PLANT ASSOCIATIONS 943 



is their succulence, which is accompanied by a very high osmotic pres- 

 sure. In temperate regions the most representative salt marsh plants 

 are herbaceous, but in the tropics extensive mangrove forests are found 

 in similar conditions ; few plants show more marked xerophy tic features 

 than do the mangroves, which have evergreen leaves with water tissue, 

 prominent palisade cells, and thick cutin. Often there is a network of 

 prop roots above the water line, and in some cases there are ascending 

 "knees" (fig. 726). 



Xerophytic associations. The characteristic features of xerophytes. 

 In most respects xerophytes are the reverse of hydrophytes in their 

 structural features. The roots frequently are strongly developed (though 

 not in cacti), possessing either considerable length or great size; roots of 

 the latter class accumulate large amounts of water and food. In some 

 extreme xerophytes the root hairs extend to the root tips, and in certain 

 cases they possess rigid thickened walls. Palisade tissue is strongly 

 developed, and the chlorenchyma in the leaves and stems commonly is 

 deeply sunken, giving them a pale tint as viewed from without; usually 

 the plastids are small and relatively immotile. 



Protective features are remarkably developed both in amount and in 

 kind, and their advantage is undoubted, owing to the great exposure of 

 xerophytes to transpiration. The transpiring surface usually is rela- 

 tively reduced, the leaves being small and thick. Many species are 

 leafless, the cylindrical stems exposing a relatively small surface to trans- 

 piration, while their vertical orientation affords some protection from 

 the intense rays of light at midday ; species with vertical leaves are sim- 

 ilarly protected. In many cases there is a temporary reduction of sur- 

 face, as in the involute leaves of grasses, as in those legumes whose leaves 

 close in dry weather, and as in the " resurrection plants." Temporary 

 reduction of surface is exhibited also by plants which shed their leaves 

 or stems during dry periods ; annuals, which die at the beginning of dry 

 periods, represent the culminating form of such behavior. Dwarf- 

 ness of habit is a prominent xerophytic feature, the resulting compactness 

 in arrangement of branches and leaves and the closeness to the ground 

 affording considerable protection. 



The more minute structural features of xerophytes are no less signifi- 

 cant than are the more obvious characters. Commonly the epidermis 

 is thick and highly cutinized (except in succulent xerophytes), and often 

 it is superficially coated with wax, resin, or varnish. In woody stems 

 there is a prominent bark development, the cork in particular being of 



