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- 
Fic. 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 
