The Romance of Modern Chemistry

CHEMISTRY AND AGRICULTURE in vegetable tissue only to a small extent, is an important and indeed essential constituent. The reader might suppose that the natural source of nitrogenous food for the plant would be the atmosphere, with its vast stock of nitrogen. It is conceivable that the leaves might take in and assimilate the nitrogen of the air, just as they deal with the carbon dioxide, which is so much more scarce. There are some who have supposed that this really takes place, but the bulk of the evidence shows that the leaves of plants generally are unable to digest nitrogen when it is presented to them in the form of the element itself. Atmospheric nitrogen, however, does ultimately reach the tissues of some plants, but by a very indirect road, via the soil and the roots. Leguminous plants, such as peas and vetches, are provided with exceptional apparatus for assimilating nitrogen, in the shape of swellings or “nodules11 on their roots. These nodules contain micro-organisms which have the power of taking in atmospheric nitrogen, and so manipulating it as to render it suitable for use as food by the plant. The majority of plants, however, are destitute of these para- sitic attendants, and are unable to utilise atmospheric nitrogen directly; they appear to find this element most digestible when it is presented to them in the form of a salt, such as a nitrate. Nitrates are readily taken up from the soil by plants, and the nitrogen is subsequently transformed into the complex nitrogenous constituents of the plant tissues by various chemical processes which at present are not within our knowledge, far less our power of imitation. In comparison with the practical chemistry which goes on in the cells of plants, the methods of the chemist are elementary and crude, and he may well feel humble in view of the complex and delicate processes 221