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F. C. STEWARD AND
Η. Y. MOHAN RAM
apex is intact. However, when the dominance is removed, by excising
the terminal shoot apex, the DNA content increases to the 4C value,
accompanied by active cell divisions in the lateral buds. Thus, the
inhibition of the lateral buds and its release to permit growth is believed
to be regulated through DNA synthesis, which, according to Patau and
Das, would be a function of balance between kinetin and auxin.
III. The Regulatory Control of Cell Growth
Enough is now known of the physiological, biochemical, and morphological characteristics of cells brought from the resting inactive state to
that of renewed rapid growth so that an appraisal of this transition can
be made. Again attention will be focused mainly on the system with
which the authors are most familiar, although many of the points in
question could be cited from other sources.
A. Activation of Quiescent Cells
1. Physiological and Biochemical Characteristics
The first obvious change as the cells pass from their quiescent stage
to that of active growth under the stimulus of chemical growth factors
is a pronounced increase in their water content. Whereas the explanted
phloem tissue as it is removed from the carrot root has a water content
of approximately 90% of the initial fresh weight, the rapidly growing
explants after some two to three weeks have as much as 95 to 97%.
One expects all forms of metabolism concerned with the application of
metabolic energy to the purposes of the growing cell to be stimulated by
the variables which induce growth. Many earlier observations of accentuated respiration when cells divide at wounded surfaces (for summary
see Steward, Wright and Berry, 1932), the tendency for cells in meristematic (cambial) regions to have higher respiration rates (Goddard,
1945), and the graded fall of respiration as tissues enlarge and mature
along the axis of the shoot and root (Erickson and Goddard, 1951;
Brown, Reith and Robinson, 1952; Sunderland et al., 1956; Brown and
Robinson, 1955), all suggest that respiration should increase in response
to the growth stimulus. This, however, is not the only point here in
question, for it now seems that the agents that increase the respiration
tend also to alter its character. The quiescent tissue, freshly removed
from the carrot root, is markedly sensitive to certain enzymes or
respiratory inhibitors—notably to cyanide and to carbon monoxide
which tend to suppress the heme-catalyzed respiratory reactions
involved in terminal oxidation. Thus, if applied prior to the growth
induction period, these two inhibitors inactivate the centres at which
F. C. STEWARD AND
Η. Y. MOHAN RAM
apex is intact. However, when the dominance is removed, by excising
the terminal shoot apex, the DNA content increases to the 4C value,
accompanied by active cell divisions in the lateral buds. Thus, the
inhibition of the lateral buds and its release to permit growth is believed
to be regulated through DNA synthesis, which, according to Patau and
Das, would be a function of balance between kinetin and auxin.
III. The Regulatory Control of Cell Growth
Enough is now known of the physiological, biochemical, and morphological characteristics of cells brought from the resting inactive state to
that of renewed rapid growth so that an appraisal of this transition can
be made. Again attention will be focused mainly on the system with
which the authors are most familiar, although many of the points in
question could be cited from other sources.
A. Activation of Quiescent Cells
1. Physiological and Biochemical Characteristics
The first obvious change as the cells pass from their quiescent stage
to that of active growth under the stimulus of chemical growth factors
is a pronounced increase in their water content. Whereas the explanted
phloem tissue as it is removed from the carrot root has a water content
of approximately 90% of the initial fresh weight, the rapidly growing
explants after some two to three weeks have as much as 95 to 97%.
One expects all forms of metabolism concerned with the application of
metabolic energy to the purposes of the growing cell to be stimulated by
the variables which induce growth. Many earlier observations of accentuated respiration when cells divide at wounded surfaces (for summary
see Steward, Wright and Berry, 1932), the tendency for cells in meristematic (cambial) regions to have higher respiration rates (Goddard,
1945), and the graded fall of respiration as tissues enlarge and mature
along the axis of the shoot and root (Erickson and Goddard, 1951;
Brown, Reith and Robinson, 1952; Sunderland et al., 1956; Brown and
Robinson, 1955), all suggest that respiration should increase in response
to the growth stimulus. This, however, is not the only point here in
question, for it now seems that the agents that increase the respiration
tend also to alter its character. The quiescent tissue, freshly removed
from the carrot root, is markedly sensitive to certain enzymes or
respiratory inhibitors—notably to cyanide and to carbon monoxide
which tend to suppress the heme-catalyzed respiratory reactions
involved in terminal oxidation. Thus, if applied prior to the growth
induction period, these two inhibitors inactivate the centres at which