152 PLANT PARASITES. 



classifications we are wont to call genera 'and species, are not such in the 

 strict sense in which these terms are used in other domains of biology. 

 That which corresponds to the generic name in the more exact vocab- 

 ularies is in ours usually the growth form which indicates the primary 

 class to which the germ belongs, as coccus, bacillus, or spirillum, or some 

 growth modification of this, as diplococcus, streptococcus, streptobacillus, 

 and the like. To this is usually appended a more or less distinctive 

 specific name, which ordinarily indicates some noteworthy physiological 

 capacity of the germ, such as its peptonizing power, the pigment which 

 it elaborates, some prominent chemical reaction which it initiates, some 

 marked effect upon an artificial culture medium, its disease-producing 

 power in men or animals, or some fact about its habitat, or the situation 

 in which it was found. All of these and other heterogeneous character- 

 istics, largely functional, which may be developed under natural or arti- 

 ficial conditions, constitute data in the life history of germs upon which 

 the classification and nomenclature of bacteria are at present based. 



As examples of names of bacteria thus derived may be cited Micro- 

 coccus luteus, Diplococcus lanceolatus, Sarcina ventriculi, Bacillus acidi lac- 

 tici, Spirillum cholera? Asiaticce. 



Notwithstanding the value of this principle of grouping and nomen- 

 clature, its inadequacy even for temporary use is becoming painfully 

 evident as research proceeds, partly because of the large variations to 

 which physiological activities are liable, and partly because we cannot 

 sharply distinguish between races, varieties, and species. 



It is not yet possible to say whether it will ever be practicable in this 

 limited field of lowly life to draw such exact distinctions between genera 

 and species as is possible among higher organisms. 



Methods of Morphological Study of Bacteria. 



The simplest mode of studying bacteria is to examine them either in the fluids in 

 which they grow or in one-half-per-cent salt solution. For the study of many of the 

 phenomena of life this method is important. 



This may be accomplished by the examination of a thin layer of the fluid under a 

 cover slip, in the usual way; or a small drop may be placed on the cover slip and this 

 inverted on a hollow slide so that the observation is made in the hanging drop. A 

 streak of vaselin painted around' the edge of the cover will prevent evaporation of the 

 fluid. 



STAINING. By far the most important aid in the morphological study of the bac- 

 teria is derived from the use of staining agents. Most of the bacteria are stained more 

 or less readily by one or more of the basic anilin dyes. The ease with which they are 

 colored varies considerably in different species and with the different dyes. The tissue 

 elements, and a variety of other materials with which the bacteria may be associated, 

 also stain more or less readily at the same time; but most of these part with their color 

 more readily than do the bacteria on being treated with alcohol or dilute acids. It is 

 thus possible to obtain a differentiation in color between bacteria and other structures. 

 The bacteria, moreover, differ among themselves in respect to the tenacity with which 

 they hold their stain in the presence of decolorizing agents, and upon this fact is based 

 one of the important methods of distinguishing between different species. 



The anilin dyes more commonly employed for bacteria staining are fuchsin, gen 

 tian-viot'it, and methylene blue. A saturated alcoholic solution of these dyes. 



