RESPONSE OF THE PLANT TO ITS SURROUNDINGS 281 



Drawing their nourishment from the loose soil in which they 

 are anchored, and lacking the support of a liquid medium, 

 they develop roots and vascular stems. The roots of plants 

 growing in swamps have difficulty in obtaining proper aer- 

 ation on account of the water, which shuts off the air from 

 them ; hence they are furnished with large air cavities, and 

 the bases of the stems are often greatly enlarged, as in the 

 Ogeechee lime (Nyssa capitata) and cypress, to give room 

 for the formation of air passages. The peculiar hollow pro- 



FIG. 418. A Southern cypress swamp, showing on the left the peculiar enlarge- 

 ments for aeration, known as " cypress knees." (From Mo. Botanical Garden Rep't.) 



jections known as " cypress knees " are arrangements for 

 aerating the roots of these trees. 



320. Xerophyte societies are adpated to conditions the 

 reverse of those affected by hydrophytes. The extreme of 

 these conditions is presented by regions of perennial drought, 

 like our Western arid plains and the great deserts of the in- 

 terior of Asia and Africa. Under these conditions plants 

 have two problems to solve, to collect all the moisture they 

 can and to keep it as long as they can. Hence, plants of 

 such regions have a diminished evaporating surface, owing 

 to the absence of foliage and the compacting of their tissues 



