82 MANUAL OF GENEEAL AGEICULTURE. 



light, oxygen, or toxic substances. A few are known to 

 cause diseases of man and animals, a rather large number 

 (about 125) are now known to cause diseases in plants. 



Some Common Bacteria. Their Structure and Nature. 

 In the material provided, the bacteria are growing on 

 slices of cooked potato exposed to the air of laboratory for 

 three minutes, covered, and then set in a warm place for 

 several days. What conditions favorable to plant growth 

 were provided? Observe: 



1. The more or less circular patches of various sizes 

 and colors on the surface of potato the colonies of bac- 

 teria. How can you tell them from molds? Note the dif- 

 ference in form and character of margins of different col- 

 onies. Each colony was formed by the multiplication of 

 a single bacterium. 



2. Compare different colonies as to nature of the 

 surface, moist, dry, shiny, dull, smooth, wrinkled, etc. 



3. Note that some colonies are covered with a skin 

 or pellicle, made up of bacteria stuck together by their 

 gelatinous wall and dried by exposure to air. With needle, 

 determine toughness of pellicle. 



4. Bacteria feed on substances dissolved from the 

 potato. Some colonies penetrate the potato, others sim- 

 ply pile up. 



5. Bacteria, in their growth, produce gases with 

 offensive odors, such as the odor from the potato. The 

 common odors of decay are of this nature. Map the sur- 

 face of the potato. 



6. With the point of the needle touch the different 

 colonies, drawing the needle slowly away. Note that 

 some are viscid, drawing out into long threads as the 

 needle is removed. Clean your needle. 



Selecting a colony with wrinkled surface, remove 

 from the smooth, glistening margin a bit on the point of 

 your needle. Stir it into a drop of water on a clean slide. 

 How does it affect the water? Why? 



7. Under low power, note the finely granular appear- 



