Agriculture Reference
In-Depth Information
The flowering plant represents the pinnacle of
evolution in the plant world and there is no doubt
that in the ornamental garden it is the contribution
of flowers which is often at the forefront of plant
selection (Figure 8.2).
Flowers are the organs of
sexual reproduction
in
flowering plants. Their structure forms the basis,
in large part, of plant classification and reflects the
different pollination mechanisms used and, in the case
of animal-pollinated flowers, the need to advertise
their wares! They are also where fertilization, the
fusion of the male sex cell in pollen and the female
sex cell in the ovule, takes place. Sexual reproduction
leads to the development of seeds and fruits, the
means by which plants spread. It also brings about
mixing of the genes contributed by each 'parent' so
that the offspring will be similar but not identical to the
parent plants and each other. This range of variation
enables plants to withstand changes in environmental
conditions as there will always be some individuals
which are likely to survive.
In contrast, plants can also reproduce
asexually
through many natural means such as runners, bulbs,
or by layering or suckering, enabling plant dispersal
(see Chapters 7 and 11). The resulting plants are
genetically identical to the parent plant (clones). It is an
alternative to sexual reproduction, useful if pollination
is poor or seed production fails, but does not provide
the genetic variation which is necessary for long-term
survival and adaptation of the species. It is, however,
very useful in vegetative propagation where the aim is
to maintain the desirable characteristics of the parent
plant (see Chapter 11).
Sexual reproduction
is the formation of new
individuals through fusion of male and female
sex cells (gametes). It results in variable
offspring.
Asexual reproduction
is the formation
of new individuals without fusion of gametes
resulting in genetically identical offspring.
Flower structure
However varied flowers appear, they all have the
same basic structure with the flower parts arranged in
four whorls (Figure 8.3). These are the:
Petal (corolla)
8
Anther
Stamen -
male organ
Filament
Stigma
Style
Carpels-
female
organ
Ovary
Ovule
Sepal (calyx)
Receptacle
Pedicel
(a)
(b)
Figure 8.3
Flower structure: (a) fl ower of (
Glaucium
corniculatum
); (b) diagram of a typical dicotyledonous
fl ower to show structures involved in the process of
sexual reproduction; (c) long section of a pumpkin
fl ower showing anthers (with yellow pollen), lobed
stigma in the centre and developing ovary with
immature seeds on the right
(c)
