In single-celled organisms e. 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 sexa process that is called conjugation. In ciliate protozoans e. 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 Cellular process of asexual reproduction. Many multicellular lower plants give off asexual spores, either aerial or motile and aquatic zoosporeswhich 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 "Cellular process of asexual reproduction," 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 flatwormsthe individual worm can duplicate by pinching in two, each half then regenerating the missing 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. 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: At some later stage in the life history of the organism, the chromosome number is again reduced by meiosis to form the next generation of gametes. The gametes may be equal in size isogamyor one may be slightly larger than the other anisogamy ; the majority of forms have a large egg and a minute sperm oogamy.
The sperm are usually motile and the egg passive, except in higher plants, in which the sperm nuclei are carried in pollen grains that attach to the stigma a female structure of the flower and send out germ tubes that grow down to the egg nucleus in the ovary. Some organisms, such as most flowering plants, earthworms, and tunicates, are bisexual hermaphroditicor monoecious — i.
All other organisms, including some plants e. Some sexual organisms partially revert to the asexual mode by a periodic degeneration of the sexual process. For instance, in aphids and in many higher plants egg nucleus can develop into a new individual without Cellular process of asexual reproduction, a kind of asexual reproduction that is called parthenogenesis. We welcome suggested improvements to any of our articles.
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