210
F.
C. STEWARD AND
Η. Y. MOHAN RAM
test, the induced parthenocarpy test, and the elongation of genetically
dwarfed shoots (viz. Zea)] all have in common an effect, or differential
effects, upon the elongation of cells. But while this is essentially true,
the individual substances known as auxins or gibberellins may act in a
dissimilar fashion in particular situations. The justification for distinguishing two classes of stimuli is thus a purely operational one and
substances or extracts which cause one or other of these responses are
held to be auxins or gibberellins respectively. A similarity in a response
is no guarantee of identity or similarity in the causal substances.
Therefore, in the case of cell enlargement as in the case of cell division,
it is now apparent that a variety of molecules may intervene, and often
all they seem to have in common is the ability to evoke the effect in
question.
Although gibberellins resemble auxins in promoting cell elongation,
many distinguishing features are noteworthy. Gibberellins do not
inhibit the growth of lateral buds or roots, do not form a callus or cause
epinasty, but they are more powerful in inducing parthenocarpy; they
neither interfere with abscission phenomena nor promote an uptake of
water (Stowe and Yamaki, 1957) as auxins do. Since the gibberellins
produce specific effects on the elongation of monocotyledonous leaves,
on the bolting of long day plants and on the removal of light inhibition
of internodal growth, Stowe and Yamaki (1957) plead for an operational
definition for these substances. These authors define gibberellins as
'that class of compounds which cause internodal elongation when
applied to certain intact genetically dwarfed plants. A promotion of
elongation of monocotyledonous leaves might provide a useful supporting definition.'
That the gibberellin-induced elongation of dwarf plants is primarily
due to cell elongation has been emphasized by Brian et al. (1954) and
Kato (1955). Contrarily, the elongation of dwarf plants as a function of
the increase in cell number has been recently reported by Sachs and
Lang (1957) and Sachs, Bretz and Lang (1959). By counting the total
number of mitotic figures observed in the most median serial sections of
the shoot apices in the biennial Hyosoyamus and the long day plant
Samolus, Sachs et al. conclude that it is the subapical part (perhaps
comparable to the rib meristem and peripheral zones of Popham, 1951)
that shows the greatest number of mitoses and not the most apical cells,
which are quiescent. At least for the first 72 hours after treatment there
is no elongation of the cells; any increase in length in this period is
solely by cell division. It is not the mere number of divisions which is
significant, but it is their transverse orientation (with regard to the
axis) which is the determining factor in vertical elongation. There have
been other reports of gibberellin-induced cell divisions (Greulach and
F.
C. STEWARD AND
Η. Y. MOHAN RAM
test, the induced parthenocarpy test, and the elongation of genetically
dwarfed shoots (viz. Zea)] all have in common an effect, or differential
effects, upon the elongation of cells. But while this is essentially true,
the individual substances known as auxins or gibberellins may act in a
dissimilar fashion in particular situations. The justification for distinguishing two classes of stimuli is thus a purely operational one and
substances or extracts which cause one or other of these responses are
held to be auxins or gibberellins respectively. A similarity in a response
is no guarantee of identity or similarity in the causal substances.
Therefore, in the case of cell enlargement as in the case of cell division,
it is now apparent that a variety of molecules may intervene, and often
all they seem to have in common is the ability to evoke the effect in
question.
Although gibberellins resemble auxins in promoting cell elongation,
many distinguishing features are noteworthy. Gibberellins do not
inhibit the growth of lateral buds or roots, do not form a callus or cause
epinasty, but they are more powerful in inducing parthenocarpy; they
neither interfere with abscission phenomena nor promote an uptake of
water (Stowe and Yamaki, 1957) as auxins do. Since the gibberellins
produce specific effects on the elongation of monocotyledonous leaves,
on the bolting of long day plants and on the removal of light inhibition
of internodal growth, Stowe and Yamaki (1957) plead for an operational
definition for these substances. These authors define gibberellins as
'that class of compounds which cause internodal elongation when
applied to certain intact genetically dwarfed plants. A promotion of
elongation of monocotyledonous leaves might provide a useful supporting definition.'
That the gibberellin-induced elongation of dwarf plants is primarily
due to cell elongation has been emphasized by Brian et al. (1954) and
Kato (1955). Contrarily, the elongation of dwarf plants as a function of
the increase in cell number has been recently reported by Sachs and
Lang (1957) and Sachs, Bretz and Lang (1959). By counting the total
number of mitotic figures observed in the most median serial sections of
the shoot apices in the biennial Hyosoyamus and the long day plant
Samolus, Sachs et al. conclude that it is the subapical part (perhaps
comparable to the rib meristem and peripheral zones of Popham, 1951)
that shows the greatest number of mitoses and not the most apical cells,
which are quiescent. At least for the first 72 hours after treatment there
is no elongation of the cells; any increase in length in this period is
solely by cell division. It is not the mere number of divisions which is
significant, but it is their transverse orientation (with regard to the
axis) which is the determining factor in vertical elongation. There have
been other reports of gibberellin-induced cell divisions (Greulach and