ALTITUDINAL RANGE OF VEGETATION. 695 



8. The Polar Zone.— This includes all lands from 72° to 82° lati- 

 tude. The mean temperature of one point in this zone, viz. Melville 

 Island, is 1J°. In the Old World, the mean temperature is 16^°. 

 .Summer temperature of the New World, 37|°, and of the Old, 38^°; 

 Winter temperature, -28° in the New, and -2^° in the Old World. 

 No trees nor bushes in this zone. Some Saxifrages, Ranunculi, 

 Potentillas, species of Draba, Dryas, Panya, and Phippsia, and 

 numerous Oryptogamic plants, as Lichens, prevail in it. 



ALTiinDiNii Eanqe op Vegetation. 



Under this head we consider the changes produced in the physiog- 

 nomy of vegetation on ascending mountains. It has reference to the 

 -distribution of plants in an altitudinal or hypsometrical point of view. 

 This geographical range is best seen in the high mountains of tropical 

 ■countries, where all gradations are met with, from the heat of the 

 torrid zone to the cold of the frigid zone. Humboldt, in describing 

 South American scenery, remarks : — " In the burning plains, scarce 

 raised above the level of the Southern Ocean, we find Bananas, Oyca- 

 ■dacese, and Palms in the greatest luxuriance ; after them, shaded by 

 the lofty sides of the valleys in the Andes, Tree Ferns ; next in suc- 

 cession, bedewed by cool misty clouds. Cinchonas appear. When lofty 

 trees cease, we come to Aralias, Thibaudias, and myrtle-leaved Andro- 

 medas ; these are succeeded by Bejarias abounding in resin, arid 

 forming a purple belt around the mountains. In the stormy regions 

 of the Paramos, the more lofty plants and showy flowering herbs 

 ■disappear, and are succeeded by large meadows covered with grasses, 

 on which the Llama feeds. We now reach the bare trachytic rocks, on 

 which the lowest tribes of plants flourish. Parmelias, Lecidias, and 

 ' Leprarias, with their many-coloured sporules, form the flora of this 

 inhospitable zone. Patches of recently fallen snow now begin to cover 

 the last efforts of vegetable life, and then the line of eternal snow begins. " 

 On the mountains of temperate regions the variety is rather less, 

 but the change is'not less striking. " We begin to ascend the Alps, 

 for instance, in the midst ot warm vineyards, and pass through a 

 succession of oaks, sweet-chestnutSj and beeches, tiU we gain the 

 ■elevation of the more hardy pines and stunted birches, and tread on 

 pastures fringed by borders of perpetual snow. At the elevation of 

 1950 feet the vine disappears; and at 1000 feet higher the sweet- 

 .chestnuta cease to grow ; 1000 feet farther, and the oak is unable to 

 maintain itself; the birch ceases to grow at an elevation of •4680, and 

 the spruce fir at the height of 5900 feet, beyond which no tree appears. 

 The Rhododendron ferrugineum (the Rose of the Alps) then covers 

 immense tracts, to the height of 7480 feet, and Salix herbacea creeps 

 ^00 or 300 feet higher, accompanied by a few Saxifrages, Gentians, 



