SYMBIOSIS, a mutually beneficial internal partnership be tween two organisms of different kinds, such as sea-anemone and unicellular algae. It differs from commensalism, where the part nership is external, as in the case of some hermit-crabs (e.g., Pagurus bernhardus) which have as constant companions certain sea-anemones (e.g., Adamsia rondeletii). It also differs from endo parasitism, which is not beneficial to the host; though it is pos sible that some symbionts or symbions, e.g., the bacteria in the root-tubercles of Leguminosae, began as parasites; and that some parasites began as symbionts. The term mutualism is often used to cover both symbiosis and commensalism; and the term com mensalism is sometimes used as equivalent to mutually beneficial symbiosis. This unnecessary confusion is increased when para sitism is called "antagonistic symbiosis." The idea of symbiosis first became clear in regard to lichens, which were shown to be mutually beneficial combinations of an alga and a fungus. This was demonstrated analytically by De Bary (1866) and others, while Rees and Stahl were subsequently successful in building up a lichen synthetically from the ap propriate alga and fungus. Symbiosis between two different plants is also illustrated by the numerous cases of mycorhiza (q.v.), where a fungus has entered into intimate and profitable partnership with the roots of other plants, such as oaks, pines and orchids. Sometimes, as in the case of heather, the partner fungus spreads through the whole plant, from root to stem, from leaf to flower, and even into the seed. It is this partnership with a fungus that enables the heather to flourish on the soil of the moorland where few other plants can survive. This mycorhiza relation is now known to be very common, and it occurs in various forms. Thus the fungus may be ectotrophic, investing the roots externally, as in the beech; or it may be endotrophic, penetrating into the internal tissue, as in orchids. Also symbiotic is the oc currence of tubercle-forming bacteria (e.g., Bacillus radicicola) in the roots of leguminous plants, a linkage of great importance, inasmuch as the bacteria are able, in some way not yet clear, to capture and fix the free atmospheric nitrogen.
A second form of symbiosis is between an animal and an in cluded unicellular green alga, such as Zoochlorella. The plant is sheltered, and it may be borne about by the animal, whose respiratory carbon dioxide it can utilize in its photosynthesis. The animal profits by the oxygen given off by its minute part ners, and it can also utilize the carbon-compounds which the algae build up. Some green animals—green because of the in cluded algae—can flourish for a long time without any extraneous food, being sufficiently sustained by what their symbionts supply.
Most animals of a green colour are green because of these partner plants, many of which are included in the genera Zoochlorella and Zooxanthella. This was first made clear by Cienkowsky, Geddes and Brandt in regard to the "yellow cells" found inside radiolarians, whose symbiosis probably contributes to the great wealth of numbers and of species exhibited by these pelagic Protozoa. While there seem to be some Protozoa, such as euglenids and Vorticella viridis, which have chlorophyll cor puscles of their own, most of the green or greenish Protozoa, e.g., Stentor, owe the colour to their symbionts. The same is true for freshwater sponges, Hydra viridis, some sea-anemones, most reef-corals, numerous Alcyonarians, some Turbellarians, and a few higher forms, e.g., a polyzoon and a sea-slug. Very interesting is the small turbellarian worm, Convoluta roscoffensis, which creeps from the sand and forms green patches On the flat beach at Roscoff, in Brittany, when the tide goes out. Its life has become intimately wrapped up with a symbiotic alga, one of the Chlamy domonadineae. As Keeble has well shown, there are four chapters in the history of Convoluta. The very young worm feeds inde pendently. Then green symbionts appear and multiply, and the worm is nourished from within as well as from without. Thirdly Convoluta ceases to take any solid food and depends entirely on the photosynthetic activity of the symbions. Finally, the animal digests its partners, and, having done so, dies.
A third type of symbiosis, especially studied by Buchner, is seen in many insects where fungoid plants, e.g., bacteria and yeasts, live in the lining cells of the food-canal and assist in some obscure way in the process of digestion. These intracellular sym bionts are known in at least seven orders of insects, especially in types that feed on dry wood and the like. The fungoid infec tion usually takes place the egg-cell. In some cases there is a special organ or "mycetome" for the multiplication of the sym bionts. In a few luminescent marine animals, e.g., some cuttlefish, (such as Sepiola elegans) there is considerable evidence that the light is produced by symbiotic bacteria.
A similar type of symbiosis is illustrated by the wood-eating species of termites, which have remarkable infusorians in their food-canal. These are essential to the health of the termites which die when bereft of them. Their symbiotic function is to digest the wood, which no higher animal is able to do. Thus there may be symbiosis between plant and plant, between plant and animal, or between animal and animal.