Pollination and Fertilisation Notes

Pollination and Fertilisation

Syllabus Overview:

This chapter covers the following topics:

• Pollination: Self and cross-pollination

  • Explanation, advantages, and disadvantages of self and cross-pollination.

  • Agents of pollination and the characteristic features of flowers pollinated by various agents such as insects, wind, and water.

  • A brief idea as to how nature favors cross-pollination.

• Fertilisation

  • Events taking place between pollination and fertilisation leading to the formation of zygote in the embryo sac.

  • A brief explanation of the terms double fertilisation and triple fusion.

• Fruit and Seed - definition and significance.

Introduction

In plants, seed production requires the union of male and female gametes. The male gamete is produced by the androecium within the pollen grain, while the female gamete is produced by the gynoecium within the ovule. The union of male and female gametes and their fusion is brought about by two processes called pollination and fertilisation, respectively.

5.1 Pollination

Pollination is the process of transfer of pollen grains from the anther to the stigma. The transfer may involve a single flower, or two different flowers on the same plant, or the flowers present on two different plants.

Based on this, pollination is basically of two types:

• Self-pollination

• Cross-pollination

5.2 Self Pollination

Self-pollination involves the transfer of pollen grains from the anthers of a flower to the stigma of the same flower or of another flower born on the same plant.

• Autogamy: If self-pollination occurs in the same flower

• Geitonogamy: If it occurs between two different flowers on the same plant.

Self-pollination generally occurs in bisexual flowers. Since the male and female gametes that take part in fertilisation are formed in the same flower, their genetic make up is identical. As a result, the newly formed plant resembles the parent plant.

Adaptations for Self pollination

Flowers show the following characteristics to ensure self-pollination:

• (i) Homogamy: The anthers and the stigma of a bisexual flower mature at the same time. This is known as homogamy. Homogamy occurs in wheat, rice, and Mirabilis.

• (ii) Cleistogamy: Some bisexual flowers never open and remain closed, so that the pollen grains can pollinate the stigma of the same flower. This condition is called cleistogamy. Underground flowers of Commelina benghalensis show cleistogamy.

Examples of plants exhibiting cleistogamy include oxalis and viola.

Advantages of Self pollination

• (i) Self-pollination in bisexual flowers ensures continuity of the race or species.

• (ii) Self-pollination helps to preserve the parental characters as the gametes from the same flower are involved.

• (iii) It is economical (for the plant) as the plants do not have to produce pollen grains in large quantity.

• (iv) Flowers need not be showy, nor they need to produce nectar or scent.

Disadvantages of Self pollination

• (i) New varieties cannot be obtained by self-pollination.

• (ii) The genetic defects of the breed cannot be removed.

• (iii) Repeated self-pollination leads to loss of vigour and vitality of the species. Seeds produced are smaller and weaker.

5.3 Cross Pollination

The transfer of pollen grains from anther of a flower of one plant to the stigma of a flower of another plant of the same species is called cross-pollination or allogamy. Cross-pollination occurs both in unisexual and bisexual flowers and results in intermixing of genetic characters of the two parental plants. It brings about variations in offspring.

Nature encourages cross pollination (Adaptations for Cross pollination)

Following conditions favour cross-pollination:

• (i) Unisexuality: The unisexual flowers (male or female) may be borne on the same plant (as in maize, castor, cucumber) or on two separate plants (as in palm, mulberry, papaya). This condition favours cross-pollination.

• (ii) Self Sterility: In this condition, the pollen grains of a flower do not germinate on the stigma of the same flower but germinate on the stigma of a flower born on another plant of the same species; thus ensuring cross-pollination.

  • Examples: Ray florets of sunflower, orchids, apple, grape, rye.

• (iii) Dichogamy: The flowers are bisexual, but stamens and carpels mature at different times. This condition is known as dichogamy which acts as a barrier to self-pollination.

There are two states of dichogamy:

• (a) Protogyny: In this case, gynoecium matures first; gyne = female): In this case, gynoecium matures earlier than the anthers of the same flower and the stigma receives the pollen grains brought from another flower.

  • Examples - Peepal, custard apple.

• (b) Protandry: When the stamens mature earlier than the gynoecium of the same flower, the pollen grains are carried over to the stigma of another flower, as in pea, sunflower, salvia.

• (iv) Herkogamy: In some bisexual flowers, some sort of barrier develops between stamens and the pistil of the same flower, thereby preventing self-pollination. For example, a hood covering the stigma acts as a barrier in pansy flowers. In Gloriosa, the stamens liberate the pollen grains on the outside, so that they are not able to land on the stigma of the same flower.

• (v) Heterostyly: Some plants bear flowers of different forms. One form bears long stamens and short style, and the other form bears short stamens and long style. This condition favours cross-pollination. Heterostyly is shown by primrose and oxalis. The anther and stigma grow at different heights in a bisexual flower.

Advantages of Cross pollination

• (i) Cross-pollination results in healthier offspring.

• (ii) Seeds produced by cross-pollination have much better germinating capacity.

