other species only by gradual adaptation. Leguminosæ which lack proper bacteria are backward in development.
If, for instance, black locusts are grown in soil which contains no bacteria or only bacteria taken from the tubercles of leguminosæ somewhat less closely allied to them, they thrive only moderately; whereas under the influence of the locust bacteria they develop luxuriantly. The organism living in the root tubercles of the alder is also, according to Hiltner, instrumental in the absorption of nitrogen. It displays its activity particularly whenever the available nitrogen of the soil begins to be exhausted on account of its vigorous consumption by the tree. The fungi which cover the suction rootlets of oaks, beeches, birches, and other cupuliferæ, as well as conifers and many other woody plants, probably also assist the trees in the taking up of nitrogen. They do not, however, absorb the atmospheric nitrogen, but only decompose, among other organic substances, the nitrogenous compounds of humus, and thus convert them into chemical combinations more accessible to the tree.
Thus, excepting the leguminosæ and a few species of other families, plants depend for their supply of nitrogen upon the nitrogenous compounds in the soil. And it is with these plants that the question as to the source of nitrogen in the soil becomes important, inasmuch as there are no minerals containing nitrogen so widely distributed as to satisfy the demand of trees for this element. Atmospheric precipitations and the product of the decay of animal and vegetable refuse are usually supposed to be the sources of nitrogenous compounds in the soil. The chemical compounds in which nitrogen occurs in the soil are chiefly nitrates, ammonia, and organic nitrogenous compounds resulting from the decomposition of organic substances. The best sources of nitrogen for green vegetation are generally supposed to be the nitrates; but for forest trees, at least, these seem to be of small account, because, as Ebermayer says, in forest and marshy soils nitrates are present, if at all, only in small traces. Also, in the interior of stems nitrogen is found in the form of nitric acid only when they are grown on a soil containing nitrates; for instance, on cultivated land. Among the compounds of nitrogen which are supplied to the soil through atmospheric precipitation, some nitric acid is found, but two to five times as much ammonia is present. The whole amount of nitrogen which is held in combination and supplied to the soil in this manner through snow and rain amounts, according to Ramann, to nine pounds per year per acre, and even less. This is insufficient to supply the demands of trees for nitrogen, as they store up a much greater quantity. Ebermayer states the average amount of nitrogen taken up by various kinds of trees per year and acre as follows:
