V.
DETERMINING
FACTORS IN CELL
GROWTH
243
D. The Transition from the Vegetative to the Reproductive State
One of the most outstanding morphogenetic events to be explained is
that of flowering. This vast topic can only be touched upon incidentally
to relate it to the main theme of this article. In flowering, an apical
meristem which is composed of cells essentially similar to those which
produce the vegetative plant body produces—instead of leaves, internodes and buds—either a terminal flower with successive whorls of
sepals, petals, anthers and carpels or an axis, bearing laterally arranged
flowers. Though the floral organs are traditionally regarded as modified
leaves, i.e. essentially as sporophylls, their distinctive form and function
require a very drastic departure from the normal pattern of growth in
the growing region. As will be seen, there is so gradual a transition from
the growing apex which produces foliar organs to that which produces
floral organs that the essential crisis has passed before much is visible
with the optical microscope. Since the constituent cells of vegetative
and reproductive growing regions are not visibly dissimilar, the ensuing
different results may be achieved by a different spacial distribution of
growth by cell division and enlargement in the growing regions that give
rise to the different kinds of primordia. This brings the problem of
flowering within the range of phenomena affected by the growthregulating mechanisms that control the growth of cells.
Flowering should not be thought of as a single morphogenetic event
in the average monoecious plant, which represents both male and female
sexual organs and cycles. One can hardly credit a single hormone-like
substance, acting in a formative way, with the ability so to modify
growth that sepals, petals, anthers and carpel and the events of sporogenesis and gametogenesis successively arise. The ultimate destiny of
flowering plants is to complete the life cycle through flowering; this is
a 'built-in' attribute of the egg and, as shoot apices organize, their
ability to form flowers must be as inherent as their ability to grow
vegetatively. From this standpoint, however, the induction of flowers
does not require a specific formative stimulus so much as a triggering
action that releases growth in centres, cells or primordia which so
modify the form of the growing point that the events of flowering ensue.
In vegetative growth however, leaves arise in such a way that they
seem, as it were, to repel each other. Whatever accumulated stimuli to
division are necessary to set off a leaf as a distinct and separately growing part are only recovered, or accumulated, at a distance sufficiently
far from the preceding growth centre. The number of leaf primordia
accommodated in the shoot apex of a dicotyledonous plant at the level
of their formation is often small, four being even a high number as in an
opposite and decussate leaf arrangement, and, thereafter, a measure of
1
A.M. I
DETERMINING
FACTORS IN CELL
GROWTH
243
D. The Transition from the Vegetative to the Reproductive State
One of the most outstanding morphogenetic events to be explained is
that of flowering. This vast topic can only be touched upon incidentally
to relate it to the main theme of this article. In flowering, an apical
meristem which is composed of cells essentially similar to those which
produce the vegetative plant body produces—instead of leaves, internodes and buds—either a terminal flower with successive whorls of
sepals, petals, anthers and carpels or an axis, bearing laterally arranged
flowers. Though the floral organs are traditionally regarded as modified
leaves, i.e. essentially as sporophylls, their distinctive form and function
require a very drastic departure from the normal pattern of growth in
the growing region. As will be seen, there is so gradual a transition from
the growing apex which produces foliar organs to that which produces
floral organs that the essential crisis has passed before much is visible
with the optical microscope. Since the constituent cells of vegetative
and reproductive growing regions are not visibly dissimilar, the ensuing
different results may be achieved by a different spacial distribution of
growth by cell division and enlargement in the growing regions that give
rise to the different kinds of primordia. This brings the problem of
flowering within the range of phenomena affected by the growthregulating mechanisms that control the growth of cells.
Flowering should not be thought of as a single morphogenetic event
in the average monoecious plant, which represents both male and female
sexual organs and cycles. One can hardly credit a single hormone-like
substance, acting in a formative way, with the ability so to modify
growth that sepals, petals, anthers and carpel and the events of sporogenesis and gametogenesis successively arise. The ultimate destiny of
flowering plants is to complete the life cycle through flowering; this is
a 'built-in' attribute of the egg and, as shoot apices organize, their
ability to form flowers must be as inherent as their ability to grow
vegetatively. From this standpoint, however, the induction of flowers
does not require a specific formative stimulus so much as a triggering
action that releases growth in centres, cells or primordia which so
modify the form of the growing point that the events of flowering ensue.
In vegetative growth however, leaves arise in such a way that they
seem, as it were, to repel each other. Whatever accumulated stimuli to
division are necessary to set off a leaf as a distinct and separately growing part are only recovered, or accumulated, at a distance sufficiently
far from the preceding growth centre. The number of leaf primordia
accommodated in the shoot apex of a dicotyledonous plant at the level
of their formation is often small, four being even a high number as in an
opposite and decussate leaf arrangement, and, thereafter, a measure of
1
A.M. I