The flower’s function is to bring about sexual reproduction (the
production of offspring following the fusion of male and female nuclei).
The male and female nuclei are contained within the pollen grain
and ovule respectively and pollination is the transfer process. Crosspollination
ensures that variation is introduced into new generations of
occurs when pollen comes from the same flower (or a
different flower on the same plant) as the ovule, common in Fabaceae
Cross-pollination occurs when pollen comes from a flower of a
different plant, with a different genetic make-up from the ovule,
common in Brassicaceae (cabbage family).
Natural agents of cross-pollination are mainly wind and insects.
|Figure 10.2 Wind-pollinated species have small, inconspicuous flowers,
calamagrostis (b) Cyperus chira (c) Luzula nivea
The characteristics of wind-pollinated flowers
are their small size, their green appearance (lacking coloured petals),
their absence of nectaries and scent production, and their production
of large amounts of pollen which is intercepted by large stigmas. They
also often have proportionally large stigmas that protrude from the
flower to maximize the chances of intercepting pollen grains in the air.
The commonest examples of wind-pollinated plants are the grasses, and
trees with catkins such as Salix
(oak). The Gymnosperma (conifers) also have
wind-pollination from the small male cones.
|Figure 10.3 Insect-pollinated flowers,
e.g. (a) Day Lily ( Hemerocallis)
(b) Digitalis stewartii
(c) Verbascum ‘Cotswold Queen’,
coloured and sometimes have guidelines in the
The characteristics of insect-pollinated
flowers are brightly coloured petals (and scent production) to attract
insects, and the presence of nectaries to entice insects with sugary food.
Insects such as bees and flies collect the pollen on their bodies as they
fly in and out and carry it to other flowers. In tropical countries, slugs,
birds, bats and rodents are also pollinators. Some floral mechanisms,
e.g. in snapdragon and clover, physically prevent entry of smaller
non-pollinating insects and open only when heavy bees land on the
flower. Other plant species, such as Arum lily, trap pollinating insects
for a period of time to give the best chance of successful fertilization.
|Figure 10.4 Structural mechanisms to encourage
pollinated flowers are shown in
(a) and (b) snapdragon
to the weight of the bee
on the lower petal and
(c) and (d)
Primula flowers where the stamens
and style are arranged
spp. have stigma and stamens of differing lengths to
encourage cross-pollination; in thrum-eyed flowers, the anthers emerge
further from the flower than the stigma, so that insects rub against them
when reaching into the flower tube; in pin-eyed flowers, the stigma
protrudes from the flower and will catch the pollen from the same place
on the insect body, so ensuring cross-pollination (see Figure 10.4).
Bees in pollination
The well-known social insect, the honey bee (Apis mellifera)
, is helpful
to horticulturists. The female worker collects pollen and nectar in
special pockets (honey baskets) on its hind legs. This is a supply of
food for the hive and, in collecting it, the bee transfers pollen from plant
to plant. Several crops, such as apple and pear, do not set fruit when
self-pollinated. The bee therefore provides a useful function to the fruit
grower. In large areas of fruit production the number of resident hives
may be insufficient to provide effective pollination, and in cool, damp or
windy springs, the flying periods of the bees are reduced.
|Figure 10.5 Bumble-bee boxes provided in
pollination of tomatoes
It may therefore be advantageous for the grower to introduce beehives
into the orchards during blossom time, as an insurance against bad weather. One hive is normally adequate to serve 0.25 ha of fruit. Blocks
of four hives placed in the centre of a 1 ha area require foraging bees
to travel a maximum distance of 70 m. In addition to honey bees, wild
species, e.g. the potter flower bee (Anthophora retusa)
bumble-bee (Bombus lapidarius)
, increase fruit set, but their numbers
are not high enough to dispense with the honey bee hives.
All species of bee are killed by broad spectrum insecticides, e.g.
deltamethrin, and it is important that spraying of such chemicals be
restricted to early morning or evening during the blossom time period
when hives have been introduced.
In commercial greenhouses, the pollination of crops
such as tomatoes and peppers is commonly achieved by
in-house nest-boxes of bumble-bees, Bombus terrestris
(see Figure 10.5). Plant breeders may use blowflies in
glasshouses to carry out pollination. They also perform
mechanical transfer of pollen by means of small brushes.
When a pollen grain arrives at the stigma of the same plant
species, it absorbs sugar and moisture from the stigma’s
surface and then germinates to produce a pollen tube
pollen tube contains the ‘ male ’ nucleus (and also an extra
‘second nucleus’). These nuclei are carried in the pollen tube
as they grow down inside the style and into the ovary wall.
After entering the ovule, the male nucleus fuses with the
female nucleus, their chromosomes becoming intimately
associated. The term ‘gamete’
is used to describe the
agents, both male and female, that are involved in
fertilization. In animals, the gametes are the eggs and
sperms. In plants, they are the ovules and pollen.
in relation to fertilization, is a genetic
mechanism that prevents self fertilization, thus encouraging crosspollination,
e.g. in Brassicaceae. The mechanism operates by inhibiting
any of the following four processes:
- pollen germination;
- pollen tube growth;
- ovule fertilization;
- mbryo development.
in relation to fertilization , is a genetic mechanism that allows
self fertilization, thus encouraging self pollination, e.g. in Fabaceae.
where fertilization does not occur before fruit
formation, is a useful phenomenon when the object of the crop is the
production of seedless fruit, as in cucumber. It is usually accompanied
by a high level of auxin in the plant and may be induced in pears by a
spray of gibberellic acid.
The fertilized ovule (zygote)
undergoes repeated cell division of its
young unspecialized cells before beginning to develop tissues through
differentiation, that form the embryo within the seed.
Additionally, however, it should be noted that there is often a second
fertilization within the ovule, which has led to the term ‘double
fertilization’ . The second fertilization involves the ‘second nucleus’ of
the pollen tube (mentioned above) fusing with two extra (‘polar’) nuclei
present in the ovule itself. The resulting tissue consequently contains
three sets of chromosomes (triploid) and is called endosperm
Endosperm is a short-term food supply used by the embryo to help its
growth. Endosperm is found in the seeds of many plant families, but
is best developed in the grass family. In maize seed, for example, the
endosperm often represents more than half the seed volume. Anyone
making popcorn will be eating ‘exploded endosperm’ .
Not all parts of a seed are derived from embryo or endosperm origins.
The outer coat (testa) is formed from the outer layers of the ovule and
is thus maternal in origin. Also, the ovary, which contained the ovules
in the flower before fertilization, develops and expands to form the fruit
of the plant (see seed structure).
In the previous sections of this chapter, descriptions have been given
of the plant processes that lead to the development of a seed. Genetics
also requires some knowledge of the microscopic details of reproductive
cells and of the biological processes in which they are involved, since
this knowledge explains how plant characteristics are passed on from
generation to generation.