The flower
Morphologically a flower is a modified shoot. The elongated axis
(internodes) of a vegetative shoot is reduced into a horizontal disc, the
receptacle. Floral leaves are arranged on the receptacle in different
whorls. The flower arises in the axial of a leaf known as bract. Usually
the flower has a short stalk or pedicel, sometimes the pedicel is absent
and the flower is then sessile. Sometimes the flowers are solitary, more
commonly they are grouped together on a flower bearing shoot known as
the inflorescence.
The different parts of the flower
Generally the flower consists of some floral leaves, which are placed on
the floral axis or the receptacle.
a. Subsidiary or sterile leaves (Perianth): its protect the
essential or fertile leaves.
b. Essential or fertile leaves (Sporophylls): these are spore
bearing and we have either:
I. Microsporophylls or male fertile leaves,
also known as stamens.
II. Megasporophylls or female fertile leaves,
also known as carpels.
Floral leaves are arranged on the receptacle in a special way known as
phyllotaxy. There are two types of phyllotaxy:
1. Spiral (Acyclic): characteristic to primitive
flowers.
2. Whorled (Cyclic): characteristic to advanced
flowers, the different types of the leaves are
arranged in distinct whorls or cycles. When the
numbers of whorls are 3, the flower is tricyclic as
in monocotyledons.
Perianth: In the majority of dicotyledons, the perianth segments are
usually differentiated into outer whorl (sepals), these are form the calyx
of the flower. The inner whorl (petals) of coloured leaves, forms the
corolla of the flower.
Perianth segments are either free or united into different patterns (funnel,
tubular, salver-shaped, bilabiate, radiate, etc..).
Aestivation
Arrangement of the floral parts in the bud.
There are different forms of arrangement in the bud:
a. Valvate: with the parts meeting by their abrupt edges
without overlapping or turning.
b. Imbricate: in aestivation of five parts one being exterior,
one interior and the rest three having one margin exterior
and other interior. Descending imbricate: posterior petal is
outer most as in pela; ascending imbricate, posterior petal is
inner most.
c. Quincuncial: in aestivation, partially imbricated of five
parts, two being exterior, two interior, and a fifth one having
one margin exterior and the other interior.
d. Twisted (Convolute): with parts rolled up in such a way that
the outer part of each covers the inner part of the one in front
of it, while in turn its inner part is covered by the one behind
it.
e. Vexillary: an aestivation when there are five petals, of
which the posterior one is the largest and it almost covers the
two lateral petals, and the latter in their turn nearly overlap
the two anterior or smallest petals.
Stamens
Androecium: the androecium of the flower consists of one or more
stamens. The stamen is usually differentiated into a filament and anther.
The anther consists of one or two lobes. Each lobe contains 2 pollen sacs,
when pollen grains are produced. Pollen grains vary greatly in shape and
size in different plants and their characters are proved to be of taxonomic
value. Each pollen grain has two walls: inner thin wall made of cellulose,
the intine and outer thick wall made of sporopollenin, the exine.
Gynoecium: The gynoecium of a flower consists of one or more
carpels. Carpels maybe free in primitive flowers, each carpel forms a
pistil. In advanced flowers, carpels are united to one pistil.
The pistil consists of a lower dilated part known as the ovary, which
carries one or more ovules on its inner surface. The ovary is extended
upwards into a cylindrical style, which is terminated by a variously –
shaped stigma. The ovules are attached to the inner wall of the ovary in
places known as placentae. Each ovule has a short stalk known as the
funicle, which attach the basal part of the ovule to the placenta. The ovule
has in the middle an embryo sac, which is surrounded by a nutritive
tissue, the nucleus. A double wall, the integuments, envelops both.
Between the integuments there is an opening, which is known as
micropyle, through which the pollen tube passes to the embryo sac during
fertilization.
Ovules have three characteristic positions:
a. Orthotropous: Funicle is straight; micropyle and chalazal
ends are on the same line.
b. Anatropous: Funicle is recurved, micropyle and chalazal
ends are on two parallel lines.
c. Campylotropous: Funicle is curved; micropyle and chalazal
ends are at right angles.
Flower terminology
Floral formulas provide an easy way to write down the important features
of flowers. Ca = Calyx; Co = Corolla; A = Androecium; G = Gynoecium;
black bar = Receptacle Apocarpous - carpels separate, not fused;
Syncarpous - carpels fused to form a single unit or compound.
Fertilization
This process starts when pollen grains come in contact with the stigma. In
that time the male gametophyte is fully developed inside the pollen grain,
also the female gametophyte within the embryo sac.
Apoolen tube comes out from the pollen grain, the male gametes move
towards the end of this tube. The pollen tube penetrates gradually the
tissues of the stigma and style and continues within the ovary to the
olvule. The pollen tube passes to the embryo sac, either through the
micropyle or through the chalaza. The pollen tube releases the two male
gametes inside the embryo sac. One of these gametes succeeds to fertilize
the egg and a diploid (2n) nucleus is formed which is known as the
zygote. The other male gamete fuse with the primary endosperm nucleus
(2n) and a triploid nucleus (3n) is formed.
Embryo and seed formation
The zygote divides repeat idly to give the embryo. In the same time the
endosperm nucleus divides extensively to form the endosperm, which is a
nutritive tissue. The developing embryo feeds on the endosperm, and if it
consumes it completely, nothing is left in the future-formed seed,
therefore known as non-endospermic seed.
If a part of the endosperm is left after the complete formation of the
embryo, the seed is known to be endospermic.
When the embryo is fully developed, the integuments are solidified to
form the testa of the seed. In the sometime certain changes take place in
the ovary and the fruit is formed.
This process can be summarized in the following diagram.
Micropyle Micropyle
Ovule
Fruit
Embryo sac Embryo
Seed
Ovary----
Integuments Testa
Wall Pericarp
1. Monoecious plant
The plant has unisexual flowers, the stamens and ovary occurring on the
same individual.
2. Dioecious plant
The flowers unisexual. The stamens occurring on one individual and the
ovary occurring on the other one.