2 MANUAL OF BACTERIOLOGY. 



the formation of other much more complex and unstable com- 

 pounds, 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 the 

 oxidation of the carbohydrates, which* may appear indirectly in 

 the form of nervous or muscular activity or warmth. 



Those plants that are devoid of chlorophyll are compelled to 

 some extent to use the same kinds of food as animals. They 

 are unable to decompose carbon dioxide (in most cases), and 

 procure their nourishment from substances derived from the 

 dead or living bodies of other plants or animals. Since they 

 have no chlorophyll, light is of no advantage to them, and is 

 often a positive detriment. Bacteria contain no chlorophyll, 

 and consequently are unable to decompose carbon dioxide and 

 to use it as food.* 



There is another well-known property, possessed by yeasts 

 especially, which may be useful in explaining the work done by 

 bacteria. It is a fact of every-day observation that alcohol and 

 gas are formed when ordinary yeast grows in fruit juice or 

 other fluids containing sugar. It not only appears that bacteria 

 sometimes form alcohol and gas from sugar, but that with 

 different kinds of bacteria and different kinds of food material 

 a great number of substances are formed, some of which are 

 powerful poisons. In most, if not in all, of the diseases caused 

 by bacteria such poisons are produced within the living body 

 of the affected individual, and the symptoms of the disease and 

 the changes produced in the body are certainly due to these 

 poisons, as a rule, rather than to the direct action of bacteria. 



On account of their extremely minute size, the bacteria can- 



* See Part II., Chapter I. 



