal photochemical reaction, but the above considerations indicate that after the induction the energy for curvature development comes from the normal growth processes.
A simple way of visualizing this induction would be to assume that
the absorbed light acts photoelectrically to give a potential across the
tip and thus to cause migration of auxin ions. Schrank (161) observed
transverse electrical polarities of the order of 10 to 20 millivolts induced
by 200 MCS unilateral illumination, giving some basis for the photopotential notion. According to Koller (162) the photoelectric process is
rarely more than 1% efficient, but if we assume for the moment that each
quantum absorbed gives one electron, then, since 1 erg of 450 τημ light
contains approximately 2.2 X 10
11 quanta, 1 erg absorbed would lead to
the emission of 2.2 X 10
11 electrons. In the most efficient possible process, then, this effect could lead to the displacement of 2.2 X 10
11 molecules of auxin. It is interesting that this approaches the required magnitude (^ΙΟ
11 molecules; cf. pp. 266-267), but it must be noted that this
assumes 100% efficiency, the absorption of 1 erg, rather than the 0.01
ergs actually required, and the participation only of the ions of indoleacetic acid and not of those of any other acid in the cells. Taking these
factors into consideration, a more realistic estimate could not exceed the
displacement of 10
7 molecules of auxin. In any event both this case and
Went's interpretation still invoke the lateral displacement of auxin,
whose subsequent action, as we have already noted, is catalytic.
In connection with the photoelectric effect it is interesting to note
Schrank's (163) finding that direct current of 5 to 20 microamperes applied transversely for 2 minutes to the 5 mm. apical segment of the
Avena coleoptile induces bending toward the positive pole. The charge
transferred here is of the order of 10~
3 coulombs or (since e = 1.6 X 10~
19
coulombs) roughly 6 X 10
15 electrons, which is several orders of magnitude greater than the numbers of electrons discussed above.
A different comparison of the energy magnitudes involved may be
derived from studies originated by Heyn (164) and by Söding (165) on
KENNETH V. THIMANN AND GEORGE M. CURRY
redistribution of the processes consuming normal metabolic energy in
growth.
In connection with the idea of a light-induced lateral transport of
auxin, Went (160) calculated that 0.001 erg is required for a lateral
transport of auxin in the tip sufficient to give curvature (10~
13 moles of
auxin displaced 1 mm.). He says (22) "the minimal amount of light
energy required for such a curvature is about 1 erg. This fact makes it
possible to view the induction of phototropic curvature as a normal
physicochemical reaction, for which the energy is supplied by the exciting light" (italics ours). There can be no doubt that light initiates
the process by a normal photochemical reaction, but the above considerations indicate that after the induction the energy for curvature development comes from the normal growth processes.
A simple way of visualizing this induction would be to assume that
the absorbed light acts photoelectrically to give a potential across the
tip and thus to cause migration of auxin ions. Schrank (161) observed
transverse electrical polarities of the order of 10 to 20 millivolts induced
by 200 MCS unilateral illumination, giving some basis for the photopotential notion. According to Koller (162) the photoelectric process is
rarely more than 1% efficient, but if we assume for the moment that each
quantum absorbed gives one electron, then, since 1 erg of 450 τημ light
contains approximately 2.2 X 10
11 quanta, 1 erg absorbed would lead to
the emission of 2.2 X 10
11 electrons. In the most efficient possible process, then, this effect could lead to the displacement of 2.2 X 10
11 molecules of auxin. It is interesting that this approaches the required magnitude (^ΙΟ
11 molecules; cf. pp. 266-267), but it must be noted that this
assumes 100% efficiency, the absorption of 1 erg, rather than the 0.01
ergs actually required, and the participation only of the ions of indoleacetic acid and not of those of any other acid in the cells. Taking these
factors into consideration, a more realistic estimate could not exceed the
displacement of 10
7 molecules of auxin. In any event both this case and
Went's interpretation still invoke the lateral displacement of auxin,
whose subsequent action, as we have already noted, is catalytic.
In connection with the photoelectric effect it is interesting to note
Schrank's (163) finding that direct current of 5 to 20 microamperes applied transversely for 2 minutes to the 5 mm. apical segment of the
Avena coleoptile induces bending toward the positive pole. The charge
transferred here is of the order of 10~
3 coulombs or (since e = 1.6 X 10~
19
coulombs) roughly 6 X 10
15 electrons, which is several orders of magnitude greater than the numbers of electrons discussed above.
A different comparison of the energy magnitudes involved may be
derived from studies originated by Heyn (164) and by Söding (165) on