Minnesota Plant Life. 1 1 7 



substances which will suffice for plants with leaf-green. It is 

 not clear why bacterial-purple is limited to so small a number 

 of tiny and lowly organisms, while leaf-green, which does much 

 the same kind of work, is present in almost all of the types of 

 higher vegetable life. 



Nitrogen bacteria. Among the bacteria described as nitri- 

 fying, there are two principal classes, those which live in the 

 soil or water, and those which associate themselves with higher 

 plants, forming a chemical partnership that suggests, a little, 

 the arrangement between the lichen fungus and its algal mate. 

 Nitrifying germs which live independently in the soil or water 

 have apparently the power of producing chemical changes, as 

 a result of which the nitrogen of the air, or of ammonial com- 

 pounds, is combined with minerals in such a way as to pro- 

 duce the nitrogen salts called nitrates or nitrites. Bacteria 

 of this character are found in guano beds, and the ripening of 

 guano into the high grade of fertilizing material, which it is 

 known to be, must be attributed to the presence of nitrifying 

 and de-nitrifying bacteria. Germs of this general group are 

 found in Minnesota in old manure piles, and in other masses of 

 decaying animal substances as well as in ordinary loam. 



Bacteria on clover roots. Still more interesting are the ni- 

 trifying bacteria which join forces with higher plants and de- 

 velop a nitrogen-fixing mechanism. If one pulls up by the 

 roots the first patch of clover which he sees, and then carefully 

 washes out the soil and examines the rootlets, he will discover 

 upon many of them little irregular nodules the size of a pin- 

 head. Upon cutting open one of these nodules and applying a 

 suitable magnification it would be seen that the swelling on the 

 root was caused by the growth in its tissues at that point of a 

 colony of nitrifying bacteria. Apparently the presence of these 

 bacteria is recognized by the plant as desirable, for an apparatus 

 known as infection-threads is produced in the root-substance of 

 the clover and prolonged into some of the root-hairs. Root- 

 hairs, with infection-threads, pick up from the soil the nitrifying 

 germs and these are then led back along the threads to the inner 

 tissues where they develop and multiply in the nodules. A plant 

 like clover, possessing such nodules, grows very much more vig- 

 orously than one without them and certain interesting agricul- 



