The creation of a life form, by a similar life is called reproduction. Reproduction is the extension of life of a species at a given time. It is a means of perpetuation of the species and also multiplies their number. By this process, the individuals transmit life to the next generation and thereby ensure the continued existence of living organisms on earth. If there was no reproduction, life on this earth would sooner or later come to an end.
New individuals are produced from a single parent without the act of fusion of gametes. The new individuals so formed are generally identical to the parent plant, as only mitotic divisions take place during their development. Regeneration of new plants from portions of vegetative organs is very common and is called vegetative propagation. For this purpose, organs like the root, stem, leaf, flower etc are variously modified.
In flowering plants, flowers have taken the place of conventional sex organs, and the ovules which later on develop into seeds are enclosed within an ovary. The sexual reproduction in angiosperms involves the fusion of two gametes, the male and the female. The male gametes are produced in the pollen grains and the female gametes are produced in the ovule. The fusion of the two gametes results in the formation of a zygote. The zygote develops into an embryo within the seed. This embryo, when the seed germinates grows into a new plant.
The pollen grains which produce the male gametes and the ovules which bear the female gametes are borne on different structures. It therefore becomes necessary that for sexual union to occur, the pollen grains must be transferred to the stigma. The transfer and deposition of pollen grains from the anther to the stigmatic surface of the flower is called pollination.
The pollen grains which reach the sticky stigma, absorb these secretions, swells up and ruptures at one of the germ pores. The thin intine emerges out in the form of a pollen tube which continues to grow and penetrates deep into the style.
After fertilization, the zygote undergoes a number of mitotic divisions and form a multicellular embryo. The primary endosperm nucleus also passes through a series of mitotic divisions and form a mass of endosperm cells which provide nourishment to the developing embryo.
In angiosperms, the fruits are usually formed after the process of fertilisation. In certain plants, however, the fruits are formed without the act of fertilisation. Such fruits are called parthenocarpic fruits and the phenomenon is known as parthenocarpy.
Reproduction occurs by vegetative propagation, asexual and sexual reproduction. Vegetative propagation through artificial method can be done by cuttings, layering and grafting.
Regeneration of new plants from portions of vegetative organs is very common. Runner, rhizome, bulbs, corns and tubers serve as means of propagation.
b) sexual reproduction