• (iii) More abundant and viable seeds are produced.

• (iv) Variations are introduced by cross-pollination.

Disadvantages of Cross pollination

• (i) Plants have to depend on external agencies for pollination, which may or may not be available at the proper time.

• (ii) The pollen grains have to be produced in large quantity to ensure pollination. Thus, lot of pollens are wasted.

• (iii) The process is less economical as various devices have to be adopted by the flowers to attract pollinating agents, like the flowers have to be large, coloured, showy, scented and nectar producing.

5.4 Agents of Cross Pollination

As the pollen grains are not capable of locomotion, they have to depend on various external agents such as wind, water or insects for their transmission

There are two main groups of agents:

• (i) Biotic agents which include animals of different types such as insects, birds, bats, and snails.

• (ii) Abiotic agents like wind and water.

Types of Pollination

• Biotic: Living agents, e.g;

  • insects - Entomophily

  • animals (zoophily)

    • Elephant - Elephantophily

    • birds - Ornithophily

    • bats - Cheiropterophily

    • snails - Malacophily

• Abiotic: Non-living agents

  • wind - Anemophily

  • water - Hydrophily

(a) Entomophily

Pollination by insects is of common occurrence and entomophilous flowers have various adaptations for attracting the insects.

1. The flowers are large-sized and brightly coloured to attract the insects.

2. When flowers are not conspicuous, other parts become coloured and showy. For example, in Bougainvillea bracts are large and colourful.

3. They produce nectar or scent which attracts insects, as in jasmine.

4. Some flowers bear markings or lines on the petals which act as nectar guides that lead the insects to the nectaries, as in pansy.

5. The pollen grains of entomophilous flowers are either rough or sticky so that they adhere to the body of insects easily.

6. The stigma is sticky, flat or lobed present inside the flower

7. When the flowers are small, they form inflorescence to make them conspicuous and attractive. Common example is sunflower.

Insect Pollination in Sweet pea (Lathyrus)

The pea plant belongs to legume family and has papilionaceous (butterfly-shaped) flowers. The flower is complete, bisexual and zygomorphic.

Calyx is green consisting of five sepals.

Corolla is brightly-coloured with five petals. It consists of a large innermost petal called standard, two lateral petals called wing petals, and two innermost petals called the keel petals. These petals together form a trough-like structure which encloses the reproductive organs.

There are ten stamens, nine of which are fused together to form a staminal tube which encloses ovary and part of style. The tenth stamen is free.

The gynoecium consists of a single carpel. The ovary is long and narrow. The style is curved structure and is hairy just below the stigma.

Pollination: The flower is well adapted to insect pollination. Bees pollinate the flowers.

(b) Anemophily

Anemophilous or wind pollinated flowers have following characteristics:

1. The flowers are usually small, inconspicuous and unattractive.

2. They do not produce scent or nectar.

3. The anthers produce large quantity of pollen grains.

4. The stamens are long and protrude out of the flower to be exposed to wind.

5. Anthers are large that protrude out of the flowers and are loosely attached to the long filaments so that they can easily be moved by the wind.

6. The pollen grains are smooth, light and dry and sometimes provided with wings as in pine so that they can be easily carried by wind to long distances.

7. Stigma is comparatively large, branched and often feathery to trap the pollen grains, e.g., maize, rice, grasses, sugarcane, pine.

In maize plant, the male flowers produce large quantities of pollen grains. As the anthers burst, the pollen grains are carried by air current and many of these are trapped by the large and feathery stigmas of the female flowers. The female flowers have long hanging styles for easy trapping of pollen to ensure cross pollination

5.1: Differences between Insect and Wind pollinated flowers

(c) Hydrophily

Pollination may also be brought about in some aquatic and submerged plants through the medium of water, e.g., Vallisneria, Hydrilla.

Pollination in Vallisneria

Vallisneria is a unisexual submerged plant. The female flowers have long coiling stalks which bring the flowers on the surface of water. The male flowers grow under the water.

When ripe, the male flowers get detached from the plant and float on the surface of water. The free floating male flowers cluster around female flowers. The anthers burst and the pollen grains get attached to the stigma of female flowers.

After pollination, the stalk of the female flower gets coiled thus, pulling the female flower once again under water.

Pollen grains are unwetable / waterproof and have specific gravity less than water so that they are able to float.

The flowers have the following characteristics:

1. Male and female flowers are born on separate plants, i.e., the plants are dioecious.

2. Flowers are small and inconspicuous.

3. Pollen grains are produced in large quantity.

(d) Zoophily (Pollination by animals)

There are only a few flowers pollinated by birds (ornithophily, as in Canna, Bignonia), bat (cheiropteriphily, as in kadam tree), or snails (malacophily, as in snake plant and Lemna).

Elephopily

In Rafflesia, pollination takes place by elephants. Rafflesia is found in Burma and Indonesia. Its flowers remain on the ground. The pollen grains get attached to the feet of the elephants and are carried to the stigma of other flowers thus, bringing about pollination.