12 MANUAL OF BACTERIOLOGY. 



sugars, as food, and exhale the stable carbon dioxide from 

 the lungs. At the same time the animal receives the bene- 

 fit of the energy resulting from the oxidation of the carbo- 

 hydrates, which may appear indirectly in the form of nerv- 

 ous 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 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 are usually classed with the fungi, which they resem- 

 ble in their inability to decompose carbon dioxide and to 

 use it as food. 1 



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 observa- 

 tion that, when yeasts grow in dilute solutions of sugar, 

 alcohol and gas are formed. 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 made, some 

 of which are powerful poisons. In most of the diseases 

 caused by bacteria such poisons are produced within the 

 living body of the patient. The symptoms of the disease 

 and the changes in the patient's body are due to these 

 poisons, rather than to the direct action of bacteria. 



The extreme smallness of the bacteria prevents us from 

 seeing them as individuals without the aid of the micro- 

 scope, although great numbers of them taken together may 

 form a plainly visible mass or growth. When they are 

 examined with the microscope they appear as little round, 



'See Chapter I, Part II. 



