326 



FOUNDATIONS OF BOTANY 



growths as occur in the rainy forest regions of the tropics. 

 On the other hand, the largest trees on earth, the "big 

 trees," or Sequoias (Fig. 32), occur in the temperate por- 

 tion of North America, along the Sierra Nevada, and 

 the taller, though less bulky, gum trees (Eucalyptus) of 

 Australia grow in a warm temperate region. 



398. Temperate Plant Societies due to Special Conditions 

 of Soil. Even where the climate 

 is a moderate one as regards tem- 

 perature and rainfall, peculiar 

 soils may cause the assemblage 

 of exceptional plant societies. 

 Some of the most notable of 

 such societies in temperate North 

 America are those of the salt 

 marshes, the sand dunes, and the 

 peat bogs. 



In salt marshes the water sup- 

 ply is abundant, but plants do not 

 readily absorb salt water by their 

 roots, so that the plants which 

 grow in salt marshes usually have 

 something of the structure and appearance of xerophytes. 

 Some of them are fleshy (Fig. 230), and some species are 

 practically leafless. 



Sand dunes, whether along the seacoast or near the 

 great lakes, offer a scanty water supply to the roots dur- 

 ing much of the year, and the soil-water contains less of 

 the raw materials for plant food than is offered by that 

 of ordinary soils. Many grasses thrive, however, in these 

 shifting sands (Plate I), and some, like the beach-grass 



FIG. 230. A Halophytic Plant 

 (Salicomia). 



