The relation which exists between the nodule bacteria and their host is an interesting one. The bacteria can live in the soil for several years with out the presence of the legume host,—how long is not known. Nor is it known what permanent bene fit the organism derives from its association with its host. There is at least a temporary gain by the rapid increase in the number of bacteria which are formed within the nodule, but the larger number of these become surcharged with the nitrogen which they fix, pass into abnormal and involution forms and die. It may be, however, that the living ones which escape again into the soil form an in crease over what the increase would be in the soil, and also that the association with the legumes may give them new vigor. The host benefits by the association from the increased nitrogeneous sub stance placed at its disposal. This is abundantly shown by experiment where there is an increase in size and product when the organism is present over that under the same conditions when the organism is absent. The few cases which have been observed under experimental conditions where the bacteroids assume a firm condition so that they cannot be dis solved by the host, cannot be taken as proof against the general and almost universal benefit derived by the host from the association with the bacteria, ex cept when the soil is already very rich in nitroge neous plant-food. Even under these conditions, although the number of nodules is smaller than in nitrogen-poor soil, there may be an increase of nitrogen in the plant, though no increase in the crop. It cannot be denied, therefore, that there is a mutual benefit derived from this association of the bacterium and the legume in the nodules. The bacterium lives within the nodular root, and thus the nodules are endotrophic mycorhiza.
This relationship of the bacterium and the legume is a good example of what is ordinarily called sym biosis, a living together. The term is now gener ally applied to those cases of symbiosis where there is a mutual benefit to the symbionts. This special kind of symbiosis is often called mutualistic or reciprocal symbiosis to distinguish it from those cases of symbiosis existing between a strict para site and its host, which is called antagonistic sym biosis. Disjunctive symbiosis has reference to the relation of flowers and insects in pollination, while contact symbiosis has reference to the relation between the bacterium, Clostrydium pasteurianum, and certain low, blue-green algfe in the soil, the alga; supplying the bacterium with carbohydrates. These carbohydrates supply the Clostrydium with the energy which enables it to assimilate free nitrogen.
Some have raised an objection against the use of the term symbiosis applied to the relation of the nodule bacterium and the legume, on the ground that the bacterium is a parasite, that certain cells in the tubercle are destroyed, and that it is difficult to see what benefit the host can derive from an association with a parasite which destroys some of its cells. It is beyond contradiction, however, that
leguminous plants do benefit from this association, in the fixed nitrogen which they are able to absorb from the dead bacteroids in the nodule, except perhaps in soils already rich in nitrogenous plant food, under which condition it is known that few nodules are formed, while in soils poor in nitroge nous plant-foods many nodules are formed and the legume profits to a great extent from the symbiosis. The 'parasitism is confined to the nodular roots or mycorhiza. This nodule serves a useful purpose for the legume, and the fact that its formation is caused by a parasite, and that some of its cells die, does not necessarily lead to the conclusion that the legume does not benefit by the association. Other normal organs of the plant, as leaves, perform special and important work for the plant, and later die. But the good they have served the plant more than balances the loss of the part or the death of its cells.
It has also been recently stated that since the early relation of the bacterium in the nodule is that of a parasite, this relation cannot be symbiosis in the sense in which DeBary used the term. Now, DeBary distinctly says in his "Die Erscheinung der Symbiose," 1879 (the following is a translation), "The best known and most exquisite phenomenon of symbiosis is complete parasitism, i. e., that arrangement by which an animal or plant goes through its entire vegetative process on or in another organism belonging to a different species. The latter serves the parasite exclusively as a dwelling place and furnishes it with its entire food material ; it is in every sense of the word its host." Literature.
H. Marshall Ward, some recent publications bearing on the question of the sources of nitrogen in plants, Annals of Botany, 1, 325-357 (1888); Atkinson, The Biology of the Organism Causing Leguminous Tubercles, Botanical Gazette, 18, 157 266, plates 12-15 (1893); Moore, Soil Inoculations for Legumes, Bulletin No. 71, Bureau of Plant In dustry, United States Department of Agriculture (1905); Pfeffer, Physiology of Plants, 1, 393-403 (1900). The literature referred to in these works will supply other references. Germ life in the soil is discussed at length in Vol. I, Chapter XIII, of this Cyclopedia, and should be read in this connec tion. Additional references to literature are given there.