12 A MANUAL OP THE CONIFER* 



own country, grows from 50 to 70 feet high; at its northern limit, 



in the Siberian Plains and Kamtsohatka, it is dwarfed to a low bush 



whose height ranges between 50 and 70 inches. The American 



White Spruce (Abies alba), which in Canada and the adjoining 



States, at about lat. 45° K, attains a height of 50 feet; at its 



northern limit, approaching the shores of the Arctic Ocean, at 65 , 



its height is less than 50 inches. Pinus Banlcsiana, which is scarcely 



botanically distinct from the Scotch Pine of our own country often 



seen upwards of 100 feet high, is a straggling shrub of from three to 



five feet high among the rocks in the dreary wastes of Labrador. 



The chief cause of the great difference just noticed is the diminished 



amount of solar heat which the dwarfed forms receive, and by which 



their growth is constantly retarded. At high elevations, this diminution 



is owing to the rarity of the atmosphere, which permits a rapid 



radiation of heat into space without affording any such checks as 



are present in the denser strata of lower altitudes and at the sea 



level, where the atmosphere is always more or less surcharged with 



vapour. In high latitudes, the diminution of solar heat is due to 



the slanting direction in which the sun's rays strike the earth, owing 



to the convexity of its surface, and whence their power is greatly 



weakened ; also the short period the sun is daily above the horizon 



during nearly half the year, owing to the ohliquity of the earth's axis. 



The size and height attained by the trunks or stems of Coniferous 



trees, and more especially of the same or allied species, are also greatly 



influenced by the amount of moisture of the climate in which the 



trees are growing, or which amounts to nearly the same thing, the 



annual rainfall of the region or district. It is . observed, in reference 



to the distribution of the Coniferse, that their abundance and rate of 



growth follow pretty nearly the general laws relative to the distribution 



of rain: — thus (1). — In the mountainous regions of the temperate zones 



more rain falls than in the level districts, because mountains arrest the 



clouds, and a condensation of vapour ensues from collision with their cold 



summits, and there are found the densest forests and most luxuriant 



growth. (2). — The precipitation of rain decreases in proceeding from the 



Tropic of Cancer to the Arctic Circle; in like manner it may be roughly 



stated that, except in maritime districts, the size attained by Coniferous 



trees and their rate of growth diminish in a like ratio. (3). — The rainfall 



also decreases in passing from maritime to inland countries ; it is also 



found that the growth of Coniferous plants is influenced by the same 



law. The same general facts are observable in England : thus in 



Cornwall and Devonshire the average annual rainfall exceeds 30 inches, 



while in the Eastern Counties it is often below 20 inches. The 



numerous reports published in the horticultural journals during the 



