Sexual reproduction in plants

In a nutshell

The process of sexual reproduction in plants consists of pollination and fertilisation. Pollination is the process of pollen landing on the stigma. Then, fertilisation is the process of the male gamete nucleus fusing with the female gamete nucleus. Flowering plants undergo a unique process of double fertilisation. 

Reproductive organs 

The reproductive organs of plants are in their flowers. Brightly coloured flowers attract insects for pollination whereas duller flowers and those with long stamens, rely on the wind to transfer their pollen during pollination. Most flowering plants are hermaphrodites. This means that they possess both male and female reproductive organs. The reproductive organs of the plant are described below.

Pollen grain formation

The anther may have two or four lobes. The lobes contain microspore mother cells. These are diploid cells that divide by meiosis to produce four haploid microspore cells. The microspores then divide by mitosis to produce two haploid nuclei. The pollen grain contains a tube nucleus and a generative nucleus. The latter is the male gamete. 

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Embryo sac formation

An ovary can contain one or more ovules. Within the ovule, the embryo sac develops. This process is outlined below.

Pollination

Pollination occurs when a pollen grain lands on the stigma of a plant. There are two types of pollination: self-pollination and cross-pollination. Self-pollination occurs when the pollen is transferred from the anther to the stigma of the same plant. Cross-pollination occurs when the pollen is transferred from the anther of one plant to the stigma of another plant. Insects and the wind can aid in the transfer of pollen during cross-pollination.

Pollen grains from different plant species have different shapes and physical characteristics. Only the pollen grain with the complementary shape will germinate once on the stigma and form a pollen tube. 

Fertilisation

Fertilisation occurs when the male and female gametes fuse together. During pollination, the pollen will land on the stigma of a plant. The pollen will swell as it absorbs water and then it will split open. A pollen tube will form from the pollen grain and grow down the stigma and style towards the ovary. The growth of the pollen tube is facilitated by the recipient plant. The tube digests the plant's tissues which provides materials for the growth of the pollen tube. The generative nucleus travels down the pollen tube and divides by mitosis to produce two haploid nuclei. The pollen tube also has its own nucleus.

The pollen tube will reach the ovary and grow through the micropyle to gain access to the embryo sac. Once in the ovary, the end of the pollen tube will break down and the pollen tube nucleus will disintegrate. 

The two haploid nuclei are the male gametes and these will travel into the embryo sac. Double fertilisation can then occur. 

The process of double fertilisation occurs only in flowering plants and it involves the fusion of one male nucleus with one female nucleus to form a diploid zygote. It also involves the fusion of the second male nucleus with the two polar nuclei in the embryo sac. This forms a triploid primary endosperm cell.

Note: Any other nuclei present in the embryo sac will break down.

The diploid zygote develops into an embryo plant. The radical is the embryonic root and the plumule is the embryonic shoot. The triploid primary endosperm forms food tissue. The embryo sac becomes the seed and the integuments become the protective layer surrounding the seed called the seed coat. The ovary walls then become the fruit.