\_______________________________________/ | PUTREFACTIVE AND DENITRIFYING BACTERIA | +--------------+----------+-------------+-------------------+ | | | | | Free Nitrogen Gases Water Ammonia [Nitrites] | [CO_{2}] and other elements | of broken-down | complex bodies. | | \_______________________/ | | | NITRIFYING BACTERIA | | | Nitrites[=Nitrous organism | | (Nitrosomans)] NITROGEN-FIXING Nitrates[=Nitric organism BACTERIA (Nitrosomonas)] [In soil and in the nodules [In soil and available for on the rootlets of _Leguminosæ_] plant life] The threefold function of plant life is nutrition, assimilation, and reproduction: the food of plants, the digestive and storage power of plants, and the various means they adopt for multiplying and increasing their species. With the two latter we have little concern in this place. Respecting the nutrition of plant life, it is obvious that, like animals, they must feed and breathe to maintain life. Plant food is of three kinds, viz., _water_, _chemical substances_, and _gas_. Water is an actual necessity to the plant not only as a direct food and food-solvent, but as the vehicle of important inorganic materials. The hydrogen, too, of the organic compounds is obtained from the decomposition of the water which permeates every part of the plant, and is derived by it from the soil and from the aqueous vapour in the atmosphere. The chief _chemical substances_ of which vegetable protoplasm is constituted are six, viz, potassium, magnesium, calcium, iron, phosphorous, and sulphur. These inorganic elements do not enter the plant as such, but combined with other substances or dissolved in water. _Potassium_ occurs in salt form combined with various organic acids (tartaric, oxalic, etc.), _calcium_ and _magnesium_ as salts of lime and magnesia in combination both with organic and inorganic acids. _Iron_ contributes largely to the formation of the green colouring matter of plants, and is also derived from the soil. _Phosphorus_, one of the chief constituents of seeds, generally occurs as phosphate of lime. _Sulphur_, which is an important constituent of albumen, is derived from the sulphates of the soil. In addition to the above, there