FUNGI 135 
preserving food are attempts to exclude bacteria that 
would induce fermentation or decay. 
(2) Bacteria that induce dtsease.—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, H), 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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