CHEMICAL WORK OF PLANTS. 453 



original source of combined nitrogen. The combined nitrogen of the air 

 may possibly be absorbed directly from the air by plants, but it is mainly 

 useful when brought down by rain, snow, dew, fog, etc. However, the 

 amount of nitrogen precipitated from the air, including both that which is 

 secondary to plants and animals and that which is originally produced, is 

 very small, only 1 or 2 kilograms per annum per acre." 



We have therefore yet to account for the main original source of 

 combined nitrogen. Recent discoveries have shown that there are certain 

 bacteria that are capable of utilizing- the nitrogen of the air. The Legunh- 

 nosae, such as peas, beans, etc., form numerous nodules upon their roots, 

 which become the hosts of these bacteria, h and finally abstract the nitrogen 

 fixed by the bacteria. Thus the bacteria abstract and the leguminous 

 plants store nitrogen compounds in the fruits, stalks, and roots. Experi- 

 ments show that where leguminous plants are grown and turned under, the 

 gain in nitrogen for the soil is rapid and the accumulation of combined 

 nitrogen great; and this gain is certainly largely due to the capacity of the 

 bacteria and leguminous plants together to use the free nitrogen of the 

 atmosphere. 



Certain bacteria are able to fix free nitrogen without the assistance of 

 other living plants." Whether the plants obtain their nitrogen from the 

 nitrates already in the soil, or, like the leguminous plants, help to produce 

 the combined nitrogen, the material is mainly built into albuminous 

 compounds or proteids. 



From the foregoing it follows that there are two original sources of 

 combined nitrogen in the soil — that combined with other elements through 

 the agency of plants, and that combined with other elements through the 

 agency of electricity. Material from each of these sources is built into the 

 body of the plants, as already seen, mainly in the form of proteids. The 

 material so long as it is in this form has no important geological effect, but 

 when plants decay these compounds become the source of the nitrogen 

 acids and salts which are important geological agents. (See pp. 465-466.) 



Besides concentrating combined carbon and nitrogen in the belt of 

 weathering, live plants have a direct chemical effect upon the rocks. It is 



« Aikman, C. 1., Manures and manuring, Win. Blackwood & Sons, London, 1894, p. 119. 

 6 Fischer, Alfred, The structure and functions of bacteria, trans, by A. C. Jones, Clarendon Press, 

 Oxford, 1900, pp. 88-97. 



« Aikman, cit., pp. 96-97. 



