248
F.
C. STEWARD AND
Η. Y. MOHAN RAM
affected by photoperiods and night temperature (Steward et al. 9 1959).
This suggests that the decisive metabolic criterion of plants, in these
different states, is the balance between such C 4 compounds as aspartic
acid and asparagine on the one hand and such C 5 compounds as glutamic
acid and glutamine on the other. This in turn may reflect some other
metabolic criterion which is even closer to the mechanism of perception
and response of the stimuli. But the work on Mentha does suggest that
the apices are in fact subjected to different metabolites from the
subjacent tissue when they give rise to the contrasting patterns of
reproductive and vegetative growth.
Another, highly suggestive approach to the metabolic basis of the
photoperiodic response of short day plants is that of Gregory and
Thimann, as summarized by Spear (1959). These authors studied the
diurnal C0 2 metabolism of Kalanchoe as affected by the photoperiodic
stimulus. Briefly this work shows that carbon dioxide fixation during
the dark period is affected by variables similar to those which evoke the
flowering response. Even though the present evidence does not justify
the conclusion that the carbon dioxide is the direct metabolic agent that
'triggers off' the flowering response, it does again indicate that the lightinduced response is accompanied by a changed biochemical milieu
which (as in the case of Mentha) could make its impact upon the growing
regions. The identification of a single causal metabolic reaction, if it
exists, however, is not possible at present.
Recognizing that floral organs are modified leaves, what determines
their different manner of growth? This is perhaps part of the larger
problem which differentiates leaf and sporophyll. Why does an anther
grow in bulk rather than in surface and, having done so, what induces a
deep-seated plug of archesporial tissue to embark upon the distinctive
events which lead eventually to pollen grains? The foliar nature of the
carpel, folded and more or less joined at its margins, may be apparent,
but, unlike the anther, it encloses a hollow space into which the ovules
grow. If one regards fertile fronds of fern as the near equivalent of
carpels, it is interesting that they form sporangia, i.e. behave as sporophylls, when they grow attached; but, when separately cultured, the
sporangia do not form (as described earlier). However, Sussex and
Steeves have demonstrated (1958) that excised fronds of some osmundaceous ferns will produce sporangia when the sucrose concentration in
the medium is raised. Thus it is not enough to say why the growth of a
primordium is changed so that a different kind of leaf-like organ
emerges; there is still the added stimulus to make it behave as a
sporophyll. This is an extension of the problems (discussed above)
inherent in determining whether primordia form buds or leaves at
definite sites on the vegetative apex.
F.
C. STEWARD AND
Η. Y. MOHAN RAM
affected by photoperiods and night temperature (Steward et al. 9 1959).
This suggests that the decisive metabolic criterion of plants, in these
different states, is the balance between such C 4 compounds as aspartic
acid and asparagine on the one hand and such C 5 compounds as glutamic
acid and glutamine on the other. This in turn may reflect some other
metabolic criterion which is even closer to the mechanism of perception
and response of the stimuli. But the work on Mentha does suggest that
the apices are in fact subjected to different metabolites from the
subjacent tissue when they give rise to the contrasting patterns of
reproductive and vegetative growth.
Another, highly suggestive approach to the metabolic basis of the
photoperiodic response of short day plants is that of Gregory and
Thimann, as summarized by Spear (1959). These authors studied the
diurnal C0 2 metabolism of Kalanchoe as affected by the photoperiodic
stimulus. Briefly this work shows that carbon dioxide fixation during
the dark period is affected by variables similar to those which evoke the
flowering response. Even though the present evidence does not justify
the conclusion that the carbon dioxide is the direct metabolic agent that
'triggers off' the flowering response, it does again indicate that the lightinduced response is accompanied by a changed biochemical milieu
which (as in the case of Mentha) could make its impact upon the growing
regions. The identification of a single causal metabolic reaction, if it
exists, however, is not possible at present.
Recognizing that floral organs are modified leaves, what determines
their different manner of growth? This is perhaps part of the larger
problem which differentiates leaf and sporophyll. Why does an anther
grow in bulk rather than in surface and, having done so, what induces a
deep-seated plug of archesporial tissue to embark upon the distinctive
events which lead eventually to pollen grains? The foliar nature of the
carpel, folded and more or less joined at its margins, may be apparent,
but, unlike the anther, it encloses a hollow space into which the ovules
grow. If one regards fertile fronds of fern as the near equivalent of
carpels, it is interesting that they form sporangia, i.e. behave as sporophylls, when they grow attached; but, when separately cultured, the
sporangia do not form (as described earlier). However, Sussex and
Steeves have demonstrated (1958) that excised fronds of some osmundaceous ferns will produce sporangia when the sucrose concentration in
the medium is raised. Thus it is not enough to say why the growth of a
primordium is changed so that a different kind of leaf-like organ
emerges; there is still the added stimulus to make it behave as a
sporophyll. This is an extension of the problems (discussed above)
inherent in determining whether primordia form buds or leaves at
definite sites on the vegetative apex.