POLLINATION
What is Pollination?
Pollination is the Process of transference of pollen grains
from the the anther to the stigma.
The stamens of a flower are the male organs. The anthers of
the stamens produce powdery material called pollen which consists of particles
called pollen grains. Each pollen grain contains nuclei that participate in
reproduction. For initiating this process of formation of fruit and seeds the
first step is that the pollen grains must reach the stigma. It may happen in
three principal ways.
1. The pollen of the same flower may fall on its stigma by itself
(autogamy).
2. The pollen of another flower of the same plant may fall on the stigma
(geitonogamy).
3. The pollen of a flower of another plant of the same species may fall on the
stigma.
This transference can occur through wind, insects, or other
agents and the term used for this transference is cross pollination.
Pollination must occur between plants of the same species.
Many different kinds of flowering plants may be growing in the same vicinity.
For example, an orchard may have trees of mango, guava, litchi, etc., and even
some wild trees like neem, jamun or sheesham may also be growing in the
neighbourhood. Similarly, various crops may be growing in neighbouring
agricultural fields—wheat, mustard, pulses, vegetables and so on. The wind may
blow away the pollen from all such plants together and thus the flowers of a
particular plant may receive all kinds of pollen of its own kind as well as of
others.
Similarly insects like bees and butterflies generally do not
discriminate between plants and collect nectar from all Kinds of flowers. In
the process, the insects are carrying different types of pollen on their
bodies. Therefore, the stigma of a flower of any one particular plant receives
pollen of its own species as well as of others. But Nature has so designed that
only the pollen of the same species of plant will survive and grow further to
continue the processes for reproduction and the rest of the types of pollen
will fail to germinate and perish.
Kinds of pollination
There are two kinds of pollination: self-pollination and
cross-pollination.
SELF-POLLINATION
Self-pollination is the transfer of pollen from the anther
to the stigma of the same flower (autogamy), or to the stigma of another flower
of the same plant (geitonogamy). Pollination between flowers of the same plant
is considered self-pollination on account of the common genetic characters—i.e.
the general qualities or features of any one plant are the same in the
different flowers borne on it. the anthers and stigma lie close to each other
which mature at the same time and self-pollination is assured.
Self-pollination can occur in bisexual flowers. in flowers
having both male and female organs, or it can also occur in such unisexual
flowers where both male and female flowers are borne on the same plant. To
ensure self-pollination, it is necessary that the anther and stigma of a flower
must mature at the same time, or if the flowers are unisexual and bore on the
same plant then they must also mature at the same time.
ADVANTAGES AND DISADVANTAGES OF SELF-POLLINATION
Advantages of self-pollination-
1.It is much surer in such bisexual flowers where stamens
and carpels mature at the same time.
2.Parental characters are preserved indefinitely.
3.There is no wastage of pollen grains. Even a small quantity of pollen will
suffice.
4.The flowers need not be large and showy.
5.Scent and nectar need not be produced by flowers.
Disadvantages of self-pollination-
1.Continued self-pollination, generation after generation,
may lead to weakening of the variety or the species. The seeds produced through
it are poor in quality and give rise to less vigorous offspring.
2.The weaker or defective characters of the variety or breed cannot be
eliminated.
3.It does not yield new varieties. The genetic traits of the same plant with no
change and without any intermixing are passed on to the next generation. Thus
there is little chance for improvement in the next generation.
CROSS-POLLINATION
Cross-pollination is the transfer of pollen from the anthers
of flowers of one plant to the stigma of a flower of another plant of the same
species. This too has advantages and disadvantages.
Advantages of cross-pollination-
1.The offspring are healthier.
2.The seeds produced are abundant and viable.
3.New varieties may be produced by cross-pollinating two different varieties of
the same species or even two species.
Disadvantages of cross-pollination-
1.The pollination is not always certain because some
pollinating agent is always needed which may or may not be available at the
proper time.
2.The pollen has to be produced in large quantity to ensure chances of
pollination which means a lot of wastage of pollen.
3.The process is uneconomical for the plant because the flowers have to be
large, coloured, scented and have to produce nectar—for attracting pollinating
agents.
The differences between self-pollination and cross-pollination
are summarised in Table
conditions which favour cross-pollination are as follows.
Unisexuality. The flowers may be either male or
female and they may be borne on separate plants. In this way cross-pollination
is the only possibility. palms and papaya. (Papaya usually has separate
male and female trees, but less frequently there may also be a hermaphrodite
tree bearing both kinds of flowers). In cucumber, gourd, etc., the male and
female flowers may be borne on the same plant.
