Note 15 bis — Chapter V · NITROGEN AND AIR

Although the numerous, and as far as possible accurate and varied researches made in the physiology of plants have proved that the higher forms of plants are not capable of directly absorbing the nitrogen of the atmosphere and converting it into complex albuminous substances, still it has been long and repeatedly observed that the amount of nitrogenous substances in the soil is increased by the cultivation of plants of the bean (leguminous) family such as pea, acacia, &c. A closer study of these plants has shown that this is connected with the formation of peculiar nodular swellings in their roots caused by the growth of peculiar micro-organisms (bacteria) which cohabit the soil with the roots, and are capable of absorbing nitrogen from the air, i.e. of converting it into assimilated nitrogen. This branch of plant physiology, which forms another proof of the important part played by micro-organisms in nature, cannot be discussed in this work, but it should be mentioned, since it is of great theoretical and practical interest, and, moreover, phenomena of this kind, which have recently been discovered, promise to explain, to some extent at least, certain of the complex problems concerning the development of life on the earth.