What is Pollination?

• 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.

• To initiate, 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:

  • The pollen of the same flower may fall on its stigma by itself (autogamy).

  • The pollen at another flower of the same plant may fall on the stigma (geitonogamy).

  • The pollen of a flower of another plant of the same species may fall on the stigma (allogamy).

    • 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 neighborhood.

• Similarly, various crops may be growing in neighboring agricultural fields—wheat, custard, 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 parts 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:

Self Pollination:

• It 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 characteristics—i.e. the general qualities or features of any one plant are the same in the different flowers borne on it.

• In some rare cases, as in pansy, some flowers growing close to the ground level, do not open at all (cleistogamy, cleisto: closed), the anthers and stigma lie close to each other which mature at the same time, and self-pollination is assured.

When can Self-Pollination occur?

• Self-pollination can occur in bisexual flowers, i.e. 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 borne on the same plant then they must also mature at the same time.

Advantages of Self-Pollination:

• It is much surer in such bisexual flowers where stamens and carpels mature at the same time.

• Parental characters are preserved indefinitely.

• There is no wastage of pollen grains.

• Even a small quantity of pollen will suffice.

• The flowers need not be large and showy.

• Scent and nectar need not be produced by flowers.

All the above advantages mean a great economy on the part of the plant.

• A lot of nutrient material that would otherwise be used in the production of pollen, nectar, scent and large petals, etc., is saved.

Disadvantages of Self-Pollination:

• Continued self-pollination, generation after generation, may lead to a weakening of the variety or the species.

  • The seeds produced through it are poor in quality and give rise to less vigorous offspring.

• The weaker or defective characters of the variety or breed cannot be eliminated.

  • 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 anthem 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:

• The offspring are healthier.

• The seeds produced are abundant and viable.

• New varieties may be produced by cross-pollinating two different varieties of the same species or even two species.

Disadvantages of Cross Pollination:

• The pollination is not always certain because some pollinating agent is always needed which may or may not be available at the proper time.

• The pollen has to be produced in large quantities to ensure chances of pollination which means a lot of wastage of pollen.

• The process is uneconomical for the plant because the flowers have to be large, colored, and scented and have to produce nectar—for attracting pollinating agents.

Nature Favors Cross Pollination:

• The vast majority of flowering plants are cross-pollinated. Some of the various devices (contrivances) or the conditions which favor 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;

    • Example: 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).

      • Examples: bhindi, sweet peas, salvia, and sunflower.

    • In some plants, the stigma of the flower matures earlier than the anthers (protogyny, protos: first, gyne: female).

      • Examples: 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 off the seeds.

      • Examples: ray florets of sunflowers, 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 do not favor self-pollination.

    • Examples: primrose, 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 (or entomophilous) flowers usually have the following characteristics :

    • The flowers are farm. They are usually brightly colored to attract insects.

    • They usually emit scents for attracting insects.

    • They produce nectar which is food for insects.

    • The pollen grains are stick or spiny to enable them to be carried by the insects easily.

    • The stigma is sticky and does not generally hang out from the flower.

  • Wind-pollinated (or anemophilous) flowers usually possess showering special features. (Example: Maize)

    • The flowers are small.

    • They are usually not brightly colored and are often dull green.

    • They do not produce scents or nectar.

    • The stamens are long and hang out of the flower to be exposed to the wind.

    • The anthers are large and left attached to the filaments so that the slightest wind may move them (versatile).

    • Pollen is produced in very large quantities.

    • Pollen grains are light, dry, and smooth so that they can easily be carried away by the wind.

    • The stigmas are feathery and hang out of the flower to trap the pollen grains.

  • Water-pollinated (or hydrophilous) flowers are found only in aquatic plants.

  • They have the following characteristics:

    • Pollen grains are produced in large numbers.

    • 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 the water.

    • 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 ornithophily.

    • Examples: 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.

    • 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 their efforts 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 peas 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 ‘keel petal’ 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 from 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 carpets are enclosed within the keel and are more or less at the same level.

• In wind-pollinated maize, pollination is brought about by the wind.

  • As the wind blows, the pollen from the outwardly hanging anthers (in the tassel) is blown away easily because the anthers are so loosely attached to the filament that the slightest wind will shake them.

    • The pollen blown away by the wed 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 there are fewer chances for self-pollination.

Fertilization:

• Fertilisation is the union/fusion of the nuclei of male and female gamete.

• The pollen grain is the male 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 been divided into a tube nucleus and a generative (male) nucleus.

    • At this stage, the pollen is transferred to the 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:

• The ovule is the inner part of the ovary.

• The ovule is destined to become the seed and the ovary to become the fruit containing the seed inside.

  • There may be

    • a single ovule producing single-seeded fruit, or

    • many ovules producing a many-seeded fruit.

  • Each ovule has one or two protective coverings, the integuments.

    • The integuments leave a small opening in the micropyle at one end (for the entry of the pollen tube).

      • Enclosed by the integuments is the nucellus (a mass of food-laden cells), and further inside the nucellus is the embryo sac.

  • The embryo sac contains seven (3+3+1) cells 3 cells at the micropylar end — one egg cell and two synergids,

    • 3 cells at the 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 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 that 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 the pollen tube all the way down now disintegrates.