Reproduction

Reproduction, process by which an organism produces offspring. In asexual reproduction parts of an organism split off to form new individuals; the process is found in some animals but is more common in plants: e.g., the fission of single-celled plants; the budding of yeasts; the fragmentation of filamentous algae; spore production in bacteria, algae, and fungi; and the production of vegetative organs in flowering plants (bulbs, rhizomes, and tubers). In sexual reproduction, special (haploid) cells containing half the normal number of chromosomes, called gametes, are produced: in animals, sperm by males in the testes and ova by females in the ovary; in plants, pollen by males in the stamens and ovules by females in the ovary. The joining of gametes (fertilization, or conception) produces a (diploid) cell with the normal number of chromosomes, the zygote, which grows to produce an individual with genes inherited from both parents. Fertilization may take place inside the female (internal fertilization) or outside (external fertilization). Internal fertilization demands that sperm be introduced into the female—insemination by copulation—and is advantageous because the young spend the most vulnerable early stages of their life histories protected inside the mother.