408 A TEXTBOOK OF BOTANY [Ch. X 



possible to cause the formation of the antitoxins in advance, 

 so to speak, by giving the patient an allied but less harmful 

 disease; and such is the principle of vaccination against 

 smallpox. 



Nitrogen-conserving Bacteria live in the soil, and produce 

 effects immensely important in agriculture (page 244). 

 Some kinds take the ammonia abundantly set free by decay 

 Bacteria, and effect its transformation to nitrates, most 

 important of all fertilizers. Others do more, for they ab- 

 sorb free nitrogen from the air, and fix it in compounds uti- 

 lizable by the higher plants, which are not able to use the 

 free gas; and these are the forms which are colonized in the 

 familiar tubercles of the Leguminosse (page 245). For the 

 most part these Bacteria are saprophytic, depending for their 

 food upon the humus in the soil, but a few are independent 

 forms, able to construct their own food from carbon dioxide 

 and water, using the energy of chemical oxidations instead 

 of the power of sunlight (page 87). Such a chemosynthetic 

 mode of food formation suggests a method by which 

 organisms may have existed before chlorophyll was de- 

 veloped. 



Other kinds of Bacteria are prominent in various ways. 

 One species contains a purple coloring matter, which seems 

 to enable its possessors to make food photosynthetically. 

 Others develop much heat through intense respiration, and 

 this is the way fermenting manures become warmed, and 

 can be utilized in hotbeds. Another kind is phosphores- 

 cent, while others set free the mixture of gases which orig- 

 inates the " will-o'-the-wisp. " The gelatinous colonies of 

 another kind become blood red on milk or bread, and formed 

 the basis of the " bleeding host " of the Middle Ages. Others 

 can decompose compounds of sulphur or of iron. Then it 

 must not be forgotten that hundreds of kinds produce no 

 prominent observable effects, but, from man's point of view, 

 are harmless and obscure, as in case of the several dozens 

 which live in the alimentary canal. No matter what the 



