2 FAMILIAR TREES 



includes some five-and-twenty species of shrubs and 

 trees of medium size, confined to tlie northern 

 hemisphere, and remarkable for their extension 

 into Arctic latitudes. The Canoe or Paper Birch 

 (B. papyra'cea Ait.), of North America reaches lat. 

 70° N., whilst another species (B. Bhojput'tra Wall.) 

 grows at an altitude of 9,000 feet in the Himalayas. 

 Our own species, Betula alba, ascends to 2,600 feet 

 in the Highlands, and is widely spread over Europe, 

 Asia, and America, extending farther north than 

 any other European tree, but only constituting an 

 essential element of forest scenery as far south as 

 45°. Together with the Alders, of which there are 

 some fourteen species, the Birches form the Natural 

 Order Betulacece, catkin-bearing trees with not more 

 than five stamens to each flower, and with neither 

 " perianth " nor " cupule " (like those of Oaks or 

 Hazels) to enclose their small compressed fruits. 

 The Birches differ from the Alders in the scales of 

 the seed-bearing catkin being chaff-like, and falling 

 together with its winged fruits, whilst those of the 

 Alders remain as a woody cone. 



The White or Silver Birch is a short-lived tree, 

 as a rule from forty to fifty feet high, though, ex- 

 ceptionally, growing to eighty feet, with a trunk 

 seldom exceeding a foot in, diameter, conspicuous 

 from its flaking, silvery-white bark. This flaking 

 is produced by the formation of alternate layers of 

 larger and sinaller cells in the " periderm " or outer 

 bark, of which the larger are the more readily 

 ruptured under the influence of variations in the 

 degi'ee of atmospheric moisture. Every careful 



