148 PLANT PARASITES. 



ing ; this process being repeated several times. In this way, although 

 the spores themselves are not killed by the heat, the bacteria into which, 

 if the conditions be favorable, they develop during the intervals are 

 killed, so that finally the medium is entirely freed from both living 

 spores and adult bacteria. Strong light is in general inimical to the life 

 and growth of bacteria, and by direct sunlight many forms are readily 

 killed. Various forms of electricity, rapid and continued vibration, the 

 action of radium, etc. , may kill bacteria. 



Motility. Some bacteria are capable of performing rapid movements, 

 others are not ; and the same form may be at one time motile and at 

 another immotile, depending upon external conditions. Movement is 

 largely confined to the rod -like and spiral forms, but has been observed 

 in the spheroidal. 



It has been shown that certain of the motile bacteria, when suspended 

 in fluids, are attracted toward, or repelled from, dissolved chemical sub- 

 stances. This is called chemotaxis (see p. Ill), and it is termed positive 

 or negative according as the organisms are attracted or repelled. 



Germicides. Certain chemical agents, when brought into contact with 

 bacteria, greatly reduce their activities or destroy their life altogether ; 

 but different species differ greatly in their capacity of resistance to 

 these agents. The spores of certain bacteria are exceedingly resistant, 

 much more so than the bacteria themselves, to the action of disinfecting 

 agents. Among the chemical substances commonly used as disinfectants 

 may be mentioned formalin, carbolic acid, and especially solutions of 

 corrosive sublimate, which is very inimical to the life of most bacteria 

 and their spores, even in extremely dilute solutions. 



The Distribution and Role of Bacteria in Nature ; Metabolism, etc. 

 The bacteria play a very important role in nature in virtue of their power 

 of feeding upon and decomposing dead organic material. A part of the 

 new chemical compounds which are thus formed may be used by the bac- 

 teria for the purposes of their own nutrition and growth, while the rest 

 are set free to serve, sooner or later, as food for other forms of plants or 

 animals. In the decompositions which are brought about in nature by 

 the bacteria those compounds of nitrogen and carbon dioxide are set free 

 which are essential for the nutrition of the higher plants. 



Without the activities of bacteria, life could not be long maintained 

 upon the earth, since the necessary carbon, hydrogen, oxygen, and nitro- 

 gen would soon be permanently locked up in unavailable form in organ- 

 ized material. Through the action of the various nitrifying bacteria in 

 the soil, ammonia is decomposed with the formation of water and nitrous 

 acid ; nitrous is converted into nitric acid. The so-called denitrifying 

 bacteria reduce nitrates to ammonia and to nitrites. In these ways, 

 among others, water percolating through the soil may be freed from 

 objectionable organic compounds. There are certain soil bacteria which 

 aid special groups. of plants to fix nitrogen from the surrounding media 

 and make it available for the uses of the plant. A large number of 

 complex chemical substances are elaborated during the growth of bac- 

 teria, their nature varying with the species of bacteria and the composi- 



