SYMBIOSIS (Neo-Lat., from Gk. cettlii(datc, life together, from are, syn, together + 3io5, bias, life.) That condition in which two or more dis similar organisms live together in an intimate life relation. Various types of symbiosis depend upon the type of relation existing between the organisms, the so-called symbionts.
(1) Mutualistie symbiosis, or mutnalism, is that condition in which each party to the sym biosis seems to derive advantage therefrom. This relation has perhaps not been proved to exist in plants. though it seems likely that mutualism exists between the legumes and the bacteria of their root tuhereles. Many be lieve that lichens show true mutualism, the fungus symbiont supplying nitrogenous sub stances or other materials absorbed from the substratum, and the alga symbiont the earbohy drates.
(2) Antagonistic symbiosis, or parasitism, is that condition where one symbiont gains at the expense of the other. There are ninny illustra tions of this in nature. For example, the para sitie fungi and seed plants may be regarded as securing food from their host plants, but not giv ing an equivalent in return, as is the case in perfect mutualisin.
(3) A somewhat intermediate condition is illustrated by contingent symbiosis, or helotism (q.v.). Sonic writers hold that lichens illus trate this type, the idea being that there is not a mutual exchange between the alga and the fungus symbiont. The fungus is supposed to gain, but the alga is supposed neither to lose nor gain.
Symbiosis may be inure or less intimate. In conjunctive symbiosis the symbionts are blended together so as to form a single body, as best illustrated by the lichens. In disjunctive sym biosis the symbiosis is more temporary, or the symbionts are less completely blended, as illus trated by eetotropic myeorrliizas, Relations which are, perhaps, not to he re garded as symbiosis, but which nevertheless bear a close similarity to it, are frequently found. Saprophytism differs from parasitism only in that the host plant, so to speak, is dead. Re cently a new life relation has been recognized, such as is found in the Indian pipe (Monotropa ) ; in which case there is a root fungus which de rives its food materials saprophytically from the soil. Monotropa, not being a green plant. gets
nearly all of its food material from the fungus. Hence the relation of the two organisms taken together is saprophytism. The mutual rela tion of the pair is, however, symbiosis, and the whole relation may be ealled symbiotic sapro phytism. This term may be applied to all of the myeorrhiza plants. By some authors lianas and epiphytes have been regarded as having symbiotic relations with the supporting plants, but the relation is not at all intimate, since they prob ably get only mechanical support. Kerner held a still broader view of symbiosis, applying this term, for instance, to the herbaeeous shade plants of the woods, which are unable to grow unless shaded by the trees.
Among animals forms of symbiosis are to he found. An example of social symbiosis is the ease of a polyp (Epizoanthus) or a sea-anemone (Adamsia), which grows on the shell inhabited by a hermit-erab. Kiebs states that when the crab changes its shell it seizes the anemone and carries it off to its new home. The term physiological symbiosis may he ap plied to cases among, the lowest organisms where certain algae (Zoiixanthella.) often live in Rad iolaria (q.v.), while similar green and yellow plants inhabit the stomach epithelium of many aetinians, corals, and worms. As Hertwig says, the Zo3xanthel1e are nourished by the earl on dioxide which is formed by the animal tissues, and breathe out oxygen, which in turn serves as food for the animal. They also form starch and other carbohydrates, and there is nothing to pre vent any surplus thus formed from becoming food for the animal. A remarkable ease of sym biosis, involving a structural iraidification, is that of a carpenter bee (of the Oriental genus Koptorthosoma) and a certain mite. A special chamber, with a small orifice for entry, exists in the abdomen of the bee, in whieh the Acari are lodged. Here it is a little difficult to draw the line between symbiosis and the parasitism, for instance, of the itch mite which burrows under the skin between the fingers.