Minnesota Plant Life. i IO/ 



triumph of agricultural science if enough could be learned about 

 the nodule-bacteria and their habits to train them to grow upon 

 wheat, oats, barley or rye. The problem of the world's food sup- 

 ply would then be solved, for every cereal crop under such cir- 

 cumstances would "do its own rotating" and instead of impover- 

 ishing the soil the crop taken from it would leave it in a better 

 condition than before. It is true, plants demand a variety of 

 other substances in their nutrition than salts of nitrogen, but 

 these are the only substances which are not present everywhere 

 in almost unlimited quantities. Silicon, sulphur, phosphorus, 

 calcium, potassium, iron, oxygen, hydrogen and carbon and the 

 other elements appropriated by plants in their food-economy 

 can usually be delivered in adequate quantities and in assimilable 

 form. The peculiar inability of green plants to make any use 

 of atmospheric nitrogen would in a few generations put an end 

 to all plant and animal life on the globe were it not for the ac- 

 tivity of nitrifying germs in the soil and water. Through the 

 agency of these the store of nitrogen salts, from which plants 

 can absorb the nitrogen they need, is constantly replenished and 

 the successive generations of life are permitted to continue on 

 the earth. 



Other root-tubercle bacteria. Plants of the pea family are 

 not the only ones that develop root nodules containing nitrify- 

 ing bacteria, for such nodules are also produced upon alders and 

 upon a plant related to the buffalo-berries of the western Minne- 

 sota prairie. Since these plants are not cultivated in fields their 

 root-nodules are of less economic importance to the agricultur- 

 ist. 



Bacteria of urine. Of quite a different class from those just 

 treated are the de-nitrifying bacteria which play an important 

 part in the economy of nature by their ability to decompose 

 urea. It is generally known that urea is the ordinary form of 

 nitrogenous waste material excreted from the animal body, but 

 this substance while a compound of nitrogen is not available for 

 the nourishment of green plants until it is decomposed and re- 

 combined into nitrogen salts. The decomposition of urea, 

 wherever it occurs, is ordinarily accomplished by the activity 

 of de-nitrifying germs. As a result of their action, ammonia 

 appears and this, again further reduced in complexity, may lib- 



