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Standard Cyclopedia of Horticulture

Symbiosis is the intimate association of two or more distinct organisms, with benefit to one only, or to both; commensalism, consortism; copartnership. In this association each organism is called a symbiont.

According to the character of the union, several kinds of symbiosis have been recognized: (1) Mutual antagonistic symbiosis (mutual parasitism), when two organisms are foes of each other, as certain bacteria and animals, the latter showing a "natural resistance;" also the syntropism of certain lichens with lichens. (2) Antagonistic symbiosis (true parasitism), when the host is partly or completely killed by the parasite, as the potato and the rot fungus (Phytophthora infestans); or galls (hypertrophies) produced on the host, as in the black-knot of plums; and in higher plants, which live at the expense of others, as the mistletoe (green) and the dodder (chlorophylless). (3) Mutual symbiosis, when there is often reciprocal advantage; (a) nutricism, when one symbiont nourishes the other without apparently receiving any return, as the mycorrhiza and the roots of forest trees (mycosymbiosis); (b) mutualism, when a mutual benefit results from the union of two organisms capable of living separately, as the bacteroid and the roots of the Leguminosae; (c) individualism, when the symbionts are so intimately connected in their growth as to suggest a single individual, as the union of alga and fungus to form a lichen. By some this relationship of alga and fungus in the lichen thallus is regarded as helotism, or slavery, where the alga lives entirely indifferent to the fungus. The views of Bruce Fink, who considers the lichen to be a fungus with an alga associated with it, are widely different from the usually accepted views on the subject. (4) Prototrophy, the wet-nurse relationship, as in the lichen Lecidea intumescens, which eventually gets its nourishment by means of a lodger, a different lichen. (5) Contingent symbiosis, when one symbiont lives in the interior of another for shelter, as Nostoc in the tissues of Hepaticae, Lemna, Cycas, Gunnera; and Anabaena in Azolla. Green plants live symbiotically with animals such as Spongilla, Hydra, and Convoluta. In Hydra, the green alga, known as Chlorella, is found in the endodermic layer and when the colorless eggs of the fresh-water polyp are almost mature a few of the green cells are found migrating into the protoplasm of the egg-cells. CH

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