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 (Hucalyptus) 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 
Fic. 230.—A Halophytic Plant roots, so that the plants which 
cas all 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 
, 
