SOIL FORMATION: MOORS. 397 



brought about by the strain on the roots from the swaying of the 

 tree. Others regard them as organs for aerating the portions of 

 the root system which are usually submerged in water or wet 

 soil. The knees catch and hold floating plant remains during 

 floods, and by the decay of this debris the fertility of the soil is 

 increased. In deeper water where the lower part of the tree is 

 constantly submerged, peculiar buttresses are sometimes formed 

 on the trunk, as shown in figure 491. 



