430 JOURNAL OF FORESTRY 



dead and rotten. This also showed that the injurious agent was a 

 soil factor and the symptoms convey the impression of a localized 

 poisonous action rather than of a general starvation effect due to lack 

 of essential food materials. 



The effect of various quantities of COo gas on Sal roots was tested 

 by growing the seedlings in water cultures and bubbling the gas through 

 the culture solutions. It was found that when the concentration of the 

 gas reached approximately 500 milligrams per liter, the delicate root 

 tips and rootlets of vigorous Sal seedlings are blackened and killed and 

 the production of new roots was inhibited, the appearance of the dam- 

 aged roots resembling that of those found in poorly aerated soil. The 

 authors suggest determining the amount of oxygen and CO2 in the 

 soil as the most reliable indication of the conditions of aeration in the 

 soil and they define a badly aerated soil as one in which there is a 

 deficiency of oxygen and an excess of COo. 



The authors show the paramount importance of soil aeration in the 

 silviculture of the Sal and contend that the various agencies which in- 

 fluence soil aeration can be controlled to a considerable extent by 

 ordinary forest practices. The quantity of water, of soil organisms, 

 and of organic matter can be regulated by varying the shade and quan- 

 tity of dead leaves added to the soil and also by the controlled use of 

 fire. Texture is also influenced by the amount of organic matter in the 

 soil and also by such factors as the grazing of live stock, both of which 

 are capable of regulation. In some cases the temporary encouragement 

 of the growth of certain grasses and other herbs, the roots of which 

 are much sought after by rodents and hogs, is advocated for improving 

 soil texture. The felling of trees and the subsequent decay of the 

 subterranean roots is also an important factor in influencing soil 

 aeration. 



American silviculture can well afford to recognize the fundamental 

 importance of soil aeration in the treatment of the heavy calcareous 

 forest soils found in certain sections of this country. 



C. F. K. 



Howard, Albert, and Hole, R. S. Recent Investigations on Soil Aeration. 

 Indian Forester, 44:187-212, May, 1918. 



Of late there has been considerable discussion 

 Sunlight among foresters of the term tolerance and the 



and Its adequacy of the definition as generally accepted 



Measurement has been seriously questioned. The light require- 

 ments of a species and tolerance can no longer 

 be accepted as synonymous. We now know that the vigor and growth 

 of a tree growing under an overwood is determined not only by com- 

 petition for light but by competition for soil nutrients and soil moisture. 



