Reproduction, process whereby all living organisms produce offspring. Reproduction is one of the essential functions of plants, animals, and single celled organisms, as necessary for the preservation of the species as eating is for the preservation of the individual.
In almost all animal organisms, reproduction occurs during or after the period of maximum growth. In plants, which continue to grow throughout their lifetime, the relationship between growth and reproduction is more complex. Individual plants have growth limitations imposed by inherited characteristics and environmental conditions; if the plant grows excessively, any of a number of reproductive processes may be stimulated (see Plant Propagation). Environmental conditions also play some part in the reproduction of higher animals, but hormonal elements are more important.
Most single-celled organisms reproduce by a process known as fission, in which the parent organism splits into two or more so-called daughter organisms, thereby losing its original identity. Cell division, which results in the multiplication of cells constituting the tissues, organs, and systems of a multicellular organism, is not considered true reproduction; it is almost identical, however, with the binary fission of single-celled organisms. In certain multicellular animals, such as the cnidarians, sponges, and tunicates, cell division often results in the production of buds that arise from the body of the parent and then later separate to develop into a new organism identical with the parent; this process, known as gemmation, is analogous to the process of vegetative reproduction or propagation in plants. Reproductive processes such as those cited above, in which only one parent gives rise to the offspring, are scientifically classified as asexual reproduction. The offspring produced are identical with the parent.
A number of single-celled organisms multiply by conjugation. In this process, which is analogous to fertilization, two similar unicellular organisms fuse, exchange nuclear materials, and then break apart. Each organism then reproduces by fission; occasionally, after conjugation, the participating organisms do not reproduce, the process in these instances seeming merely to revitalize the organisms. Conjugation is the most primitive method of sexual reproduction by which organisms having genetic characteristics derived from two parents are produced. Most multicellular animals and plants undergo a more complex form of sexual reproduction in which especially differentiated male and female reproductive cells (gametes) unite to form a single cell, known as a zygote, which later undergoes successive divisions to form a new organism. The terms fecundation and fertilization are applied to the union of the male and female cells. In this form of sexual reproduction, half the genes, the carriers of inheritable characteristics, in the zygote come from one parent and half from the other parent. See Cell.
Many lower multicellular organisms and all higher plants undergo an alternation of generations. In this process, a sexually produced generation alternates with an asexually produced generation. In some organisms that reproduce sexually, a process known as parthenogenesis occurs in which the female sex cell develops without fertilization.
In higher animals, individuals of a single species are either male or female, according to whether they produce male reproductive cells or female reproductive cells. The typical male reproductive cell, which is known as a sperm, spermatozoon, or spermatozoan, is a motile cell with a head containing the nucleus and a whiplike tail with which it swims. The typical female reproductive cell, which is known as an egg or ovum, is a rounded cell many times larger than the sperm and containing large amounts of cytoplasm surrounding the nucleus. Plant reproductive cells are roughly similar to animal cells, the male cell being known as the sperm or microgamete and the female cell as the ovum or macrogamete.
In some relatively simple animals such as the earthworms and leeches, organs producing sperm and ova occur in the same individual (see Hermaphroditism). Although such animals produce both male and female gametes, the production of sperm and ova usually occurs at different times, so that these animals generally do not fertilize themselves but rather other individuals of the same species. Certain hermaphroditic animals, such as the planarian flatworms, habitually undergo self-fertilization. Among plants, one individual may bear reproductive organs of only one sex, separate reproductive organs of both sexes, or reproductive organs containing both male and female elements (see Flower). Individuals among higher animals bear reproductive organs of only one sex.
This requires the bringing together of sperms and ova that are produced at a distance from each other. In plants, wind and insects carry the sperm to the stationary egg, or, in a liquid medium, the sperm swims to the egg. In lower animals, deposition of sperm and eggs in water is common. This method is haphazard, as only a few of the many sperm discharged reach the eggs. In higher animals, various adaptations have evolved by means of which the spermatozoa, contained in a liquid known as semen, are deposited in the lower segment of the female reproductive tract.
In vertebrates this internal deposition of semen occurs in the process of copulation, also known as coitus or sexual intercourse. During copulation animals are locked in close embrace, and the male inserts the male genital organ, the penis, into the female genital orifice, the vagina, discharging the semen therein. This process is known as insemination. Both animal and human spermatozoa can be kept alive outside the body for long periods by freezing; artificially introduced into the female reproductive tract, such spermatozoa produce pregnancy. The method, called artificial insemination, has been used in human reproduction to produce offspring where either the male or the female is sterile (see Infertility) and in animals for breeding purposes.
The union of male and female, necessary for internal fertilization, is encouraged in nature. Most lower animals have definite seasons, governed by endocrine secretions (see Endocrine System; Hormone). In most female mammals, estrus, or receptivity to mating, is operative only for short periods during the year. Animals such as the cow have several such periods of estrus during the year, the dog one or two. The human female normally has a 28-day reproductive, or menstrual, cycle, with ovulation usually occurring about 14 days before the onset of the menstruation, but sexual interest in humans is more culturally determined than tied to the reproductive cycle. In most animals, copulation is preceded by a period of courtship; the courtship ritual is stereotyped to a large extent (see Animal Behavior). In humans, courtship and mating practices have been strongly modified by social and religious codes of behavior set by society.
After fertilization of the egg, the resulting zygote undergoes cell division and differentiation to form the embryo. In most higher plants, the embryo is enclosed in a layer of nutritive material surrounded by a hard outer covering, forming the seed. In most lower animals the embryo, surrounded by the nutritive material of the former ovum, is enveloped by a leathery or calcareous shell and is extruded from the body of the female. Animals, such as birds, that lay their eggs before the young are completely developed are termed oviparous. Ovoviviparous animals are those that produce eggs in shells but that hatch the eggs within the body of the mother. The placental mammals never produce shelled eggs; the embryo is implanted in the uterus and is nourished by the mother until almost completely developed. Animals that give birth to live young without forming eggs are termed viviparous. The period during which viviparous animals carry their young internally is known as the period of gestation.
In humans the period of years during which reproduction can occur begins with the onset of puberty, and the reproductive capacity of the female ends at what is known as menopause, or when menstruation ceases.
See also Fertilization; Pregnancy and Childbirth; Reproductive System.
Contributed By:
Jean Roche
Newton E. Morton
Asexual Reproduction
The single-celled amoeba demonstrates a simple method of asexual reproduction. It divides in half by a process called fission, producing two smaller daughter cells. After a period of feeding and growth, these two daughter cells will themselves divide in half.
Oxford Scientific Films/Peter Parks