PLANT PARASITES. 151 



phytic life. Such germs are called respectively facultative parasites or 

 facultative saprophytes. Those, on the other hand, whose life is strictly 

 limited to the parasitic or saprophytic condition are called obligatory 

 parasites or saprophytes. 



Xot all the bacteria which live in or upon the bodies of men and 

 animals are in the stricter sense parasites. The terms messmates and 

 commensals have been applied to such organisms as simply live with, but 

 do not necessarily derive nutriment from, the host. 



In some cases parasitic life on the part of the micro-organism may 

 contribute to the welfare of the host. This is the case in some bacteria 

 which live upon the roots of certain leguminous plants, and to whose 

 nutrition they contribute by rendering atmospheric nitrogen directly 

 available for the host. This condition of life is called symbiosis. 



Species and Varieties. As has already been indicated, the morpho- 

 logical characters of bacteria are so little subject to permanent variation 

 under the widest diversity in the conditions to which they are subject 

 that we are justified in the belief in fixed species. But so susceptible to 

 external condition are the functional activities of many species that not 

 only is the occurrence of what may be called varieties within specific 

 limits frequent under natural conditions, but more or less permanent 

 variations may be experimentally produced. 



Almost all of the functional activities of bacteria upon which we rely 

 as descriptive characters may be experimentally altered ; thus the color- 

 producing capacity may be diminished, the peptonizing and fermentative 

 activities lowered, the pathogenic powers reduced or exalted, and even 

 the capacity for spore formation may be abolished. 



These more or less permanent modifications of function in bacteria 

 may be induced by artificial cultivation under adverse conditions of tem- 

 perature and nutrition, by the presence of deleterious chemical agents, 

 antiseptics, etc., or by association with the body cells and juices in sus- 

 ceptible or insusceptible animals. 



CLASSIFICATION OF BACTERIA. 



The beginning of the systematic study of bacteria by exact and re- 

 liable methods is of such recent date, they are so minute, and our present 

 optical apparatus reveals so few differential morphological characters 

 beyond the limits of the three primary classes already mentioned, and so 

 few withal of the many existing forms have as yet been studied, that a 

 satisfactory classification or nomenclature of the bacteria is not yet pos- 

 sible. 



Outside of the limit of the primary classes above described and based 

 upon the shape, we are obliged to use for the purposes of identification 

 and description the results of physiological activities which the special 

 forms of bacteria display when placed under diverse and usually entirely 

 artificial conditions of food, temperature, and general environment. It 

 is evident from this condition of affairs that what in our attempts at 



