FUXGI 135 



preserving food are attempts to exclude bacteria that 

 would induce fermentation or decay. 



(2) Bacteria that induce disease. Fortunately most 

 bacteria are harmless, for they are constantly present in the 

 nostrils and mouth and alimentary tract. Even those that 

 are dangerous may be resisted successfully and fail to 

 develop any symptoms of disease. When the resistance 

 has been ineffectual and the disease has developed, the 

 bacteria may produce local effects, as in diphtheria (Fig. 

 122, C) and in typhoid fever (Fig. 121, F); but the most 

 general effect is from the production of poisons (toxins) 

 which are distributed by the blood, leading to fever, de- 

 lirium, etc. These poisons are different for each disease, 

 so that a successful antidote (antitoxin) for the diphtheria 

 poison has no effect on the poison of the bacterium of 

 typhoid fever. It is hoped that antitoxins will be discov- 

 ered for all such bacterial diseases, among which are not 

 only diphtheria and typhoid fever, but also cholera (Fig. 

 121, //), tuberculosis, and pneumonia. Such eruptive dis- 

 eases as measles and scarlet fever have not yet been proved 

 to be due to bacteria. Among plants also certain bacterial 

 diseases occasion great loss, as pear blight and peach yel- 

 lows, and as yet have baffled those seeking for remedies. 



(3) Bacteria that fix nitrogen. It will be remembered 

 that green plants manufacture carbohydrates from carbon 

 dioxide and water ( 14); but that in the manufacture of 

 proteids from carbohydrates nitrogen is necessary. Al- 

 though free nitrogen constitutes nearly eight-tenths of the 

 air, plants cannot use it in that form, but must obtain it 

 through their roots from certain compounds existing in the 

 soil. As crops are removed, the nitrogen supply in the soil 

 is diminished, and presently the soil becomes so impover- 

 ished that it is said to be exhausted. To restore the 

 fertility of the soil, the farmer has learned to use nitrogen- 

 containing fertilizers. Through the removal of crops and 



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