Minnesota Plant Life. 103 



Substances formed by bacteria. Before presenting in de- 

 tail an account of some of the principal forms under each of 

 these groups, a few words may be said upon the general physi- 

 ology of the bacteria, which, if thoroughly comprehended, will 

 make clear how there may be a fundamental similarity in all the 

 apparently dissimilar processes dependent upon bacterial action. 

 Bacteria are living plant cells. As such they need food, and 

 absorb it, using it in their own way and excreting their waste 

 products which may be as various as the food substances, and 

 the methods of digestion. The waste products given off may 

 be solid, liquid or gaseous ; and among the substances excreted 

 are a great variety of organic compounds. From an economic 

 point of view the most important waste materials of bacterial 

 nutrition may be classified in three groups: i. Organic poi- 

 sons; 2. ferments; 3. nitrates or nitrites. For the moment, 

 consideration of the iron, sulphur and de-nitrifying germs is 

 omitted. If a bacterium secretes some poisonous substance in 

 the body of an animal or of man, the higher organism will be 

 poisoned just as if bitten by a snake. Disease-producing bac- 

 teria are principally those forms which, entering the animal 

 body, grow there, nourish upon its tissues and produce poisons 

 that are carried into the blood and, when in sufficient quantity, 

 produce death. Plants are subject to such bacterial poisoning 

 quite as much as animals. Pear-blight and potato-scab ; carna- 

 tion disease and cucumber-rot are well-known examples of 

 bacterial plant diseases. 



Ferments. Ferments are peculiar organic compounds which 

 are remarkable for their property of initiating changes in other 

 compounds. A very little, for example, of the ferment which 

 is capable of converting starch into sugar, if put into a starchy 

 substance will transform thousands of times its own weight of 

 starch into the form of sugar. Nor will it be destroyed in the 

 process, for it seems to take into itself the starch on the one side 

 splitting off sugar on the other, but always remaining the same 

 although it produces very great changes in the substances upon 

 which it operates. 



Nitrogen salts. Nitrates and nitrites are salts of nitrogen 

 which are demanded by most higher plants as indispensable ar- 

 ticles of food. Such organisms use scarcely any of the free nitro- 





