40 AGRICULTURAL BACTERIOLOGY 



The deep colonies that result from the growth of cells em- 

 bedded in the agar must push the culture medium aside in 

 order to make room for the mass of cells. Since the colony 

 is under considerable pressure, it can not develop freely. 

 The form of such colonies is most commonly lenticular, or 

 spherical. 



It is, of course, essential that all materials and objects 

 used in making the plate culture, as it is commonly called, 

 be sterile, with the exception of the material to be exam- 

 ined, otherwise one could not be assured of the exact source 

 of the resulting growth. It is also essential that the colo- 

 nies be not too numerous, otherwise their size will be limited 

 by the competition for food. It is usual to prepare, not a 

 single plate culture, but several plates containing varying 

 amounts of the material, so that on some one of the plates 

 the resulting colonies will not be too numerous for success- 

 ful isolation of the pure cultures. 



A minute portion of a colony may be transferred to a 

 tube of sterile agar or other appropriate media, producing 

 what is called a tube culture. The exact form of the organ- 

 ism under observation can now be observed under the 

 microscope, and its physiological properties determined by 

 seeding it on various kinds of culture media and noting the 

 resultant growth. The growth of the organism in the tube 

 culture soon ceases, owing to the accumulation of its waste 

 products or to the exhaustion of food. The cells will re- 

 main alive for varying periods of time, depending on the 

 organism. The culture can be kept alive by furnishing it 

 with fresh food through the transfer of a few of the cells to 

 a new tube of nutrient medium. Such cultures form the 

 so-called stock cultures, and are the pure seed supply of the 

 bacteriologist. 



In many instances it is impossible to isolate an organism 

 from its native habitat by the plate-culture method. This 



