V. DETERMINING
FACTORS IN CELL GROWTH
193
fusion', the second male nucleus unites with the two polar nuclei (Fig.
1(b)) to give rise to the primary endosperm nucleus which, by eventual
divisions, produces the special nutritive tissue—the endosperm (Figs.
2(a),
The physiological equivalent of the endosperm is formed in a different
fashion in gymnosperms, where it is the female gametophyte which is
independent of, and formed prior to, the actual processes of fertilization.
The endosperm of angiosperms and female gametophytes of gymnosperms, although somewhat similar in nutritional relationships, differ
fundamentally in the cytological and genetical complexity of their cells.
For, whereas the female gametophyte is characteristically haploid in its
chromosomal complement, the usual, though not invariable, condition
in the endosperm of an angiosperm is that it is triploid. Whereas the
female gametophyte of gymnosperms (Fig. 2(g)) is a nutritive organ
within which archegonia, and eventually fertilized eggs, are nourished,
for these structures develop within and surrounded by the gametophyte,
the situation in angiosperms is somewhat different.
In angiosperms, the primary endosperm nucleus, which by further
division develops the endosperm tissue, grows apace—more rapidly at
first than the fertilized egg—and it gives rise to the free nuclear condition and sometimes to a liquid endosperm. A good example of this is to
be seen in the coconut (Fig. 2(c)), in which some of the liquid endosperm
persists until the nut has reached its mature size and, even at this stage,
the embryo is relatively immature and undifferentiated (Fig. 2(a)). In
some plants the endosperm passes through a brief but similar liquid
state, in others it may be cellular from the outset, and in some cases
it may be quickly absorbed as the embryo develops. Thus, in many
angiosperms the endosperm is ephemeral, though a persistent endosperm
is a somewhat general characteristic of the monocotyledons. On the
FIG. 2. The Sources of Growth Factors and their Morphology,
(ar—archegonium;
cot—cotyledon;
do—degenerated
ovule; ec—endocarp;
emb—embryo;
end—endosperm;
fg—female
gametophyte; ge—germinating
embryo; ii—inner integument;
lend—liquid
endosperm;
η—nucellus;
oi—outer
integument;
ρ—pulp;
r—roots;
send—solid
endosperm; sh—shoot; t—crown gall tumour; ν—vesicle.) (a) L.s. fruit oí Cocos nucífera;
note the embryo embedded in solid endosperm (after Kirkwood and Gies, 1902).
(6, c) As (a), showing different stages of seed germination (after Kirkwood and Gies,
1902). (d) Free nuclear endosperm in Zea mays (after Randolph, 1936). (e) Cellular
endosperm in Zea mays (after Randolph, 1936). (/) L.s. ovule of Aesculus showing free
nuclear endosperm and vesicle at the chalazal end (reproduced from Steward and
Shantz, 1959; originally drawn by Mr. A. List), (g) L.s. ovule of Ginkgo (redrawn from
Coulter and Chamberlain, 1917). (h) Crown gall tumour on the stem of
Kalanchoe.
(i) The flower of banana (Musa sp.) at the time of shooting. (,;") Parthenocarpic fruit of
the same cut to illustrate the inner layers of pericarp which develop into pulp.
FACTORS IN CELL GROWTH
193
fusion', the second male nucleus unites with the two polar nuclei (Fig.
1(b)) to give rise to the primary endosperm nucleus which, by eventual
divisions, produces the special nutritive tissue—the endosperm (Figs.
2(a),
The physiological equivalent of the endosperm is formed in a different
fashion in gymnosperms, where it is the female gametophyte which is
independent of, and formed prior to, the actual processes of fertilization.
The endosperm of angiosperms and female gametophytes of gymnosperms, although somewhat similar in nutritional relationships, differ
fundamentally in the cytological and genetical complexity of their cells.
For, whereas the female gametophyte is characteristically haploid in its
chromosomal complement, the usual, though not invariable, condition
in the endosperm of an angiosperm is that it is triploid. Whereas the
female gametophyte of gymnosperms (Fig. 2(g)) is a nutritive organ
within which archegonia, and eventually fertilized eggs, are nourished,
for these structures develop within and surrounded by the gametophyte,
the situation in angiosperms is somewhat different.
In angiosperms, the primary endosperm nucleus, which by further
division develops the endosperm tissue, grows apace—more rapidly at
first than the fertilized egg—and it gives rise to the free nuclear condition and sometimes to a liquid endosperm. A good example of this is to
be seen in the coconut (Fig. 2(c)), in which some of the liquid endosperm
persists until the nut has reached its mature size and, even at this stage,
the embryo is relatively immature and undifferentiated (Fig. 2(a)). In
some plants the endosperm passes through a brief but similar liquid
state, in others it may be cellular from the outset, and in some cases
it may be quickly absorbed as the embryo develops. Thus, in many
angiosperms the endosperm is ephemeral, though a persistent endosperm
is a somewhat general characteristic of the monocotyledons. On the
FIG. 2. The Sources of Growth Factors and their Morphology,
(ar—archegonium;
cot—cotyledon;
do—degenerated
ovule; ec—endocarp;
emb—embryo;
end—endosperm;
fg—female
gametophyte; ge—germinating
embryo; ii—inner integument;
lend—liquid
endosperm;
η—nucellus;
oi—outer
integument;
ρ—pulp;
r—roots;
send—solid
endosperm; sh—shoot; t—crown gall tumour; ν—vesicle.) (a) L.s. fruit oí Cocos nucífera;
note the embryo embedded in solid endosperm (after Kirkwood and Gies, 1902).
(6, c) As (a), showing different stages of seed germination (after Kirkwood and Gies,
1902). (d) Free nuclear endosperm in Zea mays (after Randolph, 1936). (e) Cellular
endosperm in Zea mays (after Randolph, 1936). (/) L.s. ovule of Aesculus showing free
nuclear endosperm and vesicle at the chalazal end (reproduced from Steward and
Shantz, 1959; originally drawn by Mr. A. List), (g) L.s. ovule of Ginkgo (redrawn from
Coulter and Chamberlain, 1917). (h) Crown gall tumour on the stem of
Kalanchoe.
(i) The flower of banana (Musa sp.) at the time of shooting. (,;") Parthenocarpic fruit of
the same cut to illustrate the inner layers of pericarp which develop into pulp.