BOTAOTCAL GEOGRAPHY 



327 



(Ammophila) of the Atlantic coast and the great lakes, 

 will continue to grow upward as the sand is piled about 



them by the winds until they 

 have risen to a level of a 



hundred feet above the start- 

 ing point. 



Peat bogs are especially 

 characterized by the predominance of 

 the peat mosses (Fig. 231) from which 

 they take their name. 

 These plants and the others which associ- 

 ate with them are mostly hydrophytes, living 

 usually with a considerable portion of the 

 plant continually submerged in the bog 

 water. The water of such bogs contains 

 little mineral matter and only a very scanty 

 supply of nitrogen, in the form of nitrates 

 dissolved in it. The bog-plants, therefore, 

 must either get on with an exceptionally 

 small supply of nitrogen or they must get 

 it from an unusual source. The peat mosses 

 adopt the former alternative, while the sun 

 dews (Fig. 238), the pitcher-plants (Fig. 

 237), and some other species adopt the latter and 

 derive their nitrogen supply largely from insects 

 which they catch, kill, and digest. 



399. Arctic Vegetation. — The seed-plants of the 

 arctic flora are mostly perennials, never trees. 

 By the large bulk of the underground portion 

 as compared with that of the part above ground, they 

 are adapted to a climate in which they must lie dormant 



