INTRODUCTION. 



ANYONE who has not himself worked in a bacteriological 

 laboratory finds it difficult to form a vivid conception of 

 what bacteria are like, because among the familiar animals 

 and plants there are none with which a close comparison 

 can be made. Of the common organisms, perhaps ordi- 

 nary yeasts and moulds are most like the bacteria. Yeasts 

 and moulds, as everyone knows, grow on bread, cheese, 

 meat, syrups and the like. They flourish in moist and 

 dark places, as do mushrooms, puffballs and the other 

 fungi. All these fungi, appearing so different in some re- 

 spects, are alike in one particular, which is the absence of 

 the green color that we are apt to think of as being the 

 essential feature of vegetation. Plants that are green owe 

 their color to a substance called chlorophyll. Upon the 

 properties of this substance one of the most fundamental 

 facts in biology depends. Under the influence of sunlight, 

 by means of chlorophyll, plants are able to use as food the 

 carbon dioxide which is always present in the atmosphere 

 in small amounts. Although carbon dioxide is one of the 

 most simple and stable of compounds, the union of its com- 

 ponent elements is broken by the plant, and they are em- 

 ployed in the formation of other much more complex and 

 unstable compounds, such as starch and cellulose, which 

 enter into the plant's structure. The work of plants, it will 

 be noticed, is, in the main, precisely the reverse of that per- 

 formed by animals. Animals take the unstable carbohy- 

 drates w T ith high potential energy, such as starches and 



