EXERCISE XL 

 MOULDS, YEASTS, AND BACTERIA. 



Supplies for a Laboratory Section of Twelve. Five petri dishes; cheese; lemon; bread; filter paper or blotting 

 paper; one compound microscope; one cake of compressed yeast; molasses; seventy-four test tubes; one one-hole 

 rubber stopper to fit test tube; one elbow tube; lime water; meat; twelve potatoes; two ounces of absorbent cotton; 

 twelve 500 cc. beakers and twelve 340 cc. beakers to fit up as water baths or double boilers. One copy of Moulds, 

 Yeasts and Bacteria by Conn. Published by Ginn & Company. 



INTRODUCTION. Moulds, yeasts, and bacteria comprise those plants commonly known 

 as microorganisms, or popularly as microbes. These plants as a group are of the utmost im- 

 portance to the agriculturist. They have an important bearing in several directions: (a) 

 They are the cause of the decay and spoiling of foods and many other products, (b) They 

 are sometimes of value in the preparation of foods, (c) They are the cause of many conta- 

 gious diseases in both plants and animals, .(d) They play an important part in maintaining 

 soil fertility. 



As a group these plants lack the green color characteristics of the majority of plants. 

 The absence of this green coloring matter, chlorophyll, forces them to 

 live upon organic substances, that is, substances produced by plants or 

 animals. Since animals make use of many of the same kinds of foods as 

 does this group of plants, it is not surprising to often find them as rivals 

 of one another in nature. 



Moulds may be roughly described as filamentous or threadlike plants 

 easily visible to the naked eye. Reproduced by spores. 



Yeasts. Microscopic plants composed of oval bodies. Reproduced by 

 budding.* 



Bacteria. Extremely small microscopic plants composed of spherical, 

 rod-shaped, or spiral bodies. Reproduced by fission. 



Part A. Moulds. 



1. Place some bits of cheese, some pieces of lemon, and pieces of 

 moist bread in separate petri dishes. Place enough wet filter paper or 

 blotting paper in the dishes to keep the air moist. Set aside in a warm 

 place (80-95° F.). After two or three days moulds will appear. Record 

 notes concerning their color and sire. 



Fig. 6. A diagram 

 showing the com- 

 parative size of (a) 

 mould; (b) yeast, 

 and (c) bactera. 



2. Remove some of the mould from the bread and examine it under 

 a compound microscope. Make a careful drawing of the part containing 

 spores, also a few of the free spores. 



Part B. Yeasts. 



3. Rub a little compressed yeast in a very small amount of water. Place a drop of the 



water on a glass slide and examine it under the compound microscope, (a) Observe the shape 

 of the yeast plants. 



4. (Classroom Experiment). To one spoonful of molasses in a test tube, add ten spoonfuls 

 of water. Rub a little compressed yeast in water and put a few drops into the tube of mo- 

 lasses solution. Set aside in a warm place for twenty-four hours. Observe any changes tak- 

 ing place at the close of this period.. 



'Note: A few species of yeast reproduce by spores. 



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