THE TIIALLOPHYTA 287 



responsible for so many of the happenings of daily life. Since 

 Pasteur's day, as the result of our knowledge of bacteria, the 

 practice of medicine (and to a considerable extent that of certain 

 branches of agriculture and industry) has been radically changed; 

 and the astonishing advances in modern surgery, made possible 

 by Pasteur's discoveries and inaugurated by the great surgeon 

 Lister, have converted this branch of medicine from a dreaded 



Fig. 163. — Louis Pasteur, 1822-1895. 



last resort to a common and safe means of relief. The bacter- 

 iologist of today has developed a complicated and elaborate 

 technique whereby he can isolate individual bacteria, cultivate 

 them artificially on especially prepared and sterilized foods or 

 media of various kinds, and study the characteristic appearance 

 of individuals and masses, together with the physiological behav- 

 iour peculiar to each. Both as the enemies and as the alhes of 

 the human race, these lowly plants with which the bacteriologist 

 works are among the most important members of the vegetable 

 kingdom. 



Saprophytic Types. — The saprophytic bacteria live on dead 

 plant or animal material. They are responsible for much of the 

 fermentation which carbohydrate substances often undergo when 

 exposed to the ah- and which, as a form of incomplete respiration, 



