RESPONSE OF THE PLANT TO ITS SURROUNDINGS 285 



humus. For similar reasons the vegetation of sandy bogs 

 and sea beaches, owing to the poverty of the soil in nitrog- 

 enous matter, usually develops xerophyte adaptations, 

 even though there may be a superabundance of moisture. 

 Plants growing on high mountain tops and in cold arctic 

 bogs take on the same characteristics (Fig. 410). Such situa- 

 tions are said to be " physiologically dry/' because the 

 moisture they have is not in a condition to be readily ab- 



FIG. 421. A halophyte swamp of mangroves. Notice the tangle of adven- 

 titious prop roots assisting in the work of absorption from the brackish marsh soil. 

 (From Mo. Botanical Garden Rep't.) 



sorbed. The vegetation of arctic regions suffers more from 

 physiological drought than from cold. 



323. Halophytes include plants growing by the seashore 

 and the vegetation around salt springs and lakes and that of 

 alkali deserts. Seaweeds are in a sense halophytes, since 

 they live in salt water, but as they are true aquatic plants 

 and exhibit many of the peculiarities of hydrophytes in their 

 mechanical structure, they are classed with them. The 

 name halophyte applies more particularly to land plants 



