238
F. C. STEWARD
AND Η. Y. MOHAN
RAM
and root apices now seem to require the co-operative action of different
regions, or groups of cells, to bring about the integrated activity of the
whole. Brief reference to this field of enquiry will now be made.
While the shoot apex produces leaf primordia exogenously in a
definite order, the root meristem does not put forth any such exogenous
lateral organs. While the pattern of vascularization of the shoot and the
incidence of leaf primordia show marked evidence of reciprocal relationships, this is not so in the root, where the primary vascular tissue is laid
down before any lateral roots arise. The lateral roots, however, originate
far behind the apex, from a deep-seated layer—the pericycle. As
already noted (Section III C) the pericyclic cells which are stimulated to
divide and form the lateral root tip seem to derive this stimulus from
the region of the protoxylem.
Although a definite sequence of primordial leaves may be formed,
independently of light, while the embryo is encased within the seed,
their subsequent development does require light. Etiolated dicotyledons
form the characteristic habit of the plumular hook, no new leaf primordia
other than those already initiated are laid down while those already
formed expand only to a limited extent (Priestley, 1926; Priestley and
Ewing, 1923). (Strangely, the symptoms of complete etiolation in monocotyledons are far less divergent from their normal habit.) In many
etiolated dicotyledons the structure of the etiolated stem approaches
more nearly that of a root (weak development of cuticle, presence
of an endodermis with casparian strips, more centrally located vascular strands with a broader cortex, etc.). Characteristically, catalytic
amounts of light promote morphogenetic changes toward the normal
condition and the nature of the perception of stimulus is being currently
investigated by Hendricks and Borthwick (1959). It remains however,
to understand how this stimulus is transmitted to the growing cells and
what substances mediate this effect.
Curious differences exist between monocotyledonous plants with their
linear leaves and dicotyledons with broad net-veined leaves. The latter
are extremely sensitive to total etiolation and their laminae do not
expand; the former are relatively insensitive to total darkness and, if
anything, their laminae may elongate more than normal. In this latter
respect they resemble more closely the stems of dicotyledons, which
elongate in darkness and are suppressed by light (Williams, 1956).
The point here arises whether the light effect is upon cell number or cell
enlargement. The evidence (Priestley, 1926; Priestley and Ewing, 1923)
is that in the dicotyledonous shoots, etiolation permits elongation in the
internode and suppresses cell division in the laminae, and conversely
light retards elongation in the internodes and promotes cell division in
the laminae.
F. C. STEWARD
AND Η. Y. MOHAN
RAM
and root apices now seem to require the co-operative action of different
regions, or groups of cells, to bring about the integrated activity of the
whole. Brief reference to this field of enquiry will now be made.
While the shoot apex produces leaf primordia exogenously in a
definite order, the root meristem does not put forth any such exogenous
lateral organs. While the pattern of vascularization of the shoot and the
incidence of leaf primordia show marked evidence of reciprocal relationships, this is not so in the root, where the primary vascular tissue is laid
down before any lateral roots arise. The lateral roots, however, originate
far behind the apex, from a deep-seated layer—the pericycle. As
already noted (Section III C) the pericyclic cells which are stimulated to
divide and form the lateral root tip seem to derive this stimulus from
the region of the protoxylem.
Although a definite sequence of primordial leaves may be formed,
independently of light, while the embryo is encased within the seed,
their subsequent development does require light. Etiolated dicotyledons
form the characteristic habit of the plumular hook, no new leaf primordia
other than those already initiated are laid down while those already
formed expand only to a limited extent (Priestley, 1926; Priestley and
Ewing, 1923). (Strangely, the symptoms of complete etiolation in monocotyledons are far less divergent from their normal habit.) In many
etiolated dicotyledons the structure of the etiolated stem approaches
more nearly that of a root (weak development of cuticle, presence
of an endodermis with casparian strips, more centrally located vascular strands with a broader cortex, etc.). Characteristically, catalytic
amounts of light promote morphogenetic changes toward the normal
condition and the nature of the perception of stimulus is being currently
investigated by Hendricks and Borthwick (1959). It remains however,
to understand how this stimulus is transmitted to the growing cells and
what substances mediate this effect.
Curious differences exist between monocotyledonous plants with their
linear leaves and dicotyledons with broad net-veined leaves. The latter
are extremely sensitive to total etiolation and their laminae do not
expand; the former are relatively insensitive to total darkness and, if
anything, their laminae may elongate more than normal. In this latter
respect they resemble more closely the stems of dicotyledons, which
elongate in darkness and are suppressed by light (Williams, 1956).
The point here arises whether the light effect is upon cell number or cell
enlargement. The evidence (Priestley, 1926; Priestley and Ewing, 1923)
is that in the dicotyledonous shoots, etiolation permits elongation in the
internode and suppresses cell division in the laminae, and conversely
light retards elongation in the internodes and promotes cell division in
the laminae.