6. PHOTOTROPISM AND PHOTOTAXIS
303
the mechanical properties of bending seedlings. The later data of
Tagawa and Bonner (166) with the Avena coleoptile show that a 220
mg. weight suspended near one end of a 20 mm. section introduces a
permanent 22° bend in the section. In the course of the bending the
weight moves approximately 1 cm., so that the work done is at least
0.2 X 1 X 980 = 200 ergs. This mechanical bending may seem far removed from phototropic curvatures, but it must be recalled that auxin
is thought to act by modifying the mechanical properties of the growing
cell wall. Since at most only 1 or 2 ergs (approx. 200 erg/cm.
2 on 1
mm.
2 ) of absorbed light energy are needed to give curvatures of about
22°, it is apparent that the light could in no sense "push" or "pull" the
organism nor could it contribute sufficient energy in any photochemical
process to accomplish such a deformation.
So far our discussion has been concerned with light-induced spatial
redistributions of growth. The same conclusion, namely, that the light
energy itself does not provide the energy for growth redistribution, may
be drawn from an examination of a somewhat different effect, the lightgrowth reaction. In this case the almost doubled growth rate produced
by symmetrical light-stimulation of a Phycomyces sporangiophore is
compensated for by a subsequent fall below the normal rate (Fig. 5),
so that there is essentially a temporal redistribution of growth. Hence
Delbrück and Reichardt (45) conclude that "The stimulus does not
produce extra growth, it simply alters the distribution in time of the
growth that would have taken place during the same period in the
absence of the stimulus. The illumination program controls the rate of
utilization of materials available for growth, but does not alter the
quantity of this material."
Thus, although none of the evidence is perhaps wholly conclusive,
the totality of it points clearly to the use of a small amount of light
energy, both in tropism and in taxis, to channel a relatively large amount
of metabolic energy into a modification of growth or movement.
E. SUMMARY
If an attempt were to be made to summarize the large amount of
work carried out on such a variety of biological material one could only
point to the emergence of a very few general principles. These are: (1)
the fact that practically all organisms studied are to some extent lightsensitive; (2) the evident occurrence of more than one response to light
in each of the cases analyzed; (3) the high probability that in virtually
all organisms carotenoids participate in at least one of these responses;
(4) the almost completely unbridged gap between the reception of light
and its first biochemical effect, whether evidenced as change of growth,
303
the mechanical properties of bending seedlings. The later data of
Tagawa and Bonner (166) with the Avena coleoptile show that a 220
mg. weight suspended near one end of a 20 mm. section introduces a
permanent 22° bend in the section. In the course of the bending the
weight moves approximately 1 cm., so that the work done is at least
0.2 X 1 X 980 = 200 ergs. This mechanical bending may seem far removed from phototropic curvatures, but it must be recalled that auxin
is thought to act by modifying the mechanical properties of the growing
cell wall. Since at most only 1 or 2 ergs (approx. 200 erg/cm.
2 on 1
mm.
2 ) of absorbed light energy are needed to give curvatures of about
22°, it is apparent that the light could in no sense "push" or "pull" the
organism nor could it contribute sufficient energy in any photochemical
process to accomplish such a deformation.
So far our discussion has been concerned with light-induced spatial
redistributions of growth. The same conclusion, namely, that the light
energy itself does not provide the energy for growth redistribution, may
be drawn from an examination of a somewhat different effect, the lightgrowth reaction. In this case the almost doubled growth rate produced
by symmetrical light-stimulation of a Phycomyces sporangiophore is
compensated for by a subsequent fall below the normal rate (Fig. 5),
so that there is essentially a temporal redistribution of growth. Hence
Delbrück and Reichardt (45) conclude that "The stimulus does not
produce extra growth, it simply alters the distribution in time of the
growth that would have taken place during the same period in the
absence of the stimulus. The illumination program controls the rate of
utilization of materials available for growth, but does not alter the
quantity of this material."
Thus, although none of the evidence is perhaps wholly conclusive,
the totality of it points clearly to the use of a small amount of light
energy, both in tropism and in taxis, to channel a relatively large amount
of metabolic energy into a modification of growth or movement.
E. SUMMARY
If an attempt were to be made to summarize the large amount of
work carried out on such a variety of biological material one could only
point to the emergence of a very few general principles. These are: (1)
the fact that practically all organisms studied are to some extent lightsensitive; (2) the evident occurrence of more than one response to light
in each of the cases analyzed; (3) the high probability that in virtually
all organisms carotenoids participate in at least one of these responses;
(4) the almost completely unbridged gap between the reception of light
and its first biochemical effect, whether evidenced as change of growth,