368 EXPERIMENTS WITH PLANTS 



each minute drop of the liquid will contain not moi'e 

 than one of them, so that whenever we touch the potato 

 we shall deposit not more than one; hence each spot 

 will be the result of the growth of a single bacterium: 

 this is a pure colony: if we infect a sterilized potato 

 (or other medium) from such a colony, so as to get 

 only one kind of bacterium in the culture, we obtain a 

 pure culture. 



It is interesting to consider, in connection with this 

 mode of propagation, that, as long as an individual 

 keeps on dividing, there can be no such thing as death 

 from old age, since the parent is completely absorbed 

 into the offspring. 



Bacteria are distinguished not so much by their form 

 and general appearance under the microscope as by 

 their behavior when grown on various substances, such 

 as potato, gelatine, beef- broth, etc. They are classified 

 largely according to the size, shape, consistency, color, 

 etc., of the spots or colonies which they produce. 



One very interesting form, which may be easily 

 recognized, appears frequently on potato and bread - 

 cultures: it is bright red in color, and is of especial 

 interest as causing the " bloody -bread " of the middle 

 ages, which was believed to bleed miraculously. 



The bacteriological examination of water is so im- 

 portant and at the same time so simple that we may 

 well turn our attention to it. For this purpose we may 

 prepare some nutrient gelatin as follows: Make some 



