244
F. C. STEWARD AND
Η. Y. MOHAN RAM
internodal growth is necessary to permit the pattern to be repeated at
a new higher level. Thus, in the dicotyledons, the vegetative pattern
is often one of expanded internodes and lateral organs arising well
separated on the axis. But this trend is often reversed when flowers form,
for a larger number of lateral organs arise at a given level (often in
whorls of 5), and the successive whorls are crowded together on the axis
with very short internodes. The first visible and often the only sign of
flowering, therefore, is that the central dome of meristem changes shape,
becoming somewhat less rounded, broader and flatter, with the latera]
organs arising more on the surface and less on the sloping flanks (Fig.
5(c), (d), (g), (h)). In monocotyledonous plants a more complex organization often occurs. In the vegetative shoot the axis is often shortened to
a mere crown, leaves at their base encircle the stem, internodes are
commonly short, leaves do not so quickly diverge from the axis as
separate growing centres, and many more persist and are accommodated
at a given level or in close succession in the formative regions of the
shoot. By contrast, the flowers often have lateral organs in whorls of
three. Thus the problem of crowding more lateral organs at the apex in
the flower than in the vegetative shoot does not arise in the monocotyledonous plant. Accordingly, the floral apex is often more slender
and more pointed than the vegetative apex (Rotor and MacDaniels,
1951; cf. Fig. 5(e), (/); also see papers by Sharman, 1942, 1944).
Changes in the shape of the central portion of the shoot apex are
about all that may be seen at the time the decisive acts of flowering
occur, and this is remarkably little considering the knowledge that
is yet to be gained.
Long before the form of floral organs is apparent in the apex, their
appearance can be traced back to regions in an embryo. Cytological
abnormalities in single cells of the irradiated corn grain (Randolph,
1950) lead to recognizable differences or abnormalities that eventually
appear in sectors of the tassels, showing clearly that, 'These sectors
obviously consisted of tissue derived from single cell initials which were
present in the seed at the time of irradiation. The amount of tissue
derived from an affected cell during ontogeny determined the size of the
sectors which ordinarily varied in width from about 1 to 30 millimetres,
and in length from a few centimetres up to the full extent of the leaf and
leaf sheath.' Such observations recall the ideas of Buvat and others that
the relatively quiescent zone, composed of a few larger cells, in the
central apical dome of the shoot apex participates but little in growth
until the onset of flowering. Thus conceived, the essential requirements
for flowering would seem to be segregated in a part of the shoot meristem and, when this is released and grows, the different growth pattern
leading to floral parts ensues. Wetmore (1956) has examined shoot
F. C. STEWARD AND
Η. Y. MOHAN RAM
internodal growth is necessary to permit the pattern to be repeated at
a new higher level. Thus, in the dicotyledons, the vegetative pattern
is often one of expanded internodes and lateral organs arising well
separated on the axis. But this trend is often reversed when flowers form,
for a larger number of lateral organs arise at a given level (often in
whorls of 5), and the successive whorls are crowded together on the axis
with very short internodes. The first visible and often the only sign of
flowering, therefore, is that the central dome of meristem changes shape,
becoming somewhat less rounded, broader and flatter, with the latera]
organs arising more on the surface and less on the sloping flanks (Fig.
5(c), (d), (g), (h)). In monocotyledonous plants a more complex organization often occurs. In the vegetative shoot the axis is often shortened to
a mere crown, leaves at their base encircle the stem, internodes are
commonly short, leaves do not so quickly diverge from the axis as
separate growing centres, and many more persist and are accommodated
at a given level or in close succession in the formative regions of the
shoot. By contrast, the flowers often have lateral organs in whorls of
three. Thus the problem of crowding more lateral organs at the apex in
the flower than in the vegetative shoot does not arise in the monocotyledonous plant. Accordingly, the floral apex is often more slender
and more pointed than the vegetative apex (Rotor and MacDaniels,
1951; cf. Fig. 5(e), (/); also see papers by Sharman, 1942, 1944).
Changes in the shape of the central portion of the shoot apex are
about all that may be seen at the time the decisive acts of flowering
occur, and this is remarkably little considering the knowledge that
is yet to be gained.
Long before the form of floral organs is apparent in the apex, their
appearance can be traced back to regions in an embryo. Cytological
abnormalities in single cells of the irradiated corn grain (Randolph,
1950) lead to recognizable differences or abnormalities that eventually
appear in sectors of the tassels, showing clearly that, 'These sectors
obviously consisted of tissue derived from single cell initials which were
present in the seed at the time of irradiation. The amount of tissue
derived from an affected cell during ontogeny determined the size of the
sectors which ordinarily varied in width from about 1 to 30 millimetres,
and in length from a few centimetres up to the full extent of the leaf and
leaf sheath.' Such observations recall the ideas of Buvat and others that
the relatively quiescent zone, composed of a few larger cells, in the
central apical dome of the shoot apex participates but little in growth
until the onset of flowering. Thus conceived, the essential requirements
for flowering would seem to be segregated in a part of the shoot meristem and, when this is released and grows, the different growth pattern
leading to floral parts ensues. Wetmore (1956) has examined shoot