Zea mays

tube frequently corers the embryo sac via the
micropyle (1.32) but in rare cases may directly
penetrate the integuments. Once entry to the
embryo sac has been gained, a pore forms near the
tip of the pollen tube and the twO sperms 3re
liberated. One sperm nucleus fuses with the egg
nucleus and the other fertilises the centrallylocated
polar nudei (1.32). In most flowering
plants the cytoplasm surrounding the sperm nudei
is not transmitted at fertilisation and therefore
inheritance of chloroplasts and other organelles is
generally via the female line.
Development of the seed
llltroduction
The fertilisation of the haploid egg by a haploid
sperm gives rise to a diploid zygote which
subsequently divides repeatedly and develops in a
highly organised manner into the embryo (1.33).
The marure embryo commonly undergoes a period
of dormancy within the protccri\'e seed coat (testa)
which develops from the integuments of rhe
cnlargencd ovule (1.33). The embryo is packed
with food reserves (2.54); in albuminous seeds
further food is stored in the mass of endosperm
surrounding the embryo. In the Caryophyllaceae
little endosperm is formed but the nucellus
develops into a nutritive pcrisperm.
Embryo development
Following entry of the sperm nucleus through the
wall-free region of the egg cell, a wall is secreted
in this area of the zygote. Its nucleus then undergoes
mitosis and usually a transverse wall divides
the zygote into basal and terminal cells. The basal
cell divides mainly transversely to form a suspensor
(1.33); this pushes the terminal cell away from
the micropyle and into the endosperm which is
developing (1.33, 2.5, 8.37, 8.38) within the
expanding embryo sac. In legumes the suspensor
cells often show highly polyploid, amoeboid,
nuclei (8.39) and apparently synthesise growth
substances of importance for the development of
the embryo. The terminal cell undergoes divisions
in various planes to form a globular proembryo
(8.38) and soon an outer layer of anticlinally
dividing protoderm cells becomes established. In
dicotyledons the enlarging globular procmbryo becomes
transformed into a heart-shaped structure
(1.33) as paired cotyledons at its chalazal end
develop.
The embryo now elongates and differentiation
of the procambium and ground tissues occurs
(8.40). The radicle apex becomes demarcated at
the micropylar pole of the embryo and merges
into the hypocotyl above (8.41). :\1eanwhile, at
the other end of the hypocoryl and between the
148
cotyledons, a bulge representing the plumule
becomes apparent. In the mature seed the
plumular apex may either remain small (8.41) or
is larger and has already given rise to its first
foliage leaves (4.1, 8.42, 8.43), while the radicle
shows a root cap and apex (8.44). Depending
upon the architecture of the embryo sac the
embryo may be straight or variously curved (1.33,
8.41). In albuminous seeds abundant endosperm is
present and the cotyledons are thin and leaf-like,
in contrast to their swollen appearance in nonendospennous
seeds (8.41, 8.42).
In monocotyledons, the early development of
the embryo parallels that in dicotyledons, but only
a single lateral cotyledon is formed (1.21).
Monocotyledonous seeds are commonly albuminous
with the cotyledon apparently acting as a
digestive organ. In palms the cotyledon often
enlarges greatly and becomes haustoriaI, whilst in
grasses the scutellum, which divides the endosperm
from the embryo proper, is sometimes
regarded as a modified cotyledon. During early
germination the plumule of grasses is protected by
a cylindrical leaf-like coleoptile while the radicle is
initially ensheathed by the coleorhiza (8.45). In
some flowering plams (begonias, orchids) the
seeds are minute and the mature embryo shows
little morphological differentiarioD_
Endosperm
In the majority of angiosperms two haploid pol:u
nuclei occur in the embryo sac and their fusion
with one of the sperm nuclei leads to the development
of the triploid primary endosperm cell
(1.32). However, in some species multiple polar
nudei occur so that the resulting endosperm is
polyploid. In a few species fertilisation of the
polar nuclei does nOt occur and in others division
of the primary endosperm nucleus ceases very
early. In many albuminous seeds (8.43) the
nuceJ1us and the inner imegument degenerate as
the endosperm develops in the maruring seed.
In most flowering plants the initial divisions of
the primar}' endosperm nucleus are not followed
by cytokinesis and a coenocytic nuclear endosperm
develops (2.5, 8.37). However, subsequent
free-wall formation leads to the celluJarisation of
the endosperm (1.33, 8.32). These walls resemble
those in some in vitro cultured tissues (2.62) and
in conrrast to normal cell plate development
neither microtubules nor Golgi bodies are
apparenrly involved in their growth. The random
wall formation results in some endosperm cells
being multinucleate. In the mature fruit of Cocos
(coconut, 8.46, 8.47) the watery milk of the
enclosed seed represents the remnants of the
coenocytic cytoplasm while the white flesh results
from its cellularisation.