NTRODUCTION. 



Anyone who has not himself worked in a bacteriological 

 laboratory finds it difficult to form a vivid conception of what 

 bacteria are Hke, because among the familiar animals and plants 

 there are none with which a close comparison can be made. Of 

 the common organisms, perhaps ordinary 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 respects, 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 compoimds, the union of 

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

 ployed in the formation of other much more complex and un- 

 stable 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 performed by 

 animals. Animals take the unstable carbohydrates with high 

 potential energy, such as starches and sugars, as food, and 

 exhale the stable carbon dioxide from the lungs. At the same 

 time the animal receives the benefit of the energy resulting from 



