140 



PLANTS WITHOUT CHLOROPHYLL 



we can detect the presence of bacteria. These tiny plants, " man's 

 invisible friends and foes," are to be found " anywhere, but not 

 everywhere," in nature. They swarm in stale milk, in impure 

 water, in soil, in the living bodies of plants and animals and in 

 their dead bodies as well. Most " catching " diseases we know 

 to be caused directly by them ; the processes of decay, souring of 

 milk, acid fermentation, the manufacture of nitrogen for plants 



are directly or indirectly due to 

 their presence. It will be the pur- 

 pose of the next paragraphs to 

 find some of the places where 

 bacteria may be found and how 

 we may know of their presence. 



How we catch Bacteria to Study 

 Them. To study bacteria it is 

 first necessary to find some ma- 

 terial in which they will grow, then 

 kill all living matter in this food 

 material by heating to boiling 

 point (212) for half an hour or 

 more (this is called sterilization), 

 and finally protect the culture 

 medium, as this food is called, from 

 other living things that might 

 grow upon it. 



One material in which bacteria seem to thrive is a mixture of 

 beef extract, digested protein and gelatine or agar-agar, the latter 

 a preparation derived from seaweed. This mixture, after ster- 

 ilization, is poured into flat dishes with loose-fitting covers. 

 These petri dishes, so called after their inventor, are the traps 

 in which we collect and study bacteria. 



Where Bacteria might Grow. Expose a number of these steril- 

 ized dishes, each for the same length of time, to some of the fol- 

 lowing conditions : 



(a) exposed to the air of the schoolroom. 



(6) exposed in the halls of the school while pupils are passing. 



(c) exposed in the halls of the school when pupils are not moving. 



A steam sterilizer. 



