Agriculture Reference
In-Depth Information
X
Calyx.
X
Corolla.
X
Androecium (stamens).
X
Gynoecium (carpels).
X
The
calyx
or ring of
sepals
which initially enclose
and protect the fl ower bud. The sepals are often
green and can therefore photosynthesize. In some
plants (e.g.
Fuchsia
), the sepals may be coloured to
attract animal pollinators, while in wind-pollinated
plants they may be reduced in size.
X
The
corolla
or ring of
petals
may be small and
insignifi cant in wind-pollinated fl owers, (e.g.
many tree species), or large and colourful in
insect-pollinated species (Figure 8.4).
Nectaries
may develop at the base of the petals. These have
a secretory function, producing substances such as
nectar which attract pollinating organisms.
The colours and size of petals can be improved in
cultivated plants by breeding, and may also involve
the multiplication of the petals or
petalody
, when
fewer male and/or female organs are produced,
e.g. many 'double-l owered'
Campanula
species
and
Gardenia augusta
(Figures 3.22 and 8.5).
X
The
androecium
, the male organ, consists of
stamens
, which bear
anthers
that produce and
discharge
pollen grains
, borne on a
fi lament
(Figure 8.3). The pollen contains the male sex cells
or gametes.
X
The
gynoecium
, the female organ, is positioned in
the centre of the fl ower and consists of an
ovary
enclosing one or more
ovules
which contain the
ovum (the female sex cell or gamete). The
style
leads from the ovary to a
stigma
at its top where
pollen is captured. The basic unit of the gynoecium
is the
carpel
, made up of a stigma, style and ovary.
More evolutionarily primitive fl owers such as
Ranunculus
(buttercup) have many separate carpels
but in most fl owers the carpels are fused to form
one large ovary, style and stigma (Figure 8.3).
The flower parts are positioned on the
receptacle
,
which is at the tip of the
pedicel
(flower stalk).
Associated with the flower head or
inflorescence
are
leaf-like structures called
bracts
, which can sometimes
assume the function of insect attraction - for example,
in
Euphorbia pulcherrima
(poinsettia),
Hydrangea
spp. and some
Cornus
species (see Fig. 7.22). See
'Inflorescences' on the companion website.
In many monocotyledons such as tulips and
lilies, the outer two layers of the flower have a
similar appearance, making the sepals and petals
indistinguishable (
tepals
) (Figure 8.6).
A distinguishing feature of monocotyledons and
dicotyledons is the number of flower parts - for
Figure. 8.4
Dicotyledonous
Papaver
(poppy)
fl ower with fi ve petals and a bee seeking pollen and
nectar
Figure 8.5
Petalody in a
Gardenia
fl ower
example, sepals, petals and carpels. In monocots these
are in multiples of three, whereas in dicotyledons,
flower parts are in multiples of four or five.
The flowers of most species have both male and
female organs (
hermaphrodite
), but some have
separate male and female flowers on the same
plant (
monoecious
) (Figure 8.7), such as water lilies
(
Nymphaea
) and members of the cucumber genus
(
Cucumis
), and many trees such as walnut (
Juglans
),
alder
(Alnus
) and birch (
Betula
), whereas others
produce male and female flowers on different plants
(
dioecious
), such as holly (
Ilex
), willows (
Salix
) and
Skimmia japonica
.
