672 GEOGRAPHICAL AND GEOLOGICAL BOTANY. 



for the month of September. The southern boundary of this zone in the 

 northern hemisphere corresponds pretty nearly to the limit of distribution 

 of the Oak in Europe and the east coast of North America, the northern 

 boundary to the limit of the distribution of trees. 



The striking characteristic of this zone is, indeed, the predominance of 

 the Coniferous trees in the woods, giving place northwards to the Birch 

 and Alder, and generally alternating with Willows where the soil is 

 moist. Green pastures occur universally, especially adorned with showy 

 flowering herbs in the spring and summer. 



7. The Arctic Zone. The equatorial boundary is the isotherm of 36 -5 

 F. (2 C.) for the month of September, or the polar limit of arborescent 

 vegetation in the northern hemisphere ; the polar boundary is the isotherm 

 of 41 F. (5 C.) for the month of July. The vegetation of this zone corre- 

 sponds to what we understand commonly as Alpine shrubs, consisting 

 chiefly of prostrate shrubs, with a peculiar tortuous and compact habit of 

 growth, such as the alpine Rhododendra, Andromedae, the dwarf Birch and 

 Alder, the Bog-Myrtle and dwarf Willow, with a variety of low-growing 

 perennial herbs, remarkable for the comparatively large size and bright 

 colour of their flowers. Sedges and Cotton-grasses occur socially, in 

 some places covering extensive tracts ; but the grassv pastures of the last 

 zones are replaced to a great extent by tracts covered with Lichens. 



8. The Polar Zone. Equatorial boundary, the isotherm of 41 F. (5 C.) 

 for the month of July ; polar limit, the isotherm of 36 '5 F. (2 C.) for the 

 same month. This zone is characterized by presenting, in the four to six 

 weeks of summer, an alpine vegetation devoid of even shrubs, and con- 

 sisting of herbaceous perennials of dwarf habit, such as Saxifrages, Ra- 

 nunculi, Pyrolse, Potentillse, Dryas, Draba, &c., and possessing, moreover, 

 certain genera (such as Parrya, Phippsia, and others) which, although 

 they extend into the Arctic zone, are not met with in the alpine regions of 

 the mountains of the more southern regions. In Spitzbergen, the number 

 of Cryptogamic plants is remarkable, the Lichens alone equalling the 

 Flowering plants, and predominating even in mass as well as number of 

 species. 



Sect. 2. KEGIONS OF ALTITUDE. 



It is well known that the lofty mountains lying within the 

 tropics exhibit a graduated variation of character in their vegeta- 

 tion, and that those which rise above the limit of eternal snow 

 display more or less distinctly marked regions, representing the 

 zones lying between the plains at the foot of such mountains and 

 the eternal ice of the polar zone. 



Humboldt divided the surface of tropical mountains into three 

 zones, representing the tropical, temperate, and frigid zones of the 

 globe, and indicated the principal subdivisions of these regions. 

 Meyen attempted to lay down a more systematic representation of 

 the conditions in question, corresponding to his division of the 



