Zea mays

The stigma is the pollen receptor (1.32, 8.11);
in wind-pollinated flowers the stigmas are often
feather-like, while in other plants the stigmatic
epidermis is either papillose or hairy (8.26). The
majority of carpels possess a style (8.13 to 8.15)
but in some species this is very short (1.31). In
syncarpous species in which the styles are also
fused, there is usually one main longitudinal vascular
bundle per stylar component (8.13, 8.28).
The centre of the style consists of transmitting
tissue (8.28) and, after the pollen grains germinate
(8.29), the pollen tubes grow downwards through
this tissue towards the ovary (1.32), absorbing
nutrients from this tissue en route (8.30).
The ovules are attached to a thickened region
of the ovary wall termed the placenta (1.32). The
carpel is generally interpreted as a folded and
modified leaf with its abaxial surface outermost;
its margins are normally fused and typically twO
longitudinal placentae lie internally (adaxially),
close to the fused leaf margins (8.31). In syncarpous
ovaries axile placentation is common,
with the margins of the carpels fused at the centre
of the ovary (1.6, 8.27). However, the placentae
may also be located at the outer margin of the
ovary (parietal placentation) or basally (8.4). In
the apocarpous ovary marginal placentation is
common (8.31).
Each ovule is connected at its base (chalaza) to
the placenta via the funiculus (1.32, fLU, 8.33).
The ovule is invested by two (or sometimes one)
thin integuments enclosing the nucellus. At the
apical end of the ovule a narro\,,'" channel, the
micropyle, penetrates the integuments (1.32) to
expose the surface of the nucellus. The orientation
of the ovule relative to the funiculus is variable
but commonly the funiculus is bent through 180
degress (anatropous ovule, 1.32, 8.33). The ovary
walls and placentae are usually richly vasculated
(8.31) and a number of smaller veins branch into
the ovules (8.33) but vascular tissue rarely extends
into the nucellus.
Early in development of the ovule a megaspore
mother cell becomes demarcated at the micropylar
end of the nucellus (megasporangium, 8.34) and
undergoes meiosis. In the commonest situation
(Polygollllm, monosporic type) a vertical row of
four haploid megaspores is formed. Only the
deepest-sited (chalazal) cell develops further while
the outer three megaspores degenerate (8.35). The
single functional megaspore enlarges greatly
\vithin the expanding nucellus and develops into
the embryo sac (1.32, 8.33, 8.35). The original
single haploid nucleus divides (8.35) to give the
eight nuclei characteristic of the mature embryo
sac (megagametophyte, 1.32).
Other patterns of megaspore mother cell
division also occur. In the bisporic type, one of the
TWO derivatives of the first meiotic division
undergoes further divisions and gives rise to the
eight nuclei of the embryo sac while the other
derivative degenerates. In tetrasporic development,
the megaspore mother cell (8.34) undergoes
meiosis but all four nuclei are confined within the
common cytoplasm of the embryo sac. These
nuclei then undergo a variahle number of mitoses,
so that the mature embryo sac often contains
more than eight nudei. In the semi-mature embtyo
sac of Lilium (8.36) one haploid nucleus is located
at the micropylar pole but the other three nuclei at
the chalazaI end fuse to give a triploid nucleus.
Subsequent mitoses and rearrangement of the
nuclei lead to three haploid nuclei being situated
at the micropylar end, one haploid plus one
triploid nucleus in the centte of the embr>'o sac,
and three triploid nuclei at the chalazal end.
In the common monosporic type the eight
nuclei and associated cytoplasm hecome separated
by thin walls and ate located in various regions of
the embryo sac (1.32). Three antipodal cells are
located at the chalazaI end of the sac, two synergids
and a median egg cell lie at the micropylar
end \vhi1e the central region contains two polar
nuclei within the endosperm mother cell. The egg
cell is normally larger than the synergids; its
nucleus and mOst cytoplasm usually lie towards
the chalazal pole and here the wall is scantily
developed. In the synergids, the cytoplasm is
concentrated at their micropylar poles and their
walls may be modified into a wall labyrinth
(filiform apparatus) similar to that present in
transfer cells (2.63). The synergids apparently
transport nutrients from the nucellus to the egg
and may form absorptive haustoria within this
tissue. [n some species the synergids have a role in
fertilisation. The antipodal cells commonly
degenerate before fertilisation of the egg, but they
may persist and also develop haustoria.
Fertilisation
When a compatible pollen grain is deposited on
the stigma (1.32) a pollen tube grows out through
one of the germination pores in the pollen grain
wall (8.25) \...·here the exine is poorly developed.
The cytoplasm and the nuclei migrate into the
pollen tube which, after penetrating the stigma,
grows downwards in the stylar transmission tissue
(8.30).
In the anther, the generative nucleus of the
pollen grain may already have divided into two
sperm nuclei; otherwise this occurs within the
pollen tube (1.32). The nuclei and cytoplasm
concentrate at the tip of the elongating pollen
tube; behind this the tube is highly vacuolate and
is often sealed off by plugs of calJose. The pollen
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