144 BACTERIA 



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 the 

 three types, 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 im- 

 portant 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 

 accompanying table (p. 145). 



The threefold function of plant life is nutrition, assimila- 

 tion, and reproduction : the food of plants, the digestive and 

 storage power of plants, and the various means they adopt 

 for multiplying and increasing their species. With the two 

 latter we have little concern in this place. Respecting the 

 nutrition of plant life, it is obvious that, like animals, they 

 must feed and breathe to maintain life. Plant food is of 

 three kinds, viz., water, chemical substances, and gas. Water 

 is an actual necessity to the plant not only as a direct food 

 and food-solvent, but as the vehicle of important inorganic 

 materials. The hydrogen, too, of the organic compounds 

 is obtained from the decomposition of the water which per- 

 meates every part of the plant, and is derived by it from the 

 soil and from the aqueous vapour in the atmosphere. The 

 chief chemical substances of which vegetable protoplasm 

 is constituted are six, viz, potassium, magnesium, cal- 

 cium, iron, phosphorous, and sulphur. These 



