Sexual Reproduction in Plants

The flower is the reproductive structure of plants. A flower has different structures arranged in whorls at the top of the flower stalk or receptacle. The outermost whorl is the CALYX made up of sepals. These function to protect the flower as a bud. The next whorl is the COROLLA made of petals. These serve to attract pollinators in insect pollinated flowers with their usually bright colour and scent. The next whorl consists of the male floral structures and is called the ANDROECIUM made up of stamens. Each stamen has an anther which makes pollen grains and a stalk called filament.

In the centre are the female parts or CARPEL. They make up the GYNAECIUM which has at its base an ovary which manufactures ovules. The stigma serves to accept pollen grains and is held in position by the style which connects to the ovary.

A flower with only male parts is said to be staminate while one with female parts only is pistilate.

Formation of Pollen Grains

Pollen grains, made by anthers, contain male gametes. Each anther contains four compartments called POLLEN SACS. The walls of the pollen sacs contain several layers of cells. One of the outer layers is made up of cells with thickened walls called the FIBROUS LAYER. These help to liberate the pollen grains when mature. The innermost layer is the TAPETUM which nourishes the pollen grains. At the centre of each pollen sac, diploid POLLEN MOTHER CELLS divide by meiosis to give four haploid daughter cells. These may remain together as a tetrad or may separate.

Each haploid cell develops a tough protective wall and becomes a pollen grain. The wall has two layers, an outermost waterproof, tough EXINE and an inner INTINE. There are gaps in the exine called pits.

The haploid nucleus in pollen grains, divide by mitosis to give two haploid cells. One is the generative nucleus and the other tube nucleus.

When the pollen grains are mature, the anthers will split open at a point of dehiscence between two pollen sacs to expose the pollen grains.

Formation of the Embryo Sac

The female gametes are found in embryo sacs inside ovules produced by ovaries. Each ovule is connected to the ovary wall by a stalk called FUNICLE. The ovule has an outer covering or INTEGUMENTS surrounding undifferentiated cells called the NUCELLUS. At one end of the ovules, the integuments do not meet leaving a gap called the MICROPYLE. The other end of the ovule, furthest from the micropyle end (nearer to funicle) is the CHALAZA.

Inside each ovule, a large, diploid SPORE MOTHER CELL develops. This will divide by meiosis to give four haploid daughter cells. All but one of these degenerates and the surviving cell develops into an embryo sac.

The embryo sac will be nourished by the nucellus and grows larger. This nucleus will then undergo three successive mitotic divisions to give 8 haploid nuclei. Two will migrate towards the centre of the embryo sac with no membrane around them. Three will go towards the chalazal end and the other three towards the micropyle end of the cell. Those nearer to the chalazal end are ANTIRODAL CELLS. One of those at the micropyles end is a little larger than the others there. The larger cell is the female gamete while the other 2 are SYNERGIDS. The two nuclei in centre will fuse together to form a single diploid nucleus called the PRIMARY ENDOSPERM nucleus. The mature embryo sac in the ovule therefore has 6 haploid nuclei and a diploid nucleus.

Pollination

This is the transfer of pollen grains from the anther to the stigma of a flower of the same species. There are two types of pollination: cross pollination where pollen grains are transferred to the stigma of another flower and self pollination when it is transferred to the stigma of the same flower. While cross pollination ensures that there is more variation, the opposite happens in self pollination. Whilst it may be advantageous to be able to self pollinate, cross pollination provides more advantages and is favored. Plants therefore have several adaptations to foster cross pollination. These include:

1. Having only male or female floral structures. That is they are dioecious (separate sexes) e.g. holly trees

2. Protandry – The maturation of the anthers before stigma is ready to receive pollen, e.g. dandelions and daises.

3. Protogyny – Maturation of stigma before anthers (less common of the two), e.g. Bluebells and woodrurb

4. Self sterility – Some pollen of flower are unable to develop as a result of chemicals released when they land on stigma of same plant, e.g. clover

5. Mimicry – Where flower resembles agent of pollination, tricking them into thinking they can mate only to transfer pollen, e.g. orchid, primrose flower.

Since pollen grains or ovules are imitated, then the pollen grains must be carried by an agent to the stigma. This may be by wind or by insect or even water.

REVISIT CHARACTERISTICS OF WIND/INSECT POLLINATED FLOWERS

Once the pollen grain lands on the stigma by whatever method, pollination is complete.

Fertilization

The pollen grain absorbs water from the stigma and begins to germinate, if of same species. A part of the pollen grain pushes through one of the pits forming a pollen tube. The tube grows down the style towards an ovule. The tube nucleus remains close to the tip of the tube as it makes its way through the style. As the tube grows, the degenerative nucleus divides by mitosis to give two haploid male gametes.

The pollen tube contains to grow and enters the ovule through the micropyle (some digest and enter through chalaza). Once inside ovule, the tube nucleus degenerates and the two male gametes enter the embryo sac. One will fuse with the egg cell forming a diploid zygote while the other fuses with the primary endosperm (diploid) cell in embryo sac. This result in the triploid nucleus called the endosperm. This happens only in plants and is called DOUBLE FERTILISATION as two things are fertilized.

Seed Development

Once fertilized, the ovule becomes a seed. It remains attached to the ovary wall and receives nutrients as it develops. The zygote will divide by mitosis to form an embryo plant. The embryo develops a radicle, plumule and two cotyledons. The endosperm also divides by mitosis to form the endosperm which surrounds and provides nourishment for the developing embryo.

While this is happening, the integuments of the ovule develop into the testa as it thickens, hardens and becomes waterproof with lignin in its cell walls. The gap in integuments (testa) remains as hole in testa.

The walls of the ovary changes to become the fruit, while its wall becoming the pericarp. It may become fleshy or dry depending on type of fruit. The style and stigma disintegrate to leave the fruit with two scars.