354 A TEXTBOOK OF THEORETICAL BOTANY 



to a fresh plate it would increase into a pure culture of that strain. A mixed 

 growth of Bacteria could thus be separated into its constituent species. 



Gelatine has the disadvantage that it is liquefied by many species, and 

 it was soon replaced by a carbohydrate, agar, obtained from seaweed, 



Fig. 341. — A mixed culture of bacteria growing on agar in a 

 Petri dish showing colonies of varying forms. 



which is not easily liquefied and can be combined with any necessary 

 nutrients, generally beef extract and peptone. Both gelatine and liquid 

 media are, however, retained for some special purposes. A second great 

 step forward was the introduction of a form of flat glass basin with a loose 

 lid, called a Petri dish, after its inventor, which is used instead of the 

 inconvenient glass plates. The melted agar is poured into the dish while 

 hot and allowed to set, protected from contamination by the lid. 



Bacterial Species 



Much controversy has centred around the question of what constitutes 

 a species among Bacteria. The use of chemical criteria as a means of 

 distinguishing them is difi^erent from anything accepted in other groups, 

 and it is complicated by the tendency of Bacteria to vary considerably in 

 their chemical activity, not only between difi^erent strains but even in one 

 and the same culture at different times. 



Many Bacteria when kept in artificial cultures gradually lose some of the 

 chemical powers which they possessed when first isolated, and in addition 

 they may vary morphologically, the type of the colony changing from smooth 

 to rough, or irregular. These changes can often be reversed by appropriate 



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