Reproduction

I

Introduction

Reproduction, process whereby living plant or animal cells or organisms produce offspring. Reproduction is one of the essential functions of living organisms, as necessary for the preservation of the species as food getting 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.

II

Asexual Reproduction

Most single-celled organisms reproduce by a process known as fission, in which the parent organism splits into two or more 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 coelenterates, 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.

III

Sexual Reproduction

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.

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 whip-like 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.

A

Hermaphroditism

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. 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.