Sexual reproduction occurs in many one-celled organisms and in all multicellular plants and animals. In higher invertebrates and in all vertebrates it is the exclusive form of reproduction, except in the few cases in which parthenogenesis is also possible. Sexual reproduction is essentially cellular in nature,
Example : It involves the fertilisation of one sex cell by another, producing a new cell, which develops into a new organism. The union of two isogametes is called isogamy, or conjugation, and occurs only in some lower forms. Heterogamy is the fusion of two clearly differing kinds of gametes, distinguished as the ovum and the sperm.
Multicellular plants alternate sexual reproduction, or gametophyte, and asexual reproduction, or sporophyte, generations. The gametophyte produces gametes, and the union of gametes results in the growth of a sporophyte; the sporophyte produces spores that give rise to a gametophyte. The prominent generation in lower plants and the complex fungi is the gametophyte; in the vascular plants it is the sporophyte. The less prominent generation may be an independent plant, as is the small inconspicuous gametophyte of ferns, or a reduced organism consisting of only a few cells and dependent for survival on the prominent form, like the pollen grain, which is the male gametophyte of seed plants.
Many organisms exhibit special reproductive mechanisms to ensure fertilisation; among higher plants the process of pollination may involve extremely complex interaction between the flower and the pollen-bearing agent. Among land-dwelling animals internal fertilisation is necessary in order to provide the fluid environment essential for fertilisation.
Sexual reproduction is of great significance in that, because of the fusion of two separate parental nuclei, the offspring inherits endlessly varied combinations of characteristics that provide a vast testing ground for new variations that may not only improve the species but also ensure its survival. This probably explains the predominance of sexual reproduction among higher forms. Even in those microorganisms that reproduce asexually exchanges of hereditary material take place; in the hermaphroditic plants and animals self-fertilisation is almost always prevented by anatomical specialisations or by differing maturation times for male and female gametes.