Ch. X] THE BACTERIA 403 



do the walls ordinarily contain cellulose. They possess no 

 chlorophyll but are almost exclusively parasitic or sapro- 

 phytic, living thus in a great diversity of situations, — in 

 the bodies of animals and plants, in the soil, in the waters, 

 fresh and salt, and even in the air. They digest their 

 food outside their bodies by action of enzymes which they 

 secrete; and the resultant products are absorbed through 

 the general surface, through which also is taken the oxygen 

 used in their respiration. The conditions of their life bring 

 it often to pass that they cannot respire in the ordinary 

 way, by use of free oxygen (aerobic respiration), but must 

 use the oxygen contained in chemical compounds (anaerobic 

 respiration), as occurs conspicuously also in Yeast (page 173). 

 Some anaerobic Bacteria, indeed, are actually injured or killed 

 by contact with free oxygen. In organisms so minute the 

 absorbing surface is so great in proportion to the bulk of the 

 living protoplasm that a far higher respiratory and other 

 chemical activity is possible than in larger plants; and 

 thus in some part is explained their remarkable powers. They 

 secrete gelatinous material very copiously from their simple 

 membranous walls. Their reproduction is wholly by fis- 

 sion, each cell dividing across its middle, and often so rapidly 

 that two divisions occur in an hour ; and millions of individ- 

 uals may arise from one in a single day. Nothing re- 

 sembling fertilization or sex has been found. 



In some kinds the new cells separate immediately after 

 division; in others they remain attached in more or less 

 fragile filaments ; and very commonly they become embedded 

 in a copious gelatinous matrix secreted by their walls, 

 whereby they grow into distinctly visible colonies (Fig. 

 277). The same species, indeed, may appear in one or the 

 other of these states according to the external conditions. 

 In this gelatinous, or so-called zoogixea state, many colonies 

 possess characteristic forms and colors, whereby the species 

 may be identified. This feature is the basis of one of the 

 most important methods of technical Bacteriology, viz. — 



