364 ANNUAL REPORT OF THE Off. Doc. 



Of these acids, certain of them have existed in the rocky crust of 

 the earth, from which soils have originated, before the advent of life, 

 and hence are not of bacterial origin. These are sulphuric, phos- 

 phoric, silicic and hydrochloric acids. Others of them are products 

 of bacterial life such as carbonic and nitric acids. 



Of the bases, all except ammonia are of primordial or earth crust 

 origin. 



Of the salts, the sulphates, silicates, carbonates and chlorides, 

 which are largely' present in rocks, are not absorbed except in minute 

 quantities, the greater part of the bases being taken up as nitrates 

 and phosphates, and also as salts of the organic acids. 



In the process of nitrification the nitric acid combines with the 

 various bases present in the soil, and nitrates are produced. 



Phosphoric acid exists in the soil in the form of insoluble basic 

 phosphates, which, under the action of organic acids, are converted 

 into neutral or acid salts which are soluble. Hence the production 

 of organic acids» by bacterial fermentation renders phosphoric acid 

 available to plant roots. 



We have already spoken of the action of carbonic acid, in disin- 

 tegrating, and setting free as carbonates, the various bases locked up 

 as mineral silicates. These carbonates unite with silica to form 

 zeolites and these in turn are slowly decomposed by organic acids, 

 and their contained bases again liberated as organic salts. 



H. Carrington Bolton has shown that many minerals are slowly 

 decomposed by the action of cold citric acid, the zeolites and other 

 hydrous silicates being especially susceptible. * 



Thus by the combined production of carbonic, nitric and the va- 

 rious organic acids, through the action of bacterial life, w^e have all 

 the necessary agencies at hand for the dissolution of the mineral 

 elements of plant growth. 



VIII. THE ASSIMILATION OF ATMOSPHERIC NITROGEN. 



1. HiSTOEiCAL Summary. 



The Biology of Root Tubercles. — It is a fact familiar to all that 

 the roots of leguminous plants contain nodular swellings or excres- 

 cences known as ''root tubercles." 



These have been recognized from the earlier days of botanical in- 

 quiry, and as far back as 1615, DeL^champ used the characters of 

 the root tubercles as a basis of classification, and the same use was 

 made of them by DeCandole''^ in his System of Vegetation. 



At different periods during the earlier part of the nineteenth cen- 



