EXERCISE 75 



WHAT ARE THE SIZE, SHAPE, MOTION, AND GENERAL 

 APPEARANCE OF BACTERIA? 



Materials. Prepare materials for study of bacteria several days 

 before they will be needed. Place a handful of chopped hay in a 

 drinking-glass and cover with water. Place several beans in another 

 glass and add water sufficient to soak the beans and half submerge 

 them. Cover both glasses to prevent evaporation and stand in a 

 warm place. Microscopes are necessary. 



Directions for work. Transfer to a glass slide a small bit of 

 the scum from the water in which the hay has been soaking. 

 Examine with the high power of the microscope. 



The bacteria appear as very small and almost transparent 

 bodies, which may be stationary or in motion. It is probable 

 that several kinds of bacteria will be present in the scum. The 

 kind which is commonly most abundant is rod-shaped and 

 several times as long as wide. Others may be spherical or spiral. 



Try to find bacteria representing all three forms (see text- 

 book). The size of the bacteria cannot be measured without 

 special instruments, but they may be compared with each other 

 and with other objects which pupils have previously viewed in 

 the microscope ; or a small object, as a hair, may be introduced 

 for comparison. 



In studying the movements of bacteria it must be remembered 

 that if all the bacteria in the field of view move together in 

 one direction, it is due to currents of water rather than to any 

 proper bacterial movement. Also, the bacteria may exhibit an 

 interesting dancing movement, called the Brownian motion, 

 which is exhibited by any small particles suspended in a liquid 

 quite independently of whether the particles are living. Certain 

 kinds of bacteria exhibit a relatively rapid independent motion 

 through the water, commonly proceeding in a straight line until 

 coming in contact with some other object. 



Describe carefully each of the several kinds of bacteria found 

 as to shape, relative size, rate and character of motion, and 

 whether the individuals occur in chains or masses, or free. 



[93] 



