Trees; a handbook of forest-botany for the woodlands and the laboratory . Fig. 93. Alder, Alnus glutinosa. Venation strict-pinnate, a midrib ;b secondary; c tertiary; d quaternaries and terminals forming network,p. 258 (Ett). ending in teeth, or mostly forking at the apex, or givingoff strong outer nerves, which do so: angle 50—65 at acute angles, sinuous, not looped beneath thebi-serrate margin, and often forming complete cross-tiesbefore anastomosing. Each pair of secondaries distant 17—2 260 BIRCH about \ the length of the midrib; the lowest much shorterthan those in the
Trees; a handbook of forest-botany for the woodlands and the laboratory . Fig. 93. Alder, Alnus glutinosa. Venation strict-pinnate, a midrib ;b secondary; c tertiary; d quaternaries and terminals forming network,p. 258 (Ett). ending in teeth, or mostly forking at the apex, or givingoff strong outer nerves, which do so: angle 50—65 at acute angles, sinuous, not looped beneath thebi-serrate margin, and often forming complete cross-tiesbefore anastomosing. Each pair of secondaries distant 17—2 260 BIRCH about \ the length of the midrib; the lowest much shorterthan those in the middle of the leaf, and more divergent,and they are almost devoid of conspicuous outer rather loose, not prominent. /~1 n Leaf triangular- or rhomboid-cordate orbroad-ovate, acuminate; secondaries 5—6pairs, the lower the longest. Betula alba, L. Birch (Fig. 94). Small tree, withwhite periderm, and slender more or less drooping shoots,. Fig. 94. Birch, Betula alba, p. 260 (Ett). bearing thin foliage. Leaf variable, 4—6 x 2—5 cm.(3—9 cm.), broadly deltoid- or rhomboid-cordate to ovate-triangular, or more or less broad-ovate to broadly cuneate;with plane or angular-rounded, or rarely truncate or BIRCHES 261 slightly cordate base, and tapering acuminate more or less angular or sinuate-serrate with largeteeth again serrate or bi-serrate; entire at the to sub-coriaceous, green and somewhatshining; 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 tremblelike the Aspen. Petiole about 2—3 cm., half as long asthe midrib. Autumn leaves bright yellow, orange andreds passing to brown. Venation strict-pinnate, midrib tapering from a stoutbase, straight or somewhat zigzag to the apex: the second-aries running, slightly curved, direct to the margins
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