574 Introduction to the Study of Science 



(2) The parasites, those that depend upon living organisms, 

 both plant and animal ; 



(3) The independent bacteria, those that get their food 

 materials from inorganic substances. 



To the third class belong the nitrifying bacteria, some of 

 which, attached to leguminous plants, take nitrogen and per- 

 haps carbon dioxid directly from the air ; the iron bacteria, one 

 species of which probably produces the deposits of free iron 

 ochre ; and the sulfur bacteria, which abound in water contain- 

 ing sulfur. 



Bacteria may also be classified as to their independence upon 

 oxygen, as the aerobic and the anaerobic (page 216). 



287. Study of bacteria through their effects. The study 

 of bacteria by direct observation is beset with many difficulties 

 because of their extremely small size. Microscopes of high power 

 are necessary, and stains must often be employed to bring out 

 detailed structures. But a rough general idea may be formed 

 from examination of some of the effects of bacteria in different 

 substances. These give an acquaintance with the principal 

 forms, the rapidity of their multiplication in suitable conditions, 

 and important results of their growth. 



Exercise. The potato cultures prepared for molds, if these have 

 been preserved, may be observed first. You will probably find growths 

 upon the cut surface that are different in outline and color from the 

 molds. Many such growths are white, but there are other sorts of 

 bacteria that produce green, yellow, red, and blue. The outline forms 

 indicate the extent of the colony produced from a single bacterium. 

 Examine them with a magnifying glass. What features do you find 

 generally distinguishing them from molds? How can their presence 

 be accounted for? Why are they so late in development compared 

 with the molds? What does the difference in areas covered by the 

 colonies indicate ? 



Exercise : A culture medium. Potato medium as previously pre- 

 pared will do, but other culture media may be necessary. A simple 

 medium may be made with beef bouillon (such as may be bought in 

 a can) to which is added one per cent of peptone and ten per cent of 

 gelatine to stiffen it. Heat the mixture gradually until the gelatine 



