246
F. C. STEWARD
AND Η. Y. MOHAN RAM
apices of plants which respond to photoperiod, before and after this
stimulus, and his first comments upon the onset of cell division in the
otherwise quiescent zone are quoted below: 'Recent experience with
three species of plants: Chenopodium album (lamb's quarters), Xanthium
saccharatum (cocklebur) and Glycine mox (Biloxi soybean), with known
photoperiods, indicates that towards the close of the induction period,
buds killed, fixed, and sectioned show the large-celled central region to
be cut up into small, actively dividing cells.'
In a recent publication, Wetmore, Gifford and Green (1959) have
submitted the effect of photoperiodic stimulation upon the organization
of the shoot apex to closer analysis. Recognizing (see also Foster, 1938)
that shoot apices comprise three zones (peripheral, central and the pith
rib meristem zone), Wetmore et al., imply that the morphogenetic potentialities of these regions flow first from the factors that regulate in each,
the manner of growth by cell division and cell enlargement. Quoting :
(p. 258) '. . . then one has to admit that the cellular environment of
these central cells must be different from that of the enveloping
cells of the peripheral zone. It may well be true that the conditions
which foster frequent cell divisions in the peripheral zones are different
from those which produce only occasional cell divisions in the central
zone.'
Writing about Chenopodium album, in which the shoot apex possesses
a 'V-shaped central zone' of larger cells in the 2nd, 3rd and 4th rows,
they concluded that (p. 268): 'The first signs of initiation of flowering,
seen histologically, again appear just above the pith rib meristem
involving the bottom of the central zone; these signs take the form of
cell multiplications which show up on the second day in the greenhouse
under long days after two short day (shown in light and 16 hours in
dark) photoperiods.'
Again, writing about five photoperiodically responsive plants: (p. 270)
'In these species . . . The initial observable effect found in the five species
studied proved to be mitotic activity just below the central zone and
above the rib meristem zone.' In the subsequent readjustments that
occur throughout what were the original recognized regions of the
vegetative shoot apex: '. . . the apex becomes a parenchymatous core
of pith covered with a thin mantle of meristematic cells from which
arise the bracts, the axillary branches of the inflorescence and the
flowers as required genetically for each of the several species'. Accompanying these changes the original central zone may virtually disappear
as a visible entity (cf. Fig. 5(c), (d)).
In summary, Wetmore, Gifford and Green point to the cessation of
the apical dominance in the flowering shoot (for lateral flower buds and
even branches may arise profusely). They observe that the central zone
F. C. STEWARD
AND Η. Y. MOHAN RAM
apices of plants which respond to photoperiod, before and after this
stimulus, and his first comments upon the onset of cell division in the
otherwise quiescent zone are quoted below: 'Recent experience with
three species of plants: Chenopodium album (lamb's quarters), Xanthium
saccharatum (cocklebur) and Glycine mox (Biloxi soybean), with known
photoperiods, indicates that towards the close of the induction period,
buds killed, fixed, and sectioned show the large-celled central region to
be cut up into small, actively dividing cells.'
In a recent publication, Wetmore, Gifford and Green (1959) have
submitted the effect of photoperiodic stimulation upon the organization
of the shoot apex to closer analysis. Recognizing (see also Foster, 1938)
that shoot apices comprise three zones (peripheral, central and the pith
rib meristem zone), Wetmore et al., imply that the morphogenetic potentialities of these regions flow first from the factors that regulate in each,
the manner of growth by cell division and cell enlargement. Quoting :
(p. 258) '. . . then one has to admit that the cellular environment of
these central cells must be different from that of the enveloping
cells of the peripheral zone. It may well be true that the conditions
which foster frequent cell divisions in the peripheral zones are different
from those which produce only occasional cell divisions in the central
zone.'
Writing about Chenopodium album, in which the shoot apex possesses
a 'V-shaped central zone' of larger cells in the 2nd, 3rd and 4th rows,
they concluded that (p. 268): 'The first signs of initiation of flowering,
seen histologically, again appear just above the pith rib meristem
involving the bottom of the central zone; these signs take the form of
cell multiplications which show up on the second day in the greenhouse
under long days after two short day (shown in light and 16 hours in
dark) photoperiods.'
Again, writing about five photoperiodically responsive plants: (p. 270)
'In these species . . . The initial observable effect found in the five species
studied proved to be mitotic activity just below the central zone and
above the rib meristem zone.' In the subsequent readjustments that
occur throughout what were the original recognized regions of the
vegetative shoot apex: '. . . the apex becomes a parenchymatous core
of pith covered with a thin mantle of meristematic cells from which
arise the bracts, the axillary branches of the inflorescence and the
flowers as required genetically for each of the several species'. Accompanying these changes the original central zone may virtually disappear
as a visible entity (cf. Fig. 5(c), (d)).
In summary, Wetmore, Gifford and Green point to the cessation of
the apical dominance in the flowering shoot (for lateral flower buds and
even branches may arise profusely). They observe that the central zone