THE BACTEETA 13 



diseases as anthrax, cholera, diphtheria, leprosy, tuber- 

 culosis, and others; but we ought not to hastily con- 

 demn all Bacteria as workers of evil because some of 

 them are associated with disease. As a matter of fact, 

 we should be in a very bad case indeed were it not for 

 the beneficent activities of hosts of these exceedingly 

 minute plants, and it is reassuring to know that some 

 forms which are parasitic in the human body are prac- 

 tically harmless. This is the case with a number of 

 forms which live on the mucous membrane of the 

 mouth, and a particular species, Sarcina ventricuU, 

 found in groups, or packets, in the alimentary canal. 

 Bacteria, in general, are either parasites, in which case 

 they live at the expense of living plants or animals, or 

 saprophytes (Gr. sapros, rotten ; phyton, a plant) feeding 

 upon dead and decaying organic matter; and the sapro- 

 phytic forms have an exceedingly important place in 

 the economy of Nature, for they are the chief agents 

 in breaking up dead plant and animal bodies, and render- 

 ing their substances fit for absorption by higher plants. 

 Without the good offices of Bacteria, the higher plants 

 would certainly sufEer, and we know that animals and 

 men could not exist without the ministrations of the 

 plants. Then there are Bacteria which are actually 

 able to fix free nitrogen from the atmosphere and 

 make it available for the food of plants with which the 

 organisms are associated. Such Bacteria occur in 

 tubercles on the roots of plants of the Pea order; they 

 seem to provide the host-plants with nitrogen, which is 

 essential to the existence of protoplasm, in exchange for 

 bed and board. Thus, it happens that many plants of 

 the Pea order (leguminous plants), being infected with 



