It was thus clearly proved that _A. vinelandii_ at least does not need symbiosis with algæ to fix atmospheric nitrogen; but experiments with mixed cultures of the above bacillus and another (designated as No. 30 by Lipman) proved that when these two co-operate the absorption of atmospheric nitrogen is nearly doubled. As it is probable that this is the case also with other soil bacteria, the importance of this source of nitrogen to plants is obvious; provided of course that the proper nutritive ingredients are present in available form. Lipman shows that among the organic nutrients, besides the sugars, glycerine and the salts of propionic and lactic acids, and probably also others of the same groups, can serve as nourishment to the nitrogen-fixing bacteria. DISTRIBUTION OF THE HUMUS WITHIN THE SURFACE SOIL. The uniform distribution of the humus-contents of the surface soil, as shown in sections of the same, is by no means easily accounted for. The roots from which its substance is so largely derived are not so universally distributed as to account for it; but least of all can the rapid disappearance of the leaf-fall and other vegetable offal from the surface be accounted for without some outside agencies. Of these, the action of fungous vegetation, and of insects and earthworms, are doubtless the chief ones. _Fungi._—When we examine a decaying root, we find radiating from it a zone of deeper tint, as though from a colored solution penetrating outward. But since under normal conditions humus is insoluble, this explanation cannot stand. Microscopic examination, however, reveals that the outside limit of this zone is also the limit to which the fungous fibrils concerned in the process extend; and as these fibrils are much more finely distributed and much more numerous than the roots of any plant, it is natural that the humus resulting from their decomposition should be more evenly distributed than the roots themselves.[62] [62] Kosticheff, Formation and Properties of Humus; in abstract Jour. Chem. Soc., 1891, p. 611. Such fungous growth is not, however, confined to dead and decaying roots only. A large number of trees and shrubs, among them pines and firs, beeches, aspen and many others, also the heaths, and woody plants associated with them, appear to depend largely for their healthy development, notably in northern latitudes, upon the co-operation (“symbiosis”) of fungous fibrils that “infest” their roots, enabling them to assimilate, indirectly, the decaying organic (and inorganic) matter which would otherwise be unavailable, and at the same time converting that matter into their own substance. Fungous growths thus mediate both the decomposition and rehabilitation of the vegetable debris.