Zea mays

CHAPTER 8
Sexual reproduction
Introduction
The great majority of ferns arc homosporous, with
numerous haploid spores being produced by
meiosis in the sporangia borne on the leaves of rhe
diploid plant (sporophyte, 1.7). When the spores
are shed rhey develop into small, but free-living,
green gametophytes (8.1) on which the gametes
3rc later formed. Fusion of the motile sperm with
the egg results in a diploid embryo; this reestablishes
the sporophyte generation which is
soon independent of the gamctophyrc.
The seed plants are heterosporous and do not
show a free-living gamctophyce generation. In the
gymnosperms a single haploid megaspore matures
to form the embryo sac within the naked female
ovule (1.5, 1.32). This megaspore is formed by the
meiosis of a solitary meg:lspore mother cell
located in the diploid nucellus (megasporangium).
Only one of the four resulram spores matures
while the others abort (1.32). Both the embryo sac
and nuceUus are enclosed within an inregumenr
and these strucrures collectively constitute the
ovule (1.32) which is retained on the tree (1.5).
The haploid megaspore divides repeatedly to form
a mass of gametoph}'tic tissue within the
enlargened embryo sac, and eventually one or
more eggs become demarcated within this tissue
close to the micropyle (8.2).
Following pollination of the ovule by a microspore,
the egg is fertilised by a sperm and the diploid
embryo develops. The testa of the marure
seed is formed from the modified integument of
the ovule, while copious food reserves for the
future seedling are contained within the tissue of
the female gametophyte surrounding the embryo
(1.32,8.2).
In flowering plants the ovule is enclosed within
an ovary (1.32) in contrast to the exposed ovule in
gymnosperms (1.5). The female gamerophyte
within the embryo sac is typically reduced to eight
cells and one of these, at the micropylar end of the
embryo sac. represents the egg (1.32). In contrast
to gymnosperms double fertilisation occurs in
angiosperms with one sperm fertilising the egg to
form the diploid embryo (1.33) while the other
sperm fertilises the twO central polar nuclei to
form the triploid, nutritive endosperm (1.32)
which is initially coenocytjc.
General features of flowers
Most flowers contain both male and female sex
organs (1.1,1.31) but some flowers are unisexual
(8.3) and may occur on the same plant (8.3) or
separale individuals. The flower of Magnolia
(1.31) illustrates the general features of a
dicolyledon. The upper half of the elongate
receptacle (floral axis) is covered by a large
number of spirally arranged and separate female
organs (carpels) which collectively constirute rhe
apocarpous gynoecium. Below the carpels numerous
spirally arranged male organs (stamens
constituting the androecium) are inserted ontO rhe
receptacle by shorr filaments. These are rerminalcd
by large anthcrs containing the pollen (microspores).
Ar the base of the receptacle there are
usually nine large, petaloid, periamh members.
In contrast with Magnolia, the monocotyledonous
flower of Lilillm (1.1) has six petaloid
periamh members arranged in two alternating
whorls. These encircle six stamens, while the
syncarpous gynoecium consists of three fused
carpels. The three comparnnents of the ovary arc
clearly visible on the fcuits developing after the
flowers wither (1.1). The ovary in Li/ium is
terminated by a long. slender style which is tipped
by a slightly swollen stigma. As in Magnolia the
ovary is superior since its poinr of insertion lies
above Ihe rest of the floral parts, but in orher
flowers the ovary may be inferior (8.4). In the
early dcvclopmcnl of most flowers the organ
primordia arise in centripetal sequence so that the
petiamh is iniri