[Chap. XXIX ROOTS: DEVELOPMENT AND STRUCTURES 311 



movement of oxygen from the atmosphere into the air cavities through 

 open stomates. 



Windfalls frequently display the root systems of trees that have grown 

 on bogs and swamps (Fig. 129). Often none of the larger roots has 



Fig. 129. Major root system of an arbor vitae tree overthrown by a wind storm. 

 The tree had been growing in a bog and the uppermost layers of peat were lifted 

 with the roots. Photo by G. W. Blaydes. 



penetrated the substratum more than one foot. The roots that lived and 

 attained age were those just beneath the surface. Toward the close of 

 the last century, thousands of miles of fences were made of these flat 

 root bases of white pine set up side by side, enclosing pastures after 

 lumbering had denuded the land. Yet when white pine stumps were 

 pulled from sandy upland, they had several roots an inch or more in 

 thickness down to ten feet below the surface. 



The effects of submergence in rapidly flowing water differ from those 



