370 EXPERIMENTS WITH PLANTS 



table (Fig. 210). The same amount should be placed 

 in each bottle; this is easily managed with a sterilized 

 pipette or medicine -dropper. Wipe the mouth and neck 



of each bottle, 

 and plug with 

 cotton ; then 

 sterilize and 



210. Flat bottle containing gelatin: used for "plate cultures." n T / 



day or two. If 



no bacterial growth appears, we may proceed. Warm 

 the bottle just enough to liquefy the gelatin, remove 

 the plug, and introduce (by means of a sterilized pi- 

 pette or medicine -dropper) the same quantity of water 

 (several drops) into each bottle (a separate sterilized 

 pipette must be used for each). Replace the plug, 

 mix the contents by turning the bottle from side to side 

 (do not allow gelatin to get on the plug) and place the 

 bottle on its side in a dark place. Such cultures are 

 called plate cultures, and they serve excellently to tell 

 how many bacteria the water contains, since we may 

 count the colonies which come from a given quantity 

 of water (in this case we assume that each colony 

 comes from a single bacterium) . 



It is well, in making this experiment, to compare 

 some water from the drinking supply, some from a 

 stagnant pond or pool and some from a barnyard. In 

 the latter case we shall probably find bubbles of gas 

 forming in the gelatin. This is an indication of the 



