PLANT PARASITES. 



157 



0.01 c.c., of the milk, and mix them by gentle shaking; we now take a shallow covered 

 glass dish called a Petri plate (see Fig. 77), which has been sterilized by heat, lay it 

 upon a cold surface, and pour out the mixture of milk and nutrient gelatin in a thin 

 layer upon it. When the gelatin solidifies, the invisible germs which the milk contained 

 are caught and held in position by it, and if the whole be now set away in a sufficiently 

 warm place the living bacteria will presently commence to grow. 



After a few hours or days, from each one of the single living bacteria scattered 

 through the gelatin so many new germs may have developed that they form a mass, 

 called a colony, large enough to be visible to the naked eye. As different species grow 

 in different ways, some giving rise to colonies of one shape or outline, some to another; 

 some forming colored colonies, some fluidifying the gelatin, some growing much more 

 rapidly than others (see Fig. 78), we can usually recognize the difference in species 

 either with the naked eye or iinder the microscope, and with a fine, sterilized platinum 

 needle can pick out portions of the different colonies and transfer them to the tubes of 

 nutrient media of one kind or another which we have prepared, and study their growth 

 there in the form of pure cultures. 



The transfer of the germs to the tubes is made by plunging the needle which has 

 touched the plate colonies down into the gelatin or agar, or drawing it over the surface 



of the potato or agar (Figs. 79, 80, and 81). 

 This is called inoculating the culture media. 



Not infrequently it is necessary to use the 

 agar culture medium instead of gelatin for 

 plate cultures, because many disease-producing 

 forms of germs do not grow at a temperature 

 below that of the body, at which gelatin fluid- 

 ifies. 



In many cases, especially in agar plate 

 cultures, the material to be studied is spread 

 in very thin streaks, with a sterilized platinum 

 needle, over the surface of the already cooled 

 nutrient film, the colonies developing along the 

 surface streaks (see Fig. 82). 



Anaerobic germs may be cultivated in an 

 atmosphere of hydrogen, the air in the closed 

 culture receptacles being replaced by this gas. 

 Or the oxygen may be removed from this con- 

 fined portion of air in contact with the cul- 

 tures by chemical means. A description of the 

 various simple and complex devices for anae- 

 robic cultivations falls beyond the scope of 

 this work. 



The most scrupulous care is required in 

 sterilizing the nutrient media and the utensils 

 and instruments used, and the greatest caution 

 should be exercised, in transferring the bac- 

 teria from one receptacle to another, to pre- 

 vent contamination. A large experience in 

 this sort of manipulation is necessary before 

 reliable results can be obtained in original in- 

 vestigation, since the slightest error or care- 

 lessness in handling, or failure to observe the occurrence of contamination, is liable 

 entirely to vitiate the results of long series of experiments. It is only by an extended 

 preliminary training in the cultivation of some of the more characteristic and easily 

 recognizable forms, under a variety of conditions, in a perfectly pure state, through a 

 .series of generations, that one can be assured of his capacity to carry on researches in 

 this most difficult and intricate field. 



It is wiser for one purposing to carry on bacterial researches to gain a practical 



Fi. 81. A TUBE OF SOLID TRANSPARENT 

 NUTRIENT GELATIN. 



Showing growth of bacteria with formation 

 of gas along the line of inoculation by a nee- 

 dle plunged into the solid gelatin and with- 

 drawn. The bacterial masses are held fast 

 where they grow, and the gas bubbles cannot 

 escape through the solid media. 



