Page:Popular Science Monthly Volume 62.djvu/442

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ORMAL development and growth of forest plants is possible only when the plants are able to obtain a sufficient amount of nutritive substances from their medium.

Among the substances indispensable for the nutrition of plants nitrogen occupies a conspicuous place. On an average, sixteen per cent, of this element is included in the composition of albuminous matter. Nitrogen is necessary for the formation of protoplasm of vegetative cells, and when it is absent no formation of protoplasm can take place; hence, no development of organic life in general is possible. Plants derive nitrogen from the soil, where it is found in a free state (air), or combined in the form of nitrogenous compounds.

If plants had the capacity of absorbing atmospheric nitrogen and assimilating it like carbon, the question as to the presence of nitrogen in the soil would have no interest, since the air would be a sufficient source of nitrogen. While some plants have the capacity of absorbing atmospheric nitrogen, the majority draw their supply from the salts of nitrogen found in the soil.

The consumption of atmospheric nitrogen has been fully determined for the wild acacia (black locust) and for the white and black alders. Thus in quartz sand completely free from nitrogen, seeds of black locust, while developing into seedlings, increased their contents of nitrogen from 0.0024 gr. to 0.092 gr., or more than 38 times, between May 1 and September 10. The locust in this respect resembles all the other leguminosæ, which have long been known to agriculturists as accumulators of nitrogen. On examining plants which absorb the free nitrogen contained in the soil, it was found that the roots of these plants have tubercles inhabited by organisms. These organisms are bacteria, which, attracted by the excretions of the roots, immigrate from the soil and cause the formation of root-tubercles which are invariably present in the papilionaceæ. The bacteria absorb the atmospheric nitrogen and transmit it to the plant. A great many bacteria cause tubercles on the roots; they can be classified according to the species which they choose as their host, more conveniently than by their external appearance. Each species of the leguminosæ seems to possess its own race of bacteria, which can be made serviceable to