BIRCHES 261 



slightly cordate base, and tapering acuminate apex. 

 Margin more or less angular or sinuate-serrate with large 

 teeth again serrate or bi-serrate ; entire at the base. 

 Membranous to sub-coriaceous, green and somewhat 

 shining; paler and glandular, dotted and viscid below, 

 especially when young. Slightly pubescent when young, 

 usually becoming glabrous. Stipules broad and con- 

 spicuous. The leaf is often hung so lightly as to tremble 

 like the Aspen. Petiole about 2 3 cm., half as long as 

 the midrib. Autumn leaves bright yellow, orange and 

 reds passing to brown. 



Venation strict-pinnate, midrib tapering from a stout 

 base, straight or somewhat zigzag to the apex : the second- 

 aries running, slightly curved, direct to the margins and 

 ending in teeth or lobes. Tertiaries fine and connecting, 

 coming off at acute angles on both sides; not looped 

 beneath the irregularly serrate margins. Each pair of 

 secondaries separated by a distance about equal to ^ the 

 length of the midrib ; upper secondaries simple, coming 

 off at angles of 40 50 degrees ; the lower the longest and 

 at angles of 60 75 degrees, and with several prominent 

 outer branches. Reticulation distinct, meshes chiefly oval. 



[Several varieties occur, differing in the details of size, 

 shape, toothing and pubescence, viscidity, &c. of the leaves, 

 more or less drooping habit and so forth.] 



Betula nana, L., the Dwarf Birch, is an Arctic shrub, 

 with erect shoots and dark green, very small, sub-orbicular, 

 glabrous leaves, usually broader than long, 5 10 x 6 

 12 mm. (4 12 x 5 15 mm.), crowded, obtuse, coarsely 

 crenulate-dentate, glabrous, with venation reticulate and 

 salient and gland-dotted below, and sub-sessile or on very 

 short petioles (1 2 mm.). Secondaries 2 4 pairs. Other- 

 wise similar to B. alba. 