Dichogamy. (Different timings of maturation of
androecium and gynoecium). In many bisexual flowers the anthers and stigma of
the same flower mature at different times.
In some plants anthers of the flower mature earlier than the
stigma (protandry). Example- bhindi, sweet pea, salvia, sunflower.
In some plants, the stigma of the flower matures earlier than the anthers.
Example- custard apple, peepal.
In either case, cross-pollination is the only possibility.
Self-sterility. This is a condition in which
even if the stigma receives pollen from the anthers of the same flower, the
pollen fails to undergo further growth. In such cases, only the pollen from
another plant of the same species, can effectively complete the process of
setting of the seeds. Example- ray florets of sunflower, orchids, etc.
Herkogamy. (Mechanical or Structural barriers)
In some flowers the pollen of a flower cannot reach the stigma of the same
flower.
For example, a hood covering the stigma acts as a mechanical
barrier in Pansy flowers, Iris, etc.
Heterostyly. In such flowers the stigma and
anthers grow at different heights which does not favour self-pollination.
Example- prim rose, oxalis.
AGENTS OF CROSS-POLLINATION
The two commonest agents of cross-pollination are insects
and wind. But, some flowers are also pollinated by certain animals and birds,
like squirrels, bats, etc. or even by water in the case of some
aquatic plants. Each category has some special features to promote chances of
pollination.
Insect-pollinated have the following
characteristics:
1. The flowers are large.
2. They are usually brightly coloured to attract insects.
3. They usually emit scent for attracting insects.
4. They produce nectar which is food for the insects.
5. The pollen grains are sticky or spiny to enable them to be carried by the
insects easily.
6. The stigma is sticky and does not generally hang out from the flower.
7. The flowers tend to be in clusters to make them conspicuous, especially in the
cases where individual flowers are small, Example- Dahlia.
Wind-pollinated flowers usually possess the
following special features. Example Maize.
1. The flowers are small.
2. They are usually not brightly coloured and often dull green.
3. They do not produce scent or nectar.
4. The stamens are long and hang out of the flower to be exposed to wind.
5. The anthers are large and loosely attached to the filaments so that the
slightest wind may move them (versatile).
6. Pollen is produced in very large quantities.
7. Pollen grains are light, dry and smooth so that they can easily be carried
away by wind.
8. The stigmas are feathery and hang out of the flower to trap the pollen
grains.
Water-pollinated flowers are found only in
aquatic plants. They have the following characteristics:
1. Pollen grains are produced in large numbers.
2. In some plants the pollen grains have a specific gravity almost equal to
that of water so that they remain floating below the surface of water.
3. In some special cases male flowers are such that they float on the surface
of water till they meet female flowers Example- Vallisneria.
Some flowers are pollinated by birds Example- Bignonia, canna.
Elephophily is the pollination affected by elephants.
Elephophily is found in Rafflesia whose flowers are very large and are found at
ground level. The pollen grains of one flower get attached to the feet of
elephants and may be carried to the stigma of another flower when trampled by
those feet.
Artificial pollination When man himself
transfers pollen to the stigma it is called artificial pollination. In the
ancient civilization of Babylonia, it was a common practice to sprinkle “male
flowers” of palms on the “female” flowers of course, at that time they neither
knew the sexuality of palm flowers nor anything about pollination.
In modem times, artificial pollination (also called artificial crossing) is a standard practice adopted by plant breeders and scientists in to evolve new varieties. They remove the anthers in young flowers (emasculation) and cover these flowers with plastic bags. Later, they pollinate such flowers with the pollen from the plants of the desired variety.
SOME EXAMPLES OF POLLINATION
In insect-pollinated sweet. Pea.
the insects such as the bee, alight on the conspicuous
corolla. The bee thrusts its long tongue in search of nectar. In this struggle,
the ‘wing petals’ of the flower are depressed. The ‘wing petals’ along with the
‘keelpetal’ are forced down and the stamens and stigma touch the underside of
the body of the insect. If this insect has already visited another flower it
may have some pollen sticking on its body. The pollen is now transferred to the
stigma of this new flower thus - affecting pollination. When the insect flies
away, it may, in turn, carry some pollen of this flower to other similar
flowers. However, if cross-pollination does not take place, there is every
chance of self-pollination in this case, since both the stamens and the carpels
are enclosed within the keel and are more or less at the same level.
