QUALITATIVE EXAMINATION 119 



with the air-pump, and exhausted as much as possible. The method can be applied 

 in many different ways (for example, with pyrogallic solution, as in Bulloch's 

 apparatus). 



Of these various methods it is on the whole best to choose either the hydrogen 

 method, the vacuum, or the plan of absorption by grape-sugar or pyrogallic. In 

 anaerobic plate cultures grape-sugar agar plus 0'5 per cent, of formate of soda may 

 be used. The poured inoculated plate should be placed over pyrogallic solution 

 under a sealed bell-glass and incubated at 37 C. Pasteur, Roux, Joubert, Chamber- 

 land, Esmarc-h, Kitasato, and others have introduced special apparatus to facilitate 

 anaerobic cultivation, but the principles adopted are those which have been 

 mentioned. 



The Qualitative Examination of Bacteria in the Soil. We 



may now turn to consider the species of bacteria found in the soil. 

 They may be classified in five main groups ; the division is somewhat 

 artificial, but convenient : 



1. The Denitrifying Bacteria. This group, whose function has 

 been elucidated largely by the investigations of Professor Warington, 

 is held responsible for the breaking down of nitrates. With its 

 members may be associated the Decomposition or Putrefactive 

 Bacteria, which break clown complex organic products other than 

 nitrates into simpler bodies. 



2. The Organisms of Nitrification. To this group belong the two 

 chief types of nitrifying bacteria, viz., those which oxidise ammonia 

 into nitrites, and those which change nitrites into nitrates. 



3. The Nitrogen-fixing Bacteria, found mainly in the nodules on 

 the rootlets of certain plants. 



4. The Common Saprophytic Bacteria, whose function is at present 

 but imperfectly known. Many are putrefactive germs. 



5. The Pathogenic Bacteria. This division includes three types, 

 the bacilli of tetanus, malignant oedema, and quarter evil. Under this 

 heading we shall also have to consider in some detail the intimate 

 relation between the soil and such important bacterial diseases as 

 tubercle and typhoid. 



To enable us to appreciate the work which the "economic 

 bacteria " perform, it will be necessary to consider shortly the place 

 they occupy in the economy of nature. This may be perhaps most 

 readily accomplished by studying the scheme shown on p. 120. 



The threefold function of ordinary plant life is nutrition, assimi- 

 lation, and reproduction, i.e., the food of plants, the digestive and 

 storage power of plants, and the various means they adopt for multi- 

 plying and increasing their species. With the two latter we have 

 little concern in this place. Eespecting the nutrition of plant life, 

 it is obvious that, like animals, plants must feed and breathe to 

 maintain life. Plant food is of three chief kinds, viz., water, inorganic 

 salts, and gases. Water is an actual necessity to the plant as a direct 

 food and as a food-solvent, i.e. as the vehicle of important inorganic 

 materials. The hydrogen, too, of the organic compounds is obtained 



