254 Oxygen and Carbon Dioxide 



the black mangrove, common along the margins of Florida lagoons, 

 root branches, known as pneumatophores, grow vertically upward 

 through the mud and water until they project into the air, where they 

 have ready access to oxygen. Other special features of plant growth 

 found in permanent swamps are the "knees" and enlarged trunk bases 

 displayed by such trees as the bald cypress (Fig, 7.7). The knees 



Photo U. S. Fuitst Siiricc 



Fig. 7.7. Enlarged trunks and "knees" of cypress trees growing in permanent 

 swamp conditions in the Francis Marion National Forest, South Carolina. 



were formerly believed to provide aeration for the roots, but evi- 

 dence now indicates that these root excrescences and the enlarged 

 trunks represent excessive cambial growth resulting from the com- 

 bination of abundant water and oxygen supply for the tissue at the 

 water line. The roots of a few vascular plants without observable 

 adaptation for securing outside oxygen, such as the willow, are able 

 to remain active during prolonged flooding; the root tissues in these 

 exceptional instances are presumed to be capable of anaerobic 

 existence. 



Most cryptogamic plants and soil animals depend upon aerobic 

 respiration and hence are excluded from soil layers devoid of oxygen. 

 A few of the higher forms, such as the earthworm, may be able to 



