The Vegetation of North America 313 



two or three months of summer. Consequently the only plants 

 that thrive are the low plants that have shallow roots, like grasses 

 and sedges, or that grow entirely on the soil surface, like the 

 mosses and lichens. Northward, the tundra is limited by the 

 polar seas and the areas of perpetual ice and snow ; southward, 

 by the northern evergreen forest. Even farther south, among 

 the forests, are patches of tundra-like vegetation that remain in 

 bogs and on bare rock outcrops. On the higher mountain 

 summits everywhere are alpine areas covered with vegetation 

 that is closely related to that of the tundra. 



The winter season is characterized by intense cold, violent 

 dry winds, and very light snows. The short growing period 

 during the summer, the low soil temperature, poor drainage, and 

 consequent scanty aeration of the soil are important factors in 

 excluding most plants, particularly trees, from this region. The 

 better-drained stony uplands are covered with grasses ; the low 

 places, with mosses, sedges, shrubs, and flowering herbs. 



Most of the flowering plants are only a few inches in height. 

 Many have leathery leaves and creeping or reclining stems, and 

 are typical xerophytes. Cranberries, crowberries, and snow- 

 berries are examples of common low shrubs. Many of these 

 plants are evergreen, and many of them are small compared with 

 those of temperate regions. The willows are represented by 

 several dwarf species that rise only a few inches above the soil. 



The northern evergreen forest formation. Stretching from 

 Newfoundland and Labrador to Alaska by way of the St. Law- 

 rence Valley and the lower Hudson Bay region is the northern 

 evergreen forest. This forest is composed of the white spruce, 

 black spruce, paper birch, aspen, balsam poplar, tamarack, 

 balsam fir, white pine, red pine, jack pine, arbor vitae, and hem- 

 lock. The last five of these trees^ are found mostly east of 

 Winnipeg. All the trees attain their greatest size in the region 

 between northern Minnesota, Maine, and eastern Quebec. On 

 the western plains of Canada, where the rainfall is reduced to 



