V.
DETERMINING FACTORS IN
CELL GROWTH
199
an alternative source of this activity, but with one important difference.
In the coconut (Fig. 2(a), (b) and (c)) the liquid and solid endosperm develop precociously and store a great reservoir of nutrients in advance of
the slow-growing embryo. In Zea, however, immediately after fertilization the endosperm grows rapidly (Fig. 2(d) and (e)), and in the familiarly
known 'milk stage' possesses a marked activity to induce cell division
in the carrot tissue. However, the corn embryo develops rapidly, and as
its growth proceeds, the ability of the endosperm to stimulate carrot
cell growth declines concurrently. The residual activity of the endosperm
to induce cell divisions in carrot tissue is at a maximum two weeks after
pollination (Steward and Caplin, 1952); and at maturity of the grain,
i.e. in the fully formed caryopsis, it has virtually disappeared. In
contrast, the ability to induce growth by cell division in carrot tissue
persists in the coconut milk at any stage of development so long as the
embryo is dormant. Therefore, one associates this activity in the liquid
endosperm with the nutritive arrangements that supply the immature
embryo, and, as indicated above, the probability is that the effect is at
a maximum before the embryo has formed the cotyledon.
The search for a comparably rich source of these cell division stimuli
in endosperms of dicotyledonous plants was not at first successful.
Eventually rich sources of similar stimuli were discovered, first in fruits
of Juglans (Steward and Caplin, 1952) and later in Aesculus (Steward
and Shantz, 1959), harvested before the cotyledons had fully absorbed
the liquid endosperm which bathes them. Here again the stimulus to
cell division resides in a liquid endosperm which normally fosters the
growth of the young embryo (Fig. 2(f)). Parallel effects have been sought
and found in similar morphological situations. In Ginkgo, a gymnospermous tree, the female gametophyte (sometimes called endosperm)
is a large fleshy structure which nourishes the growth, first of the
archegonia and later of the embryo (Fig. 2(g)). Extracts of the female
gametophyte of Ginkgo will also induce cell division in explants of
carrot root (Steward and Caplin, 1952).
But often stimuli to cell division arise in other circumstances. In some
plants, notably certain hybrids of tobacco, the induction of growth in
cells that would normally remain quiescent may cause the spontaneous
formation of tumours (Kehr and Smith, 1952). In still other situations
a tumour (Fig. 2(A)) may be induced by the intervention of an organism, such as that which causes crown gall (Smith, 1920), or of a virus
(Black, 1947).
It is interesting that hybrids of tobacco, which spontaneously form
tumours, are also imbalanced with respect to those factors which tend
to promote cell division on the one hand, and those which tend to
suppress it as cells mature (Steward et al., 1955). Thus, an extract of
DETERMINING FACTORS IN
CELL GROWTH
199
an alternative source of this activity, but with one important difference.
In the coconut (Fig. 2(a), (b) and (c)) the liquid and solid endosperm develop precociously and store a great reservoir of nutrients in advance of
the slow-growing embryo. In Zea, however, immediately after fertilization the endosperm grows rapidly (Fig. 2(d) and (e)), and in the familiarly
known 'milk stage' possesses a marked activity to induce cell division
in the carrot tissue. However, the corn embryo develops rapidly, and as
its growth proceeds, the ability of the endosperm to stimulate carrot
cell growth declines concurrently. The residual activity of the endosperm
to induce cell divisions in carrot tissue is at a maximum two weeks after
pollination (Steward and Caplin, 1952); and at maturity of the grain,
i.e. in the fully formed caryopsis, it has virtually disappeared. In
contrast, the ability to induce growth by cell division in carrot tissue
persists in the coconut milk at any stage of development so long as the
embryo is dormant. Therefore, one associates this activity in the liquid
endosperm with the nutritive arrangements that supply the immature
embryo, and, as indicated above, the probability is that the effect is at
a maximum before the embryo has formed the cotyledon.
The search for a comparably rich source of these cell division stimuli
in endosperms of dicotyledonous plants was not at first successful.
Eventually rich sources of similar stimuli were discovered, first in fruits
of Juglans (Steward and Caplin, 1952) and later in Aesculus (Steward
and Shantz, 1959), harvested before the cotyledons had fully absorbed
the liquid endosperm which bathes them. Here again the stimulus to
cell division resides in a liquid endosperm which normally fosters the
growth of the young embryo (Fig. 2(f)). Parallel effects have been sought
and found in similar morphological situations. In Ginkgo, a gymnospermous tree, the female gametophyte (sometimes called endosperm)
is a large fleshy structure which nourishes the growth, first of the
archegonia and later of the embryo (Fig. 2(g)). Extracts of the female
gametophyte of Ginkgo will also induce cell division in explants of
carrot root (Steward and Caplin, 1952).
But often stimuli to cell division arise in other circumstances. In some
plants, notably certain hybrids of tobacco, the induction of growth in
cells that would normally remain quiescent may cause the spontaneous
formation of tumours (Kehr and Smith, 1952). In still other situations
a tumour (Fig. 2(A)) may be induced by the intervention of an organism, such as that which causes crown gall (Smith, 1920), or of a virus
(Black, 1947).
It is interesting that hybrids of tobacco, which spontaneously form
tumours, are also imbalanced with respect to those factors which tend
to promote cell division on the one hand, and those which tend to
suppress it as cells mature (Steward et al., 1955). Thus, an extract of