184 



THE PLANT WORLD 



(/) in a city park. 



(g) in a factory building. 



(h) to dirt from hands. 



(i) to contact with scrapings from the interior of the mouth. 



(j) to contact with decayed vegetable or meat. 



(k) to contact with dirty coin or bill. 



(I) to contact with two or three hairs from a pupil's head. 



Cover the dishes securely and place them in a warm dark place. 



After three to five days, note the conditions of the various plate 

 cultures. Each day count the number of spots (colonies) of bacteria 

 and molds growing on the culture medium. Make a table to show 

 your results. 



Petei Dish Exposed 



Number of Colonies of Bacteria 



1st 

 Day 



2nd 

 Day 



3rd 

 Day 



4th 

 Day 



5th 

 Day 



6th 

 Day 



7th 

 Day 



8th 

 Day 



(a) Air of schoolroom 



















(6) Busy halls of school 



















(c) Quiet hall of school 



















(d) Busy city street 



















(e) Etc. 



















Where are bacteria found in abundance? What are the factors in 

 your environment by means of which bacteria may get to your body? 

 Is it true that " bacteria are found anywhere but not everywhere "? 



Where bacteria are most numerous. As the result of our 

 studies, we may draw some inferences concerning the presence of 

 bacteria in our own environment. They are evidently present j 

 in all air, and in greater quantity in air that is moving than in j 

 quiet air. Why ? That they stick to particles of dust was proved 

 by exposing a sterile culture dish in a schoolroom. Bacteria are 

 present in great numbers where crowds of people live and move. 

 The air from dusty streets of a populous city contains more bac- 

 teria than does the cleaner air of a village street. The air of a 

 city park contains relatively few bacteria when compared with the 



