PLANT PARASITES. 147 



cases, the only absolute guarantee of the spore nature of these bodies, 

 though staining methods are useful. Another mode of sporulatiou 

 arflirogenoushas been described, but its nature is not well Understood. 



Conditions of Life, Growth, and Multiplication. The bacteria require 

 for their nutrition carbon, hydrogen, oxygen, and nitrogen, and certain 

 mineral salts. These they can obtain from proteids and carbohydrates. 

 Free oxygen is necessary for the growth and activities of some forms of 

 bacteria and for others not. 



Those bacteria which require free oxygen are called aerobic. Those 

 which do not grow in its presence are called anaerobic. But between 

 these extremes there are forms which make shift to grow without oxygen 

 under favorable conditions, though they make use of it when present ; 

 others grow in its presence, though flourishing best in its absence : these 

 are called facultative aerobes or facultative anaerobes, in distinction from 

 those first mentioned, which we call obligate aerobes or anaerobes. 



Nitrogen may be obtained by some bacteria from inorganic salts of 

 ammonia, from nitrites and nitrates. Bacteria grow best as a rule in an 

 organic food medium, especially soluble albuminous material which is 

 neutral or slightly alkaline. Most of these materials are rendered avail- 

 able as food by the action of enzymes inverting, sugar-splitting, proteo- 

 lytic, etc. , often given off by the organisms and acting upon the albu- 

 minous materials. 



Bacteria are active only in the presence of moisture. When this and 

 other conditions favoring their activity fail they do not necessarily die, 

 but some forms may remain, either as spores or as fully developed organ- 

 isms, for long periods dry and inert, but capable of resuming their ac- 

 tivity whenever they are again restored to favorable conditions. 



Some bacteria are and some are not very sensitive to changes of tem- 

 perature. At a temperature below + 5 C. they are incapable of marked 

 activity or proliferation. At + 7 C. a slow growth has been observed 

 in various species. Many forms may remain alive for long periods frozen 

 in ice, while some are not killed by a temperature of 250 C. As the 

 temperature is raised their activities increase up to a certain point. It 

 may be said in general that they are most active at about the temper- 

 ature of the body, although species differ considerably in this respect. 

 In fluids many bacteria are killed by a prolonged exposure to a temper- 

 ature of from 50 to 70 C. or even less. On the other hand, certain 

 species grow at a temperature of from 60 to, 75 C. Such are called 

 thermophilic bacteria. When dry they resist much higher temperature 

 than when moist. All known bacteria, save a few very invulnerable 

 spore-forming species, are killed by a short exposure in the presence 

 of moisture to a temperature of 100 C. The spores are, as a rule, more 

 resistant to high temperatures than the bacteria themselves, some having 

 been exposed, dry, to a temperature of 140 C. without destruction of 

 life. Fluids containing the spores of bacteria which resist very high tem- 

 peratures may be sterilized by boiling for a short time, then being allowed 

 to stand at ordinary temperatures for several hours, and then again boil- 



