V.
DETERMINING FACTORS IN CELL GROWTH
239
These examples suggest that the causal factors that promote cell
growth and cell division in monocotyledons may contrast with those
that obtain in dicotyledons. In fact, submerged leaves of dicotyledons,
even in light, often strikingly resemble the etiolated condition (dissected
or reduced laminae) and even structurally they resemble monocotyledons. Such well known contrasts in the responses of monocotyledons and dicotyledons to growth factors like 2,4-D are also relevant
here. Older ideas, now extensively modified, attributed the maximum
activity in growth of the shoot to the superficial layers at its tip, which,
by forming surface more rapidly than bulk, caused the shoot growing
point to form folds which became the leaf primordia (Schüepp, 1916).
In turn, the leaf primordia exert a formative basipetal influence upon
the position of developing vascular strands. By contrast, in the root,
growth was also conceived to occur at its tip, though here the growth
was more deep-seated and added to the length without increasing the
surface; and, in the root, somewhat sharply delimited zones of cell
division, cell enlargement and maturation were recognized along the
axis of the root. Amongst the vascular cryptogams the existence of well
defined initiating cells, or groups of cells, due to the activities and
divisions of which the mature tissues emerge, seem well established.
Ideas about the location in the growing regions of the greatest frequency
of cell division and the role of those regions which may now seem to be
less actively engaged in cell division than was hitherto supposed to be
the case are now being questioned.
On physiological grounds, Brown and his collaborators called in
question the idea that the tip of the root was necessarily the region of
the greatest physiological activity, if the criteria of such activity were
expressed upon a per cell basis instead of the then more usual segmentweight basis. Briefly, Brown located the region of most active general
physiological activity (synthesis of protein, enzyme activity, etc.)
not at the extreme tip but backward near the point of maximum
enlargement of the average cell. In an attempted analysis of the shoot
apex from a somewhat similar point of view Sunderland et al. (1956,
p. 89) regard it as clothed by a mantle of tissue of relatively low
metabolic activity which surrounds a core of tissue of greater metabolic
activity, and this presumably supplies the needed and elaborated
metabolites to the surrounding regions. (In a somewhat similar fashion
Brown and Wightman (1952) regard the root tip as being dependent on
metabolites from the more mature tissue behind.) Thus the incidence of
actual growth by cell division in the apex and the possible dependence
of this region upon others calls for some re-examination.
There have been put forth various concepts during the past century
regarding the organization of the shoot apex in angiosperms, the details
DETERMINING FACTORS IN CELL GROWTH
239
These examples suggest that the causal factors that promote cell
growth and cell division in monocotyledons may contrast with those
that obtain in dicotyledons. In fact, submerged leaves of dicotyledons,
even in light, often strikingly resemble the etiolated condition (dissected
or reduced laminae) and even structurally they resemble monocotyledons. Such well known contrasts in the responses of monocotyledons and dicotyledons to growth factors like 2,4-D are also relevant
here. Older ideas, now extensively modified, attributed the maximum
activity in growth of the shoot to the superficial layers at its tip, which,
by forming surface more rapidly than bulk, caused the shoot growing
point to form folds which became the leaf primordia (Schüepp, 1916).
In turn, the leaf primordia exert a formative basipetal influence upon
the position of developing vascular strands. By contrast, in the root,
growth was also conceived to occur at its tip, though here the growth
was more deep-seated and added to the length without increasing the
surface; and, in the root, somewhat sharply delimited zones of cell
division, cell enlargement and maturation were recognized along the
axis of the root. Amongst the vascular cryptogams the existence of well
defined initiating cells, or groups of cells, due to the activities and
divisions of which the mature tissues emerge, seem well established.
Ideas about the location in the growing regions of the greatest frequency
of cell division and the role of those regions which may now seem to be
less actively engaged in cell division than was hitherto supposed to be
the case are now being questioned.
On physiological grounds, Brown and his collaborators called in
question the idea that the tip of the root was necessarily the region of
the greatest physiological activity, if the criteria of such activity were
expressed upon a per cell basis instead of the then more usual segmentweight basis. Briefly, Brown located the region of most active general
physiological activity (synthesis of protein, enzyme activity, etc.)
not at the extreme tip but backward near the point of maximum
enlargement of the average cell. In an attempted analysis of the shoot
apex from a somewhat similar point of view Sunderland et al. (1956,
p. 89) regard it as clothed by a mantle of tissue of relatively low
metabolic activity which surrounds a core of tissue of greater metabolic
activity, and this presumably supplies the needed and elaborated
metabolites to the surrounding regions. (In a somewhat similar fashion
Brown and Wightman (1952) regard the root tip as being dependent on
metabolites from the more mature tissue behind.) Thus the incidence of
actual growth by cell division in the apex and the possible dependence
of this region upon others calls for some re-examination.
There have been put forth various concepts during the past century
regarding the organization of the shoot apex in angiosperms, the details