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sexual reproduction in plants.explain it.

Sexual Reproduction in Plants:

When a small pollen grain from a male flower lands on the stigma of a female flower, it enters inside and reaches the ovule which in inside the ovary.

The embryo sac present inside a matured ovule of a flowering plant contains seven cells.

Among these one cell remains in the middle called central cell with two nuclei.

Remaining six cells divide into two parts and each part with three cells move to the opposite corners of the sac.

The three cells closest to the opening of the sac (micropyle) differentiate into one egg cell and two synergids. These three cells together are called “three celled egg apparatus of embryo”. 

The other three antipodal cells degenerate.

The pollen nuclei (two) after entering inside the embryo sac fertilize the egg and the central cell. The fertilized egg produces zygote and the fertilized central cell produces endosperm.

Thus fertilization of flowering plant is also known as double fertilization.

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Sexual reproduction in plants involves male and female plant organs. The female structures invovled in sexual reproduction are the stigma, the styleand the ovary. The stigma is the sticky portion of the pistil that captures pollen. The style is long and slender and supports the stigma. The ovary is composed of one or more ovules and is responsible for housing the eggs. The male structures involved in sexual reproduction are the filament and the anther. The filament supports the anther which is responsible for storing and producing pollen. Pollination is the transfer of pollen from an anther to a stigma. Wind, water, insects, birds, and small mammals all aid in the pollination of plants. After pollination, one nuclei of the pollen grain forms a tube down through the style to the micropyle of the ovary. The second nuclei travels down the tube and splits into two sperm nuclei that fertilize the egg and combine with polar bodies to form the endosperm(stored fruit). 

flower
 
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Sexual Reproduction in Flowering Plants

 
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Flower structure

Sexual reproduction in flowering plants centres around the flower. Within a flower, there are usually structures that produce both male gametes and female gametes.

 

Pollination

Many plants favour cross-pollination, so pollen must be transferred to the stigma of another plant if sexual reproduction is to take place. Some flowers rely of the wind to carry pollen grains others rely on insects.

Self-pollination is where the pollen is transferred to the stigmas of the same flower or the stigma of another flower on the same plant. Self-pollination is obviously more reliable, particularly if the nearest plant is not very close.

 


Fertilisation

If the pollen grain lands on a compatible stigma, a pollen tube will grow so that eventually the egg cell, hidden away in the embryo sac, can be fertilised. A tube emerges from the grain, its growth being controlled by the tube nucleus at the tip of the tube. It may grow downwards in response to chemicals made by the ovary (a response known as chemotropism).

During the growth and extension of the tube, the generative nucleus, behind the tube nucleus, divides by mitosis to produce 2 male haploid gametes. The pollen tube enters the ovule through the micropyle and penetrates the embryo sac wall. The tip of the tube bursts open, the tube nucleus dies and what follows is calleddouble fertilisation.

1 male gamete fuses with the egg cell to produce a diploid zygote.

1 male gamete fuses with both the polar nuclei to produce the triploid primary endosperm nucleus.

Immediately after fertilisation, the ovule is known as the seed.

The following happens:

  1. The zygote divides many times by mitosis to produce an embryo. It differentiates to become a plumule(young shoot), radicle (young root) and either 1 or 2 cotyledons (seed leaves). It is attached to the wall of the embryo sac by a suspensor.
  2. The primary endosperm nucleus divides many times by mitosis to produce endosperm tissue. In some seeds this endosperm is a food store for later use by the seed. In others it may gradually disappear as the cotyledons develop.
  3. To accommodate all this growth the embryo sac expands and the nucellus is crushed out of existence, giving its nutrients to the embryo and endosperm.
  4. The integuments surrounding the embryo sac become the tough and protective testa (seed coat). The micropyle remains though so that oxygen and water can be taken in at germination.
  5. The water content of the seed decreases drastically so the seed is prepared for dormancy.
  6. The ovary wall becomes the pericarp - the fruit wall, the whole ovary now being the fruit. The function of the fruit is to protect the seeds and to aid in their dispersal, e.g. by an animal. That is why they can be brightly coloured and sweet; animals will eat them and scatter the seeds either at the time of eating or when they are passed out of the gut in defecation, unharmed.

 

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Haploid gametes are produced through meiosis; male cells are produced which are encapsulated in a pollen grain, female cells form ovules held within an ovary. 

Through the process of pollination, male pollen grains are transferred to the stigma of a receptive flower. 

The pollen grain germinates and a pollen tube grows down the style and into the ovary, where it releases the male gamete - this then fuses with the ovule forming a diploid zygote (which after further mitosis develops into an embryo). 

The embryo is encapsulated within a testa and forms a seed as the ovary ripens (to form the fruit). 

Seed are released and germinate. 

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Pollination

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Transfer of pollen grains to the stigma is called pollination. If the pollen grains are transferred to the stigma of the same flower, the pollination is called self-pollination or autogamy. If the pollen grains are transferred to the stigma of another flower of the same species, the pollination is called cross-pollination or allogamy.

Cross pollination is brought about by various agencies like wind, water, bees, birds, bats and other animals including man.

 

cross pollination process in flowers

 

A Bee Entering a Flower

The anthers on maturity burst open with force and this is called dehiscence. This releases the pollen grains with force which are then carried by wind and water to other plants. In other plants, the flowers are brightly coloured and scented to attract the birds, bees, etc. The insect or the bird enters the flower to suck the nectar produced by glands at the base of the flower. The pollen grains present on the dehisced or open anther, stick to the legs or abdomen. When the same insect visits other flowers the pollen grains are transferred to the stigma of those flowers.

Fertilisation

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On reaching the stigma, the pollen grains put out a tube. This is called germination of the pollen grain.

process of fertilisation in flowering plant

Fertilisation in a Flowering Plant

The tip of the tube contains the male nuclei. The tube grows and enters the ovule where it bursts at the tip releasing the male gametes. One of the male gametes fuses with the egg, the female gamete. The fusion of the male gamete with the female gamete is called fertilisation. This results in the formation of zygote that is diploid. The zygote develops into the embryo. The other male gamete fuses with the polar nuclei. This results in the formation of a triploid nucleus called the endosperm nucleus. Since the process of fertilisation involves two fusions, it is called double fertilisation.

The divisions of the endosperm nucleus result in the formation of the endosperm that nourishes the growing embryo. The ovule then becomes the seed and the ovary changes into fruit.

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