BACTERIA. 83 



the species, either in diluted air or even in the absence of 

 oxygen ; examples may be found among the lactic acid bacteria. 

 It is a well-established fact that with one and the same species 

 the demand for oxygen varies according to the other life con- 

 ditions. The heat-loving (thermophilous) bacteria form a 

 typical example, for they can grow at a high temperature in 

 the presence of air, whilst at a low temperature they grow only 

 in the absence of air. Of the obligatory aerobes,many, when well 

 nourished, can develop in air containing only traces of oxygen. 



Whilst a few bacteria are motionless (e.g., lactic acid and, 

 in part also, acetic-acid bacteria), the majority show a capacity 

 for free movement ; such bacteria are commonly met with 

 in decomposing fluids. This motion, which is not to be 

 confounded with the Brownian molecular movement, usually 

 consists of a forward swimming action, together with a rota- 

 tion round the longer axis. The organs of motion, which only 

 become visible on staining, consist of fine protoplasmic hairs 

 flagella or cilia which are connected with the plasma of the 

 cells through holes in the cell-wall. A few species have only 

 a single cilium, attached to one end of the cell ; others have a 

 bunch of cilia at one end, whilst cilia are distributed over the 

 entire surface of others e.g., the hay bacillus, in certain stages 

 of development, and some of the common putrefying bacteria. 



By means of these organs of motion, bacteria are enabled 

 to penetrate to that part of the nutritive fluid which offers 

 the most favourable conditions for existence. Thus, Engel- 

 mann has proved that aiirobes move to the stratum of liquid 

 which is richest in oxygen, whilst the anaerobes move in a 

 contrary direction. Similarly, Pfeffer has shown that bacteria 

 move to these parts of a fluid that contain nutriment of suitable 

 concentration. The rate of motion is conditioned by tem- 

 perature ; thus, Bac. subtilis moves more rapidly at 37 C. 

 than at 20 C., whilst other bacteria cease to move at the 

 former temperature. 



The propagation of bacteria takes place by division. It 

 has been observed in detail in the larger species. The cells 

 expand, fine transverse lines appear, which gradually increase 

 in thickness and split into two leaflets ; after this the organism 

 separates into smaller rods, which sometimes remain united, 