In wind-pollinated maize.
pollination is brought about by wind. As the wind blows, the
pollen from the outwardly hanging anthers (in the tassel) is blown away easily
because theanthers are so loosely attached to the filament that the slightest
wind will shake them. The pollen blown away by the wind may fall on the
feathery stigmas of the female flowers which have a large surface for this
purpose. The male flowers mature earlier than the female ones, so that there
are less chances for self-pollination
FERTILIZATION
Fertilisation is the union/fusion of the nuclei of male and
female gamete.
In flowering plants
The pollen grain is the male gamete.
The ovule inside the ovary is the female gamete.
The Pollen Grain
The mature pollen grain is a cell with a double wall — the
outer exine and the inner intine. Its nucleus has already divided into a tube
nucleus and a generative (male) nucleus. At this stage, the pollen is
transferred to stigma (pollination). Further changes in pollen grain occur only
if it has fallen on the stigma of a plant of the same species.
The Ovule
Ovule is the inner part of ovary. Ovule is destined to
become the seed and the ovary to become the fruit containing the seed inside.
There may be e asingle ovule producing single seeded fruit, or e many ovules
producing a many-seeded fruit. Each ovule has one or two protective coverings,
the integuments.
The integuments leave a small opening the micropyle at one
end (for the entry of pollen tube). Enclosed by the integuments is the nucellus
(a mass of food laden cells), and further inside the nucellus is the embryo
sac.
Two fertilisations (double fertilisation)
1. Two polar nuclei unite to form secondary nucleus and then
one sperm nucleus from the pollen tube unites with the secondary nucleus, to
form endosperm cell. (This is described as triple fusion, involving 2 polar
nuclei and 1 sperm nucleus. (First FERTILISATION)
2. The other sperm nucleus enters egg cell through the
assistance of synergids which then degenerate, and unites with its nucleus
(Second FERTILISATION)
The embryo sac contains seven (3+3+1) cells.
3-cells at micropylar end — one egg cell and two synergids,
3 cells at opposite end, called antipodal cells, and
1 large central cell. The central cell is different containing two nuclei
called polar nuclei.
Germination of pollen grain
Pollen grain germinates only if it falls on the stigma of
the same plant species, otherwise, it disintegrates.
The pollen grain on falling on the stigma is stimulated to
germinate due to the secretion of sugars by the stigma. Through a point in the
exine a pollen tube grows out of the pollen grain, carrying at its tip the
generative nucleus and the tube nucleus generative nucleus divides into two
nuclei (male gamete nuclei also called sperm nuclei). Thus there are three
nuclei which are not separated by cell walls, they share a common cytoplasm.
The pollen tube grows through the stigma and style by
dissolving the tissues with the help of enzymes and reaches the ovary. There,
it pushes through the micropyle and reaches the embryo sac. The tube nucleus
which had directed the growth of pollen tube all the way down, now
disintegrates.
Now, the pollen tube enters one of the synergids and
releases its two sperm nuclei. Of these, one sperm nucleus fuses with the egg
cell nucleus to form the zygote while the other sperm nucleus moves towards the
two polar nuclei in the central cell and fuses with them (thus 3 nuclei fuse together/triple
fusion to produce the endosperm nucleus). All together two fertilizations have
occurred and hence termed double fertilization.
*Fate of floral parts after on After fertilization the
flower has served its purpose.
*The petals, stamens, style and stigma wither and generally fall off.
*The calyx may either fall off or may remain intact in a dried and shrivelled
form. Apple and guava show such dried sepals very clearly. in brinjal it
remains.
*The ovary enlarges to form fruit, the ovarian wall forming the fruit wall. The
ovary wall may either form a dry and hard fruit wall or a fleshy fruit wall.
*Ovules become the seeds. The fruit contains one or more seeds which are
developed from one or more ovules attached to the ovary by means of
placenta(e). When the fruit is ripe the seeds contained inside are released by
one or the other method and grow into new plants in suitable conditions.
The fate of various parts of the ovary after fertilization
during the formation of fruit is as follows:
Ovary - Fruit
Ovary wall - Pericarp
Ovule - Seed
Placenta - Stalk of the seed
Outer integument - Testa
Inner integument - Tegmen
Secondary nucleus - Endosperm
Egg cell Synergids - Embryo
Antipodal cells - Disorganised