Bacterial life in several ways is able to reclaim from the atmosphere this free nitrogen, which would otherwise be lost. The first method to which reference may be made is that involving _symbiosis_. This term signifies "a living together" of two different forms of life, generally for a specific purpose. It may be to mutual advantage, a living for one another, or it may be, by means of an interchange of metabolism or products, finally to produce or obtain some remote chemical result. It is convenient to restrict the term symbiosis to complementary partnerships such as exist between algoid and fungoid elements in lichens, or between unicellular algæ and Radiolarians,[45] or between bacteria and higher plants. The partnerships between hermit crabs and sea-anemones and the like are sometimes defined by the term _commensalism_ (joint diet). Symbiosis and commensalism must be distinguished from _parasitism_, which indicates that all the advantage is on the side of the parasite, and nothing but loss on the side of the host. The distinction between symbiosis and commensalism cannot be rigid, but between these conditions which are advantageous to the partners and parasitism, there is an obvious and radical difference. Association of organisms together for increase of virulence and function should be distinguished from symbiosis, and mere existence of two or more species of bacteria in one medium is not, of course, symbiosis. Most frequently such a condition would result in injury and the subsequent death of the weaker partner, an effect precisely opposite to that defined by this term. The example of bacteriological symbiosis with which we are concerned here is that partnership between bacteria and some of the higher plants (_Leguminosæ_) for the purpose of fixing nitrogen in the plant and in the surrounding soil. [Illustration: ROOTLET OF PEA WITH NODULES] _The Nitrogen-fixing Bacteria_, the third group of micro-organisms connected with the soil, exist in groups and colonies situated inside the nodules appearing, under certain circumstances, on the rootlets of the pea, bean, and other _Leguminosæ_. It was Hellriegel and Wilfarth who first pointed out that, although the higher chlorophyllous plants could not directly obtain or utilise free nitrogen, some of them at any rate could acquire nitrogen brought into combination under the influence of bacteria. Hellriegel found that the gramineous, polygonaceous, cruciferous, and other orders depended upon combined nitrogen supplied within the soil, but that the _Leguminosæ_ did not depend entirely upon such supplies.