SOIL FORMATION: ROCK DISINTEGRATION . Wj 



But no less important are the influences of plants on soil con- 

 ditions on the hills, and in more level areas. Old plant parts, 

 and plant remains, by decay add to the bulk, fertility, and 

 changing texture and physical condition of the soil. 



493. The bald cypress (Taxodium distichum). — Very char- 

 acteristic are the formations presented by the forests of the bald 

 cypress of the South, which grows in swampy or marshy places. 

 The "knees" on the roots of this cypress make grotesque 

 figures in the cypress forest. These take the form of upright 

 columnar outgrowths, broader at the base or point of attach- 

 ment to the horizontal root, and possess a fancied resemblance 

 to a knee. These knees are said to occur at points on the 

 horizontal root above and opposite the point where a root 

 branch extends downward into the soft marsh soil. They thus 

 give strength to the horizontal root at the point of attachment 

 of the branch which penetrates into the soft soil, and during 

 gales they hold these root branches more rigidly in position 

 than would be the case if the horizontal root could easily bend 

 at this point. The knees thus are supposed by some to 

 strengthen the anchor formed by the root in the loose soil. 

 Their development may be the result of mechanical irritation 

 at these points on the horizontal root, 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, and in this 

 sense the knees are sometimes termed pneumatophores. The 

 knees catch and hold floating plant remains during floods, and 

 by the decay of this debris the fertility of the soil is increased. 



