THE BIRCH — continued. 
marshy ground or even of sour peaty soil. It is also remarkable for its power of 
holding its own, and even of spreading, amongst heather, where most other species 
are stifled. This, and its enormous seed-production, make it the speedy covering of 
northern forest clearings. At the same time, no forest tree so much requires light — 
a requirement reflected in its rapidity of growth and the slight shade that it casts. 
It is in general a short-lived tree, seldom exceeding fifty feet in height, or a 
foot in diameter. The main stem extends tapering gradually to the summit of the 
tree, whilst its most striking characteristic is the flaking silvery bark. This polished 
rind will be noticed to be traversed at frequent intervals by transverse blackish 
ridges extending partly round the stem. These are the lenticels, which have 
stretched, as the stem has increased in girth, from mere spots into long lines. The 
Canadian Indian in making canoes or moccassins from the bark of B. papyracea has 
to sew up these gaps with fibre. The flaking of the bark is produced by the 
alternation of layers of large and small cells in the “periderm” or outer bark which is 
constantly being added to from an inner cork-cambium. The larger cells are more 
readily ruptured under the influence of variations in atmospheric moisture. This 
constant throwing off of its outer bark enables the Birch to withstand the smoky 
atmosphere of our cities. 
Under Limit’s Bttula alba two distinct species are seemingly confused : 
B. pendula Roth and B. tomentosa Reith and Abel. These may, however, occur side 
by side and have thus become hybridised, and they seem also to have given rise to 
simple variations. B. pendula Roth, named later B. verrucosa by Ehrhart, has longer, 
more pendulous branches, with white resinous tubercles on their bark ; and 
rhomboidally triangular leaves with straight bases, and veins projecting on the upper 
surface. B. tomentosa Reith and Abel, which is B. glutinosa of Fries or B. pubescent 
of Koch, is often a mere shrub and has rounded or heart-shaped bases to its leaves, 
with the veins projecting from their under surfaces. 
The male catkins of the Birch are borne at the ends of the shoots of the 
previous year, generally in threes. Each catkin-scale has in its axil three flowers 
each with two sepals and two deeply-divided stamens, bearing four half-anthers 
without the tufts of hair that occur in Hornbeam and Hazel. 
The female catkins are shorter and are, at first, erect. Here, too, there are 
three flowers in the axil of each scale. The scale coalesces with the two bracteoles 
into a small trilobed body, and the fruits have a relatively broad membranous wing. 
When they are ripe the whole female catkin falls to pieces. 
