Sexual Reproduction in Plants

The term sexual reproduction includes participation of two gametes for generation of offspring. Sexual reproduction in plants involves different phases which consist of pre-fertilisation events, fertilisation and post-fertilisation events. The pre fertilisation events consist of microsporogenesis, megasporogenesis and transfer of gametes. Post-fertilization events consist of development of embryos after formation of zygote. In lower groups of plants like algae, bryophyte, reproduction is quite simple though steps are the same. According to the morphology of the gametes formed, sexual reproduction is isogamous, anisogamous and oogamous. 

Sexual Reproduction in different plants:

Algae: In case of algae, sexual reproduction occurs by the formation of gametes by the process of meiosis, though it is controlled by different environmental events. When an alga is reproducing sexually, it consists of two phases in their life cycle. One is the haplontic phase and another is the diplontic phase. 

Bryophytes: Sexual reproduction in plants like bryophytes includes all the phases. The microspores are immotile and need water for transport. These plants show alternation of generation in their life cycle and possess haplontic life cycle.

Pteridophytes: A wide range of pteridophytes reproduce sexually by formation of microspore and megaspore. This also shows alternation of generation in their life cycle, but have diplontic life cycle. 

Pre- fertilization Events:

Formation of megasporangium:

The cells of the ovule differentiate to form megaspore mother cells in the micropylar end. Now, one megaspore mother cell divides mitotically to form four megaspores. One of the megaspores becomes functional while the other degenerates (monosporic development). Now the nucleus undergoes mitotic divisions without cytoplasmic division. Two nuclei move towards two poles and undergo two more successive divisions to form an 8 nucleated stage. Two nuclei from each pole come to the middle to form polar nuclei. The ovum with two synergids forms egg apparatus in the micropylar end whereas their antipodal cells accumulate in the chalazal end.

Formation of microsporangium

The structure of microsporangium is quite interesting! It has a dithecous tetrasporangiate structure i.e. Each sporangium is covered by four less- epidermis, endothecium, middle layer, and tapetum, from outer to inner respectively. The tapetum covers sporogenous tissue and provides nourishment. Others provide protection and also help in dehiscence. The sporogenous tissue divides mitotically to form microspore tetrads. Each gets separated from the other and bears the potential of forming a male gamete. Callase secreted from tapetum helps in the proper maturation of pollen grains as well as in dissociation. Each pollen grain has two layers- exine and intine. The exine is made up of the most resistant material, sporopollenin, The intine is made up of pectin and cellulose. Within the pollen grain, two cells are present- larger vegetative cells and a smaller generative cell. The smaller cell undergoes mitotic division to form male gametes. 

Pollination:

The process of pollination includes the transfer of pollen grains from the anther to the stigma of the pistil. Different types of pollination include autogamy, geitonogamy, xenogamy. Autogamy includes the transfer of pollen grain from another to the stigma of the same flower. Geitonogamy includes the transfer of pollen grains within the different flowers of the same plants. These two processes do not entertain mutation and evolution. In the case of xenogamy, the transmission of pollen grains occurs from another to the stigma of flowers of different plants.

Fertilization:

Pollen-pistil interaction: After the transfer of pollen grains, stigma can be examined chemically before the formation of the pollen tube. This chemical interaction is known as pollen-pistil interaction. The pollen tube is allowed to form only if the pollen grains are compatible.

Fertilization: After the formation of the pollen tube, the pollen enters the egg sac through the micropylar end with two male gametes and is guarded by the filiform apparatus of synergids. One male gamete is guarded toward the egg whereas the other one moves to fertilize the polar nuclei. As two fertilizations are taking place simultaneously, this is known as double fertilization. The egg forms a zygote whereas the polar nuclei give rise to 3n endosperm. 

Post- fertilization events:

The polar nucleus along with the male gamete forms an endosperm which acts as a food reservoir for the embryo. In some cases like coconut, the PEN performs successive divisions to form free nuclei known as free-nuclear endosperm. The zygote forms an embryo in the micropylar end. The ovule develops to form the seed.

Sexual reproduction in plants examples

Unisexual flowers are produced by corn, cucumber and papaya and therefore, perform xenogamy.

Conclusion:

All these are the different processes of reproduction in plants. It is possible to form seeds without fertilization. The process is known as apomixis. In some species, several embryos are present in each seed known as polyembryony. Different hybrid varieties are formed nowadays by the cultivators for their profit considering different tools.