176 THE STUDY OF PLANT COMMUNITIES * Chapter VII 



tat. Most species growing in such habitats have adaptations that 

 serve to counteract poor aeration. Many have large, continuous 

 spaces— lacunar tissue— in their stems and roots permitting storage 

 and free movement of gases within the plant. In emergent and 

 floating-leaved species, these spaces are connected directly with 

 the atmosphere through the stomata. Submerged leaves of aquatics 

 are invariably finely dissected or extremely delicate, conditions 



Fig. 85. Cypress swamp (Taxodium distichum) in the coastal plain of 

 South Carolina. Note buttressed, somewhat planked bases of trees and an 

 abundance of cypress knees, whose uniform height marks average high-water 

 level.— U. S. Forest Service. 



that bring a majority of the cells in contact with the water, from 

 which they must obtain oxygen in dissolved form. A few sub- 

 merged species produce pneumatophores, or special branches that 

 extend above water and give direct connection with the air through 

 lacunar tissues. In addition to shallow root systems, a number of 

 swamp trees have other characteristics in common. Enlarged or 

 buttressed bases and plank roots are frequent, especially in south- 

 ern swamps, and the "knees" of cypress are in the same category. 

 That these structures facilitate aeration has not been conclusively 

 demonstrated, but their formation seems to be in response to alter- 

 nate inundation and exposure to air. 150 



