6. PHOTOTROPISM AND PHOTOTAXIS
261
itself in equilibrium; it is proportional to the light stimulus necessary
to evoke a given light-growth reaction.
In darkness this adaptation level is at a minimum (zero); after a
brief exposure to light it increases temporarily and then decreases again
to this minimum (45). There is good reason to believe that the decrease
is exponential (52). As can be seen in the figure, however, its level increases steadily in continuous light and correspondingly the light-growth
reaction returns to zero. If, however, the illumination is not symmetrical,
a phototropic curvature begins after about the same reaction time as
that of the light-growth reaction, but it continues to increase linearly
and is still increasing when the light-growth reaction has died away
(Fig. 5). This discrepancy is indeed hard to explain and it indicates
that the relation between light-growth reaction and curvature is very
complex.
In Avena the situation is also complex, though perhaps better understood. Table III summarizes the pertinent information about its lightgrowth reactions, following symmetrical illumination, and compares them
with its phototropism.
The existence of two light-growth mechanisms, differently distributed
in the plant, is apparent here. Indeed the existence of a rapid and a
slow light-growth reaction was made clear as long ago as 1925 by Went
(53). With intermediate doses or prolonged low intensity exposures it
is probable that both systems operate simultaneously. Analysis of such
mixed responses accounts for the wide array of facts and theories concerning the connection between light-growth reactions and curvature.
It is quite clear that the two phenomena are closely related. The
light-growth reactions produced by long exposures or high doses, although they are not proportional to the product I X t, in other respects
correspond well to Blaauw's concept, in the sense that they occur more
or less independently throughout the growing regions and that their
effects are additive. In this respect they are closely allied to the Phycomyces reactions. These reactions can be most simply interpreted as
direct, light-produced changes in growth rate at, or very close to, the
site of light absorption, i.e., localized reactions. The changes are transient
and are soon compensated, as if light temporarily unbalances the normal
sequence of events in the growth system.
The "tip responses," however, seem to depend on the special properties of the tip and not to occur in the plant as a whole. The growth
changes produced are, in part, spatially removed from the site of light
absorption, i.e., transmitted reactions. They probably result from a
more long-lasting interference by light in the production and distribution of a growth-controlling factor, generated in the tip and active in
261
itself in equilibrium; it is proportional to the light stimulus necessary
to evoke a given light-growth reaction.
In darkness this adaptation level is at a minimum (zero); after a
brief exposure to light it increases temporarily and then decreases again
to this minimum (45). There is good reason to believe that the decrease
is exponential (52). As can be seen in the figure, however, its level increases steadily in continuous light and correspondingly the light-growth
reaction returns to zero. If, however, the illumination is not symmetrical,
a phototropic curvature begins after about the same reaction time as
that of the light-growth reaction, but it continues to increase linearly
and is still increasing when the light-growth reaction has died away
(Fig. 5). This discrepancy is indeed hard to explain and it indicates
that the relation between light-growth reaction and curvature is very
complex.
In Avena the situation is also complex, though perhaps better understood. Table III summarizes the pertinent information about its lightgrowth reactions, following symmetrical illumination, and compares them
with its phototropism.
The existence of two light-growth mechanisms, differently distributed
in the plant, is apparent here. Indeed the existence of a rapid and a
slow light-growth reaction was made clear as long ago as 1925 by Went
(53). With intermediate doses or prolonged low intensity exposures it
is probable that both systems operate simultaneously. Analysis of such
mixed responses accounts for the wide array of facts and theories concerning the connection between light-growth reactions and curvature.
It is quite clear that the two phenomena are closely related. The
light-growth reactions produced by long exposures or high doses, although they are not proportional to the product I X t, in other respects
correspond well to Blaauw's concept, in the sense that they occur more
or less independently throughout the growing regions and that their
effects are additive. In this respect they are closely allied to the Phycomyces reactions. These reactions can be most simply interpreted as
direct, light-produced changes in growth rate at, or very close to, the
site of light absorption, i.e., localized reactions. The changes are transient
and are soon compensated, as if light temporarily unbalances the normal
sequence of events in the growth system.
The "tip responses," however, seem to depend on the special properties of the tip and not to occur in the plant as a whole. The growth
changes produced are, in part, spatially removed from the site of light
absorption, i.e., transmitted reactions. They probably result from a
more long-lasting interference by light in the production and distribution of a growth-controlling factor, generated in the tip and active in