V.
DETERMINING FACTORS IN
CELL GROWTH
237
smaller than those mentioned above, will not grow normally under any
known combination of nutrients.
The facts obtained from vegetative propagation, and from the
culturing of relatively large segments, present abundant evidence
that the shoot can grow when severed. The suggestions are that
the subjacent tissues, probably the growing cambial regions, furnish
essential elaborated metabolites to the shoot tip and, since the appearance of roots is commonly associated with a stimulus to growth, there
may well be a reciprocal dependence of the shoot upon formative
substances from the root. A remarkable feature is, however, that the
extreme fastidiousness (with a few exceptions, see Ball, 1946) of the
angiosperm shoot in its requirements for growth in culture contrasts
with the ease with which fern apices will grow in media that are not
competent for the growth of angiosperm shoot tips. Clearly, however,
the initial requirements of the growing tip of the plumule itself are met
in the ovule, and here again one can only point to the formative
substances supplied from endosperm via the growing primordial
cotyledons. (Parenthetically, it may be suggested that the constituents
of endosperm, e.g. coconut milk, which have not proved effective for the
culture of isolated shoot tips, might prove to be effective if the isolated
tip were placed upon, or grafted upon, a bit of cotyledonary tissue
which is bathed by, or growing in, a medium containing the coconut
milk.)
C. Centres of Growth in the Vegetative Plant Body
Once established in the embryonic stage, the markedly contrasted
organization of shoot and root apices is persistent. Again, the gathering
impression is that the behaviour of the growing regions is as much the
product of organization as of any innate potentialities of constituent
cells. If cells are inherently totipotent, as now seems probable, their
different behaviour in the intact growing regions must be due to superimposed restrictions and controls which are features of the organization
of which they are a part. In other words, living cells with the same
inherent genetic potentialities may, by virtue of their position, be
induced to perform different metabolic tasks, or functions. This means
that inherent capacities for growth—ultimately traceable to the 'built-in
capacity of the embryo to grow'—are modulated by the environment of
the cells in the plant body. Such effects would seem to have a chemical
basis, since, as already shown, cells may be caused to grow by a variety
of cell division or enlargement factors, and they also may be withheld
from so doing by inhibitors which act reversibly upon part of the system
(see Section II C). Modern ideas about the organization of shoot and root
apices seem applicable here, for interpretations of the behaviour of shoot
DETERMINING FACTORS IN
CELL GROWTH
237
smaller than those mentioned above, will not grow normally under any
known combination of nutrients.
The facts obtained from vegetative propagation, and from the
culturing of relatively large segments, present abundant evidence
that the shoot can grow when severed. The suggestions are that
the subjacent tissues, probably the growing cambial regions, furnish
essential elaborated metabolites to the shoot tip and, since the appearance of roots is commonly associated with a stimulus to growth, there
may well be a reciprocal dependence of the shoot upon formative
substances from the root. A remarkable feature is, however, that the
extreme fastidiousness (with a few exceptions, see Ball, 1946) of the
angiosperm shoot in its requirements for growth in culture contrasts
with the ease with which fern apices will grow in media that are not
competent for the growth of angiosperm shoot tips. Clearly, however,
the initial requirements of the growing tip of the plumule itself are met
in the ovule, and here again one can only point to the formative
substances supplied from endosperm via the growing primordial
cotyledons. (Parenthetically, it may be suggested that the constituents
of endosperm, e.g. coconut milk, which have not proved effective for the
culture of isolated shoot tips, might prove to be effective if the isolated
tip were placed upon, or grafted upon, a bit of cotyledonary tissue
which is bathed by, or growing in, a medium containing the coconut
milk.)
C. Centres of Growth in the Vegetative Plant Body
Once established in the embryonic stage, the markedly contrasted
organization of shoot and root apices is persistent. Again, the gathering
impression is that the behaviour of the growing regions is as much the
product of organization as of any innate potentialities of constituent
cells. If cells are inherently totipotent, as now seems probable, their
different behaviour in the intact growing regions must be due to superimposed restrictions and controls which are features of the organization
of which they are a part. In other words, living cells with the same
inherent genetic potentialities may, by virtue of their position, be
induced to perform different metabolic tasks, or functions. This means
that inherent capacities for growth—ultimately traceable to the 'built-in
capacity of the embryo to grow'—are modulated by the environment of
the cells in the plant body. Such effects would seem to have a chemical
basis, since, as already shown, cells may be caused to grow by a variety
of cell division or enlargement factors, and they also may be withheld
from so doing by inhibitors which act reversibly upon part of the system
(see Section II C). Modern ideas about the organization of shoot and root
apices seem applicable here, for interpretations of the behaviour of shoot