PRINCIPAL GROUPS OF PLANTS. 13 



etc. They serve a useful purpose in many technical operations, as 

 in the making of cheese, acetic acid, fermentation of tobacco, 

 curing of vanilla and many vegetable drugs, and in soil nitrification, 

 helping to change ammonia into nitrates one of the sources of 

 the nitrogen used by plants. Many of them are disease-producing, 

 or pathogenic, and are the cause of a number of infectious dis- 

 eases in man and the lower animals, and plants as well. They are 



FIG. 7. Bacillus subtilis (hay bacillus), a, Small rod-like organisms such as are 

 found in an infusion of hay, or bouillon; b, zooglcea or mass of bacilli forming the "skin" 

 on the surface of infusions; c, chains of organisms forming spores; d, individual bacilli 

 showing flagella, which are only seen after staining. After Migula. 



injurious in two ways : in one case they consume the tissues of the 

 host, as in tuberculosis, and in the other they produce powerful 

 poisonous substances, or toxins, as in diphtheria. 



Classes of Bacteria. In order to study Bacteria they are 

 grown upon nutrient media, such as sterile bouillon, potato, milk, 

 etc. They are divided into a number of classes, depending for 

 the most part on the shape of the cell: (i) The Sphaerobacteria, 

 or Cocci, are those whose cells are spherical or spheroid, and in 



