240 



STATE BOARD OF AGRICULTURE 



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Fig. 21. Coloniea. 



Counting, spots— pin points. If we wish to count the number of micro- 

 organisms present in one cubic centimeter of water, we would 

 add this amount to a liquefied gela- 

 tin tube and make a plate of it as 

 before, using however only one .| 

 plate and making no dilutions at all. ^i-jx; j^ 



The plate would then be allowed to "'^^^,| >,^ 

 develop its colonies and these col- ^""^ 

 onies would represent the number 



of micro-organisms present in the Fig. aa. counting apparatus. 



one cubic centimeter of water. If they are very thick we use a hand mag- 

 nifying glass and place over the plate a piece of glass measured off into 

 small squares. The numbers of colonies included in several squares are 

 averaged. The area of the plate is then computed by the number of 

 squares and multiplied by the average number of bacteria in each square. 

 This will give the entire number of bacteria in the plate and in one cubic 

 centimeter of water. 



Cultures. If pure cultures of bacteria are desired the colonies are care- 



fully studied and those presenting different appearances are 

 taken for transplantation. Those having the same characteristics are sup- 

 posed to have been produced by bacteria of the same species. To trans- 

 plant, a colony is selected under the microscope and the straight platinum 

 needle sterilized. The colony is watched through the microscope and the 

 wire introduced into it without coming in contact with any other colonies, 

 or with any apparatus, in fact with anything but the colony; for, if it is 

 touched by anything else, it is useless for producing a pure culture. After 

 the colony has been taken up on the end of the needle, it is introduced 



