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ENCYCLOPÆDIA BRITANNICA


reproduction

process by which organisms replicate themselves.

In a general sense reproduction is one of the most important concepts in biology: it means
making a copy, a likeness, and thereby providing for the continued existence of species.
Although reproduction is often considered solely in terms of the production of offspring in
animals and plants, the more general meaning has far greater significance to living organisms.
To appreciate this fact, the origin of life and the evolution of organisms must be considered.
One of the first characteristics of life that emerged in primeval times must have been the ability
of some primitive chemical system to make copies of itself.

At its lowest level, therefore, reproduction is chemical replication. As evolution progressed, cells
of successively higher levels of complexity must have arisen, and it was absolutely essential
that they had the ability to make likenesses of themselves. In unicellular organisms, the ability
of one cell to reproduce itself means the reproduction of a new individual; in multicellular
organisms, however, it means growth and regeneration. Multicellular organisms also reproduce
in the strict sense of the term--that is, they make copies of themselves in the form of
offspring--but they do so in a variety of ways, many involving complex organs and elaborate
hormonal mechanisms.




Reproduction of organisms

In single-celled organisms (e.g., bacteria, protozoans, many algae, and some fungi),
organismic and cell reproduction are synonymous, for the cell is the whole organism. Details of
the process differ greatly from one form to the next and, if the higher ciliate protozoans are
included, can be extraordinarily complex. It is possible for reproduction to be asexual, by
simple division, or sexual. In sexual unicellular organisms the gametes can be produced by
division (often multiple fission, as in numerous algae) or, as in yeasts, by the organism turning
itself into a gamete and fusing its nucleus with that of a neighbour of the opposite sex, a
process that is called conjugation. In ciliate protozoans (e.g., Paramecium), the conjugation
process involves the exchange of haploid nuclei; each partner acquires a new nuclear
apparatus, half of which is genetically derived from its mate. The parent cells separate and
subsequently reproduce by binary fission. Sexuality is present even in primitive bacteria, in
which parts of the chromosome of one cell can be transferred to another during mating.

Multicellular organisms also reproduce asexually and sexually; asexual, or vegetative,
reproduction can take a great variety of forms. Many multicellular lower plants give off asexual
spores, either aerial or motile and aquatic (zoospores), which may be uninucleate or
multinucleate. In some cases the reproductive body is multicellular, as in the soredia of lichens
and the gemmae of liverworts. Frequently, whole fragments of the vegetative part of the
organism can bud off and begin a new individual, a phenomenon that is found in most plant
groups. In many cases a spreading rhizoid (rootlike filament) or, in higher plants, a rhizome
(underground stem) gives off new sprouts. Sometimes other parts of the plant have the
capacity to form new individuals; for instance, buds of potentially new plants may form in the
leaves; even some shoots that bend over and touch the ground can give rise to new plants at
the point of contact.

Among animals, many invertebrates are equally well endowed with means of asexual
reproduction. Numerous species of sponges produce gemmules, masses of cells enclosed in
resistant cases, that can become new sponges. There are many examples of budding among
coelenterates, the best known of which occurs in freshwater Hydra. In some species of
flatworms, the individual worm can duplicate by pinching in two, each half then regenerating the
missing half; this is a large task for the posterior portion, which lacks most of the major
organs--brain, eyes, and pharynx. The highest animals that exhibit vegetative reproduction are
the colonial tunicates (e.g., sea squirts), which, much like plants, send out runners in the form
of stolons, small parts of which form buds that develop into new individuals. Vertebrates have
lost the ability to reproduce vegetatively; their only form of organismic reproduction is sexual.

In the sexual reproduction of all organisms except bacteria, there is one common feature:
haploid, uninucleate gametes are produced that join in fertilization to form a diploid, uninucleate
zygote. At some later