Sexual reproduction in flowering plants 45
germinating
pollen grain
2
stigma
male gametes-
style
tube nucleus
pollen tube
3 antipodal cells
space (locule).
integuments
nucellus
ovule
- embryo sac
2 synergids
diploid nucleus formed by fusion of polar nuclei
ovum
funicle
micropyle-
2 male gametes (nuclei) derived from the generative nucleus
tube nucleus
Figure 3.13 LS carpel at fertilisation.
le
Development of the embryo and seed
Immediately after fertilisation, the ovule is
Te-termed the seed. Thus
a seed is a fertilised ovule.
The following changes take place during development of the seed (you will need to refer to figures
3.7, 3.14-3.16 when studying these). The example
chosen to illustrate the changes in detail is
shepherd's purse.
1 The integuments become the testa. This is a
thin, tough, layer around the seed. It protects
the seed from mechanical damage and also
often contributes to the dormancy mechanism
as described in chapter 5.
2 The nucellus disintegrates as the seed develops
supplying nutrients for growth of the embryo
and the endosperm in endospermous seeds
(see below).
3 The triploid endosperm nucleus divides repeat
edly by mitosis to form the triploid endosperm.
The nuclei become separated from one another
by thin cell walls. In some seeds (endospermous
seeds) such as cereals, this remains as the food
store for the seed. In non-endospermous seeds
such as the pea and shepherd's purse, the
cotyledons (see below) grow at the expense of
the endosperm, which may then disappear
altogether.
4 The zygote grows by repeated mitotic divisions
to become an embryo and a suspensor with a
basal cell' at its base. The suspensor is a short