134 AN INTRODUCTION TO SCIENCE 



used as manure for his hops has often been repeated of late 

 years. Fearing that the dirty rags, which at one time were 

 invariably applied to hop gardens, might be a source of 

 danger to his family and labourers, he had them cooked in a 

 caldron before they were dug into the ground ; but found to 

 his astonishment that they no longer acted as a stimulant to 

 the hops. The rags were useless as manure when freed 

 from the flakes of epidermis and other germ-bearing rem- 

 iniscences of their sometime wearers. Although this 

 story will hardly bear scientific criticism, it points a moral. 

 The soil is prepared for the rootlets of plants by three sets 

 of bacteria: (a) those which reduce organic matter to simple 

 salts which plants can absorb; (b) those which oxidize 

 nitrogenous (ammonia) compounds into nitrites and nitrates ; 

 and (c) those which fix the nitrogen of the atmosphere. 

 Among the most interesting of the latter are the nitrogen- 

 fixing bacilli which grow in minute nodules on the roots of 

 leguminous plants. This life is an illustration of genuine 

 symbiosis, the bacteria being housed by the higher plant in 

 specially made excrescences, for the sake of the services 

 which they are able to render in return. The nodules on 

 the root of a pea are easily seen even without a lens. If 

 one of them is cut a creamy fluid escapes, which is found 

 upon microscopic examination to be loaded with bacilli. 

 How the bacilli do their work has not been ascertained as 

 yet, but it is certain that they fix the nitrogen of the air 

 which circulates in the interstices of the soil. Farmers have 

 long known that peas, .vetches and clovers, better than any 

 other crops, prepare the land for wheat. They were aware, 

 too, of the importance of well stirring the soil to admit air. 

 Now that the explanation has been found, it is probable that 

 scientific agriculture will discover means of replacing the 

 nitrogen which is constantly escaping from the soil, without 

 recourse to artificial manures. Experiments have been made 

 on a large scale in Germany in cultivating nitrogen-fixing 

 bacilli and introducing them with the seed. The use of these 



