Sexual Reproduction In Plants

Flower – a captivating organ of angiosperms

Human beings have a close relationship with flowers. Flowers are related to social, religious, aesthetic and cultural values – they have always been worn as symbols to convey important human emotions such as happiness, love, affection, and grief.

Even before the flower appears on the plant, the decision that plant will bear the flower has taken place. A number of hormonal and structural variations begin which lead to the differentiation and development of the flower. The floral buds are borne by inflorescences which further develop into a flower. You will find the reproductive organs of the angiosperms in flower. The male reproductive organ is stamen (androecium), and female reproductive organ is pistil (gynoecium).

The flower can be unisexual, example – watermelon, papaya when it consists of either carpels or stamens or bisexual, example – mustard or Hibiscus when it consists of both carpels and stamens.

Carpel

  • It is present in the centre portion of the flower.
  • It is a female reproductive organ
  • It is further divided into three parts –
    Ovary – the bottom swollen part. It contains ovule which further consists of an egg cell. The pollen grain produces male germ cells which fuse with the egg cell in the ovule. This fusion gives rise to zygote which grows into a new plant.
    Style – Middle and an elongated portion
    Stigma – It is the terminal portion. It can be sticky.

In order to do fertilization, it is essential to transfer pollen grains from stamen to stigma.

There are two main types of pollination in general terms

  • Self-pollination – Transfer of pollen from the anther to the stigma of the same flower
  • Cross – Pollination – If the transfer of pollen is transferred from one flower to another, it is known as cross-pollination. The transfer of pollen grains can be attained by agents like water, wind and animals.

Kinds of Pollination

On the basis of the source of pollen, it can be categorised into three kinds:

  • Autogamy – In this, the pollination is attained within the same flower. There is a transfer of pollen grains from the anther to the stigma of the same flower. However, in normal conditions, complete autogamy is very difficult. Autogamy in such flowers needs coordination in the release of pollen and stigma receptivity.  In order to achieve autogamy every time, the stigma and anthers need to lie in proximity to each other. Some plants like Oxalis, Viola, and Commelina bear two types of flowers:
    • Chasmogamous flowers – These flowers are similar to the flowers of other species with exposed anthers and stigma.
    • Cleistogamous flowers – The flowers do not open at all which is why the anthers and stigma remain nearby. When anthers burst open in the flower buds, the pollen grains come in contact with the stigma and completes pollination. Therefore, these flowers are truly autogamous as the alien pollen cannot reach the stigma, and no cross-pollination can occur.
  • Geitonogamy – In this, pollen grains are transferred from anther to stigma of another flower of the same plant. Although, it is a kind of cross-pollination and involves a pollinating agent, genetically it is identical to autogamy since the pollen grains are from the same plant.
  • Xenogamy – In this method of pollination, the pollen grains from the anther of the different plant come in contact with the stigma of another plant of the same species. This type of pollination enables the changes in genetics and give rise to dissimilarities in the offspring from its parent.

Agents of Pollination

There are abiotic (wind and water) and biotic (animals) agents that help in pollination. Most of the plants use animals for pollination. Only a small group of plants use wind and water as their agents. In the case of wind and water, pollen grains come in contact with stigma only by chance. To compensate for this situation of uncertainty and loss of pollen grains, the flowers bear numerous pollen grains.

Pollination by Wind

Pollination by wind is the most common method in abiotic pollinations. Wind usually carries lightweight and non-sticky pollen grains with it. The stamens of the flowers that use wind pollination are generally well exposed so that the pollens can be dispersed easily. The female counterpart contains feathery stigma that can easily trap the pollen grains moving in the air. These kinds of flowers usually contain a single ovule in each ovary, and numerous flowers are packed to form an inflorescence. Example – corn. Wind pollination is most commonly seen in grasses.

Pollination by Water

Water is not the most preferred agent in the plant kingdom for pollination. It is mostly seen in monocotyledons. However, the main mode of transport when we see the lower plant groups like algae, pteridophytes, and bryophytes. The distribution of these plants is limited due to the need for water for the transfer of male gametes and fertilization. Some examples are Hydrilla and Vallisneria. Some seagrasses such a Zostera also pollinate and fertilize through the water.

Note – Not all aquatic plants pollinate through the water. Example – Water hyacinth and water lily. Also, both water and wind-pollinated flowers are not quite attractive and colourful.

Pollination by Animal (Insects)

Most of the flowering plants use insects or other animals as pollinating agents. Bees, flies, butterflies, wasps, moths, beetles, bats and birds are the most common pollinating agents. Bees are the most dominating biotic pollinating agents. In some cases, large animals like tree-dwelling rodents or reptile, primates (lemurs) have also been reported to be involved in pollination.

Flowers are usually adapted to in a certain way for a particular species of animal. Most of the insect-pollinated flowers are colourful, fragrant, large and rich in nectar so that they can attract insects or animals.

Fertilization in flower

After the pollen reaches a suitable stigma, it tries to reach the female germ cells present in the ovary. For this, a small tube-like structure appears that grows out and enters the style to finally reach the ovary. After fertilization, the egg and the male gamete forms zygote which divides many times to form an embryo inside the ovule. A tough coat is developed around the ovule and is the whole organ converts into a seed. There is rapid development of ovary that forms a fruit. Meanwhile, the sepals, petals style, stigma, and stamens shrivel and fall off leaving only fruit behind. However, in some cases, sepals can be seen even after the formation of fruit.