244 ECOLOGY 



air cavities, and the bases of the stems are often greatly 

 enlarged, as in the Ogeechee lime {Nyssa capitaia) and 

 cypress, to give room for the formation of air passages. 

 The peculiar hollow projections known as " cypress knees " 

 are arrangements for aerating the roots of these trees. 



350. Xerophyte Societies are adapted 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 interior of Asia and Africa. Under these conditions 



469. — "Switch plants "of the alkali desert, condensed into mere green skeletons 

 of vegetation, and thus adapted to extreme xerophyte conditions. 



plants have two problems to solve ; to collect all the mois- 

 ture they can and to keep it as long as they can. Hence, 

 plants of such regions diminish their evaporating surface 

 by reducing or getting rid of their foliage and compacting 

 all their tissues into the Stem, like the cactus (Sec. 209), 

 or they compress their leaves into thick and fleshy forms 

 fitted to resist evaporation and retain large amounts of 

 moisture, as in the case of the yucca and century plant. 

 They also frequently develop a thick, hard epidermis, or 

 cover themselves with protective hairs and scales. 



351. Examination of a Xerophytic Plant. — Examine a 

 joint of the common prickly pear {Optmtid), if it grows in 

 your neighborhood, or use a potted cactus, and give your 

 reasons for regarding it as a stem and not as a leaf. 



