QUERCUS ViRGINIANA. 119 



possibly be the cause of the trouble when the water content is 

 too high. If the soil be too wet it is not only in poorer physical 

 condition, but many or most of its interspaces are full of water 

 and the ])lant roots are shut off from the soil air with which they 

 are usually in contact. It is not yet known how necessary this 

 soil air is to the plant but it is probably more or less so, and its 

 exclusion may be a material cause of low producti\ity. Simply 

 the quantity of water in the soil might therefore produce the 

 observed optimum entirely aside from physical factors. Too 

 little water would be unfavorable because of a two slow supply 

 of needed water; too much water would be unfavorable because 

 of the exclusion of the air. The maximally favorable conditions 

 would be found over the middle range where both water and air 

 were supplied in adequate quantity. 



We have, then, t'to apparently efficient causes of the ob- 

 served phenomenon of the optimum, one purely physical, the 

 other partly biological and related to the supply of water and of 

 air. It is probable that the latter factor of water supply and 

 aeration simply determines the rajxge within which the optimum 

 must lie, while its exact position within this range is probably 

 determined by the physical soil condition and is always co- 

 incident with the critical moisture content whenever that value 

 lies within the required range. 



Bureau of Soils, 



U . S. Department of Agriculture, 



Washington, D: C. 



THE SEEDLING OF QUERCUS VIRGINIANA. 



Isaac M. Lewis. 



For our knowledge of the mode of germination and seed- 

 ling structure of the oaks, we are indebted to the contributions 

 of Sir John Lubbock,* Rowlee and Hastings,! and Kerner and 

 Oliver. 1 1 These writers have described the germination and 



•Lubbock, Sir John. A Contribution to our Knowledge of Seedlings. Vol. II, S34. 

 JRowlee, W. W. and Hastings, George. The Seeds and Seedlings of some Araentiferae; 

 Bot. Gaz. 28: 349. 1898. 

 llCemer and Oliver. Tbe Natural History of PlanU. Vol. I, 607. 



