V.
DETERMINING
FACTORS
IN CELL
GROWTH
209
example in which 'growth' and responses to light and gravity may be
interpreted in terms of cell enlargement. In many fruits the conspicuous
phases of growth in the ovary wall are often stimulated by fertilization
and/or by the presence of a viable embryo; these events may in large
part involve cell enlargement almost to the exclusion of cell division. It
is in this area that chemically induced parthenocarpic development may
play an important role (Luckwill, 1957, 1959). The stimuli that cause
the formation of fleshy organs of perennation (tubers, bulbs, corms and
rhizomes) operate in large part through the control of growth of large
parenchyma cells that contain stored food. In all these situations,
therefore, a chemically controlled mechanism of cell enlargement, often
photoperiodically stimulated (Tincker, 1925; Tincker and Darbishire,
1933), can be seen as exercising a measure of regulatory control, and
this is an area to which the auxins, natural and synthetic, should be
applicable. But inasmuch as the elongation of cells, or internodes, is a
prominent feature of early differentiation, as for example of vascular
tissue, the stimuli to cell enlargement have also been invoked as stimuli
to cell differentiation. Jacobs (1952, 1954), Wetmore and Sorokin (1955)
have seen in the effect of auxins and a sugar a causal explanation of
the events that occur behind the shoot apex. Where these events are
genetically limited by blocks to cell or internode elongation, with the
production of genetic dwarfs, the well known role of the so-called
'gibberellins' may be specifically invoked to relieve this condition.
Similar to those stimuli which operate to bring about cell enlargement,
there must also be a set of factors (e.g. a changing pattern in
enzyme activity) leading to an 'arresting mechanism' which tends to
stop enlargement, as postulated by Brown et al. (1952).
Because of the success which has attended the interpretation of
certain of the above phenomena by invoking the role of substances
whose primary effect is upon cell enlargement (auxins or gibberellins),
their regulatory role has been extended, often with less justification,
into other areas of morphogenesis such as the control of flowering and
fruiting. This could only be valid in those cases in which the effect of
the primary stimulus to flowering or fruiting, whatever it is, was
arrested by block which operated through cell enlargement. In such
cases, and only in such cases, could one reasonably expect the auxins or
gibberellins to have marked effects. However, while recognizing that the
factors that regulate cell enlargement should not be extended beyond
their normal sphere of operation, they do represent a major part of the
system of regulatory control over growth, and, as such, their effects will
now be summarized.
The staple tests, or assays, for the activity of auxins and/or gibberellins
[the coleoptile curvature and straight growth tests, the split pea stem
DETERMINING
FACTORS
IN CELL
GROWTH
209
example in which 'growth' and responses to light and gravity may be
interpreted in terms of cell enlargement. In many fruits the conspicuous
phases of growth in the ovary wall are often stimulated by fertilization
and/or by the presence of a viable embryo; these events may in large
part involve cell enlargement almost to the exclusion of cell division. It
is in this area that chemically induced parthenocarpic development may
play an important role (Luckwill, 1957, 1959). The stimuli that cause
the formation of fleshy organs of perennation (tubers, bulbs, corms and
rhizomes) operate in large part through the control of growth of large
parenchyma cells that contain stored food. In all these situations,
therefore, a chemically controlled mechanism of cell enlargement, often
photoperiodically stimulated (Tincker, 1925; Tincker and Darbishire,
1933), can be seen as exercising a measure of regulatory control, and
this is an area to which the auxins, natural and synthetic, should be
applicable. But inasmuch as the elongation of cells, or internodes, is a
prominent feature of early differentiation, as for example of vascular
tissue, the stimuli to cell enlargement have also been invoked as stimuli
to cell differentiation. Jacobs (1952, 1954), Wetmore and Sorokin (1955)
have seen in the effect of auxins and a sugar a causal explanation of
the events that occur behind the shoot apex. Where these events are
genetically limited by blocks to cell or internode elongation, with the
production of genetic dwarfs, the well known role of the so-called
'gibberellins' may be specifically invoked to relieve this condition.
Similar to those stimuli which operate to bring about cell enlargement,
there must also be a set of factors (e.g. a changing pattern in
enzyme activity) leading to an 'arresting mechanism' which tends to
stop enlargement, as postulated by Brown et al. (1952).
Because of the success which has attended the interpretation of
certain of the above phenomena by invoking the role of substances
whose primary effect is upon cell enlargement (auxins or gibberellins),
their regulatory role has been extended, often with less justification,
into other areas of morphogenesis such as the control of flowering and
fruiting. This could only be valid in those cases in which the effect of
the primary stimulus to flowering or fruiting, whatever it is, was
arrested by block which operated through cell enlargement. In such
cases, and only in such cases, could one reasonably expect the auxins or
gibberellins to have marked effects. However, while recognizing that the
factors that regulate cell enlargement should not be extended beyond
their normal sphere of operation, they do represent a major part of the
system of regulatory control over growth, and, as such, their effects will
now be summarized.
The staple tests, or assays, for the activity of auxins and/or gibberellins
[the coleoptile curvature and straight growth tests, the split pea stem