Periodical Literature. 325 



The much mooted problem of how nitrogen 

 Nitrogen is made available to plants is, step by step, 



Fixation being solved. It is now certain that an 



in aerobic bacterium, Azotobacter chrooc- 



Soil. occum, is responsible for it. A Russian, 



Krzeminiewski, contributes further knowl- 

 edge of the activity of this bacterium. Humus does not serve 

 either as a source of nitrogen or carbon, but it acts as an im- 

 portant stimulant : the addition of nitrogen-free media multiplies 

 the amount of nitrogen fixed by the bacteria many times. Humus 

 from different soils yields different results. On the other hand, 

 the addition of nitrogenous compounds had an inhibitory in- 

 fluence. 



The importance of humus is further accentuated by these find- 

 ings. 



Untersuchungen tiber Azotobacter chroococcwm. Botanical Gazette, 

 June, 1909, p. 475. 



Mr. G. P. Burns continues an account of his 

 Peat Bog investigations of the Huron River Valley, 



Investigations. and the present paper deals with the de- 

 scription of some of its greyest peat bogs, 

 which were investigated in detail by use of a borer, to establish 

 their relation to original post-glacial lake contours. The author 

 concludes that the chief factor determining the position of the 

 greatest amount of peat deposit and the width of the zones of 

 plants — lily, bog sedge, bog shrub, tamarack, maple-poplar, in 

 definite orderly succession — is the depth of the water in the 

 different parts of the original post-glacial lakes. The position of 

 open water is determined by depth : given time enough, the open 

 water will disappear from all bog lakes. Where water is shal- 

 low, the bog flora cannot establish itself because of the wave 

 action induced by the winds, and on account of the shore-ward 

 push of the ice. 



Botanical Survey of the Huron River Valley. VII. Botanical Gazette, 

 June, 1909, pp. 445-453- 



