12 THE STEM. 



to the apex, perfectly erect except where thrown out of the per- 

 pendicular by the action of the wind, and attaining dimensions varying 

 from a few inches* to more than 300 feet in height,f and with 

 diameters generally small in proportion to the height, but in this 

 respect the Yew, the Cedar of Lebanon and the deciduous Cypress 

 afford occasional exceptions. The size attained by stems of the same 

 species is far from being uniform except under like conditions, the 

 growth being greatly influenced by soil, situation or climate, or by ti 

 combination of these causes. Some species of Pinus and Abies, for 

 example, having the slopes of mountains for their habitat, at and near 

 their lower limit grow from 60 to 100 feet high, or even more; but 

 this height is found to diminish in proportion to the elevation at 

 which they grow, so that at the highest point, often at the limits of 

 perpetual snow, they are dwarfed to a more scrubby bush over which 

 a man may step. A similar change is observed in species whose 

 habitat extends over many degrees of latitude ; thus, the Cembra Pine 

 on the Swiss Alps, and under cultivation in our own country, grows 

 from 50 to 70 feet high ; at its northern limit, in the Siberian plains 

 and Kamtschatka, it is dwarfed to a low bush the height of which 

 ranges between 50 and 70 inches.^ The American Tideland Spruce, 

 Picca sitchensis, which in the swampy littoral tracts of Oregon grows to 

 a height of 250 feet, is reduced to a low scrubby bush at its extreme 

 northern limit in Alaska. Pinus Banksiana, which is botanically allied 

 to the Scots Pine of our own country and 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 liori/on 

 during nearly half the year, owing to the obliquity 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 tin- 

 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 

 * Juniper us rntit(iii>> comprcssa, native of the Pyrenees. 

 t Sequoia Wellington ia, the Mammoth tree of California. 



i This form is described as a distinct species under the name^of Pinus jnuniln l>y 

 Dr. Heinrich Mayr, in " AMetineen des Japanischen Reichs," p 80." 



