118 PLANTS WITHOUT CHLOROPHYLL 



To prepare the nutrient agar-agar for use, it may be poured while 

 hot into Petri dishes which have been previously sterilized with dry 

 heat for several hours and then kept in a dry place free from dust. 

 It is well to sterilize the plates once or twice after they are coated, 

 using a steam sterilizer. 



Test tubes partially filled with the nutrient jelly are also useful. 

 Immediately after pouring the hot jelly into the test tubes they 

 should be plugged with absorbent cotton and then placed in the 

 steam sterilizer. 



Proble^n 97 : To demonstrate a pure culture. 



Materials. — Culture media in Petri dishes, one dish containing 

 colonies of bacteria ; sterile needle. 



Method. — The instructor transfers some of a colony of bacteria 

 on the point of a sterile needle to the sterile surface of a new Petri 

 dish which has in it nutrient jelly. Watch the growth of the 

 colony on subsequent days. 



Observations. — How long before colonies appear on the sur- 

 face? Are these colonies all alike in appearance? 



Conclusion. — Have you obtained a pure culture? If so, how 

 did you do this? 



To THE Teacher. — In this and all the problems that follow, the teacher 

 should be ready to start the experiments at least three or four days before they are 

 to be used in class laboratory for demonstration. Pupils should be led to notice 

 the conditions at the beginning of an experiment which may be several days be- 

 fore any notes or drawings are expected from their observations. Interest will be 

 held by discussing beforehand the nature of the problem and by making sure 

 that the pupil knows the aim of the experiment. Far too much work in our labora- 

 tories is blind, unreasoning, busy work following directions that lead nowhere. 

 At the beginning of these experiments in bacteriology, the instructor should make 

 sure by demonstration that the pupil knows what to work for on a plate and in a 

 tube. The making of a pure culture should be shown, not so much because it will 

 be a Tpure culture as to impress at the start the need for extreme care in making all 

 of these experiments. Pupils at the outset should be taught to recognize bacteria 

 by (a) odor, e.g., decay ; (6) change in appearance of nutrient media, e.g., cloudiness 

 of bouillon ; and (c) appearance of colonies. Microscopic demonstration is inter- 

 esting but unnecessary with young students. A scale drawing on the board or on 

 the chart means much more to the average pupil. 



Problem 98: To determine where bacteria may be found. 

 Materials. — A number of covered Petri dishes containing sterile 

 agar. 



