Sexual Reproduction

In sexual reproduction, offspring are produced by fusion on two different sex cells which usually come from two different parents. The sex cell are know as gametes.

    Gametes are formed by a kind of cell division called meiosis in which the chromosomes member is halved. Thus, the gamete cell is said to contain a haploid number (n) of chromosomes.

   The fusion of two haploid gametes is known as fertilization. It results in the formation of a single celled zygote which had a diploid number 2n) of chromosomes. The zygotes then divides and develops to produce the offspring whose somatic cells are all diploid.

   If the male and female gametes comes from two different parents, the offspring shoes traits that cone from both of them. Sexual reproduction, therefore, produces new variations in the offspring.

    In some organism, the zygote becomes enclosed in a resistant cell wall, and is known as a zygospore. The zygospore only develops when environmental conditions are favourable for growth. Sexual reproduction thus provides a growth. sexual reproduction thus provides a resting stage which helps organisms to withstand unfavourable conditions like drought, cold and scarcity of food. 

    In some unicellular organisms, sexual reproduction is very simple. The whole cell may act as a gamete. This may then pair with another similar whole cell and exchange nuclei. Such a sexual process is known as conjugant. Conjugation also occurs in Spirogyra. Here , two Spirogyra filaments come to lie side by side. Conjugation tubes are formed between them. The cells in one filament act as female gametes. The male gametes migrate and fuse with the female gametes to form thick walled zygospores.

    In the unicellular chlamydomonas, gametes are formed when a mature parent cell loses its flagella and divides into thirty two portions. Each of these portions acquires a pair of flagella and becomes a gamete. The gametes are smaller then the ordinary individuals and do not have cell walls. When they are liberated into the water, gametes from different individuals duse in pairs to form  thick walled zygospores.

    In the paramecium, fusion only occurs with an exchange of nuclei. In the chlamydomonas the gametes are structurally alike. However, gametes produced by one individual can only fuse with those produced by certain types of individuals. Such organisms show matting types. In the Spirogyra, although the gametes are alike, those produced by one member of a conjugant pair are motile.

    In higher organisms, there is a definite difference between male and female gametes. The gametes are also produced in special structures called gonads.

    In animals, male gametes are known as spermatozoa or sperms and are produced in tests (singular: testis). A sperm is a small , motile gamete with a head and a long tail. female gametes are usually known as ova or eggs, and are produced in ovaries. An ovum is a large round, non motile gamete with a store of food. Some lower animals like the hydra are hermaprodites, I.e. both the testis and ovary are present in the same individual. However, the sperms of one individual usually fuse only with the ova of another individual. Numerous sperms are produced by the testis, bit the ovary produces one egg cell at a time. The egg cell is fertilized by only one sperm.

     In most animals, such as echinoderms, arthropods and all vertebrates, male and female gonads are found in separate individuals. Generally, in aquatic animals , fertilization is external, while in land animals it is internal. When fertilization is internal, the zygote may develop outside the female parents body as in birds or within its body ad in mammals.

    In lower plants like algae, mosses and ferns, motile male gametes are usually produced in sex organs. These swim to the egg cell and one of them fertilizes it. These plants also show alternation of generation. 

   In seed bearing plants, the male gamete is a cell in the pollen grain, while the female gametes is a cell in the ovule. In flowering plants, the flower is the reproductive organ. It bears the stamens that produce the pollen grain, and the pistil which contains the ovule in the ovary. Most flowering plants have bisexual flowers, I.e. flowers that both stamens and pistils. In a few plants, separate male and female flowers are borne on the same plant, e.g. maize and oil palm. In a few others , like the pawpaw, there are separate male and female plants.

   In flowering plants, the pollen is carried by external agents such as wind,  water  and  animals , especially insects to  the stigma of the pistil. The pollen then develops a tube that leads to the egg cell In the ovary. A tube cell leads the male gametes which are carried  into he tube passively. Fertilization occurs when one of the male gametes fuses with the egg cell to form a zygote. The fertilized ovule develops into seed, while the ovary develops into a fruit. The fruit and seeds are dispersed by the various means. Under suitable conditions, the seed germinates to produce a young plant.