4. Electricity fixes, and may in the future be made to fix more, nitrogen. If a strong inductive current be passed between terminals, the nitrogen from the air enters into combination with the oxygen, producing nitrous and nitric acids.
5. Abundant evidence has now been produced in support of the fact that there is considerable fixation by means of bacteria.
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
Rootlet of Pea with Nodules 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.
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.
It was observed that in a series of pots of peas to which no nitrogen was added most of the plants were apparently limited in their growth by the amount of nitrogen locked up in the seed. Here and there, however, a plant, under apparently the same circumstances, grew luxuriantly and possessed on its rootlets abundant nodules. The experiments of Sir John Lawes and Sir Henry Gilbert at Rothamsted[46] demonstrated further that under the influence of suitable microbe-seeding of the soil in which Leguminosæ were planted there is nodule formation on the roots, and coincidentally increased growth and gain of nitrogen beyond that supplied either in the soil or in the seed as combined nitrogen. Presumably this is due to the fixation, in some way, of free nitrogen. Nobbe proved the gain of nitrogen by non-leguminous plants (Elœagnus, etc.) when these grow root nodules containing bacteria, but to all appearances, bacteria differing morphologically from the Bacillus radicicola of the leguminous plants.
| —Cellular sheath of Rootlet forming capsule of nodule. —Colonies of bacteria in situ. | |
| Nitrogen Fixing Bacteria in Situ in Nodule on Rootlet of Pea × 400 |
| Nitrogen Fixing Bacteria in Situ in Root-Nodule of Pea (Section of Nodule) × 500 | Nitrogen Fixing Bacteria in Situ in Root-Nodule of Pea (Section of Nodule) × 600 |