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Betula
pendula Roth, B. pubescens Ehrh.
Standardized Common
Name: Birch
Other Common Names: Common Birch, Downy
Birch, European White Birch, Silver Birch, White Birch
Family: Betulaceae
Taxonomy: Betula
includes about 35 species of trees, which are native to temperate habitats in
Eurasia and North America. Betula pendula and B. pubescens are
among four European species. The name B. alba L., sometimes applied to
both of these, has been rejected by taxonomists because it has been too widely
misapplied. Both species, especially B. pubescens, are quite variable; a
dozen other obsolete synonyms exist, and a number of infraspecific taxa have
been described for each. The two also hybridize freely and occasionally
intergrade, but the consensus opinion is that they should be treated as
separate species.
Description: Trees, sometimes
multiple-trunked, or large shrubs, with whitish bark separating from trunk in
large pieces. Branchlets of B. pendula hanging downward from branches,
bearing resin glands on surface; branchlets of B. pubescens spreading
outward, pubescent. Leaves alternate or clustered beneath catkins; shape
variable within and between species, 3–5(–7) cm long, deltoid to ovate or
rhombic; base truncate to cuneate or rounded; apex acuminate or acute; margins
doubly or irregularly serrate; venation with prominent midrib and with several
pairs of primary veins running from midrib toward teeth, parallel and slightly
curving. Inflorescences wind-pollinated catkins, unisexual, cylindrical, to
several cm long, consisting of numerous tiny reduced flowers, without perianth
and subtended by bracts. Male flowers reduced to small scale and 4 stamens;
female flowers reduced to single ovary. Fruit a small, wind-dispersed samara.
Parts
in Commerce:
Leaves
Identification: Though hybridization
between these two species does occur, they can generally be distinguished from
one another by leaf morphology.
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Betula pendula
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Betula pubescens
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Leaf
shape
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Ovate-deltoid,
broadest near base, sometimes rhombic
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Variable;
often deltoid or rhombic, broadest near the middle
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Leaf
length
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(2–)3–7
cm
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Usually
3–5 cm; 1.5–3 cm in some European varieties
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Leaf
base
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Truncate
to cuneate
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Rounded
to truncate
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Leaf
apex
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Acuminate
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Acute
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Leaf
margins
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Biserrate,
with few large teeth bearing numerous small teeth
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Irregularly
toothed
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Leaf
pubescence
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Glabrous
or nearly so, except for small round oil glands on both surfaces
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Slightly
pubescent with very short hairs, especially in axils of main veins beneath
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Several
characters are common to both species:
- Petiole not more
than half as long as blade
- Blade
thin-textured
- Lower surface
paler green
- With prominent
midrib from which up to 7 pairs of primary veins arise; primary veins run
in parallel almost to the teeth, curving slightly; venation most
conspicuous below
- Taste weak,
slightly bitter
References:
Gardiner AS, Pearce NJ. Leaf-shape as an indicator
of introgression between Betula pendula and B. pubescens. Trans
Bot Soc Edinburgh. 1979;43:91–103.
Walters SM. Betula. In: Tutin TG, Heywood VH, Burges NA, et al.,
eds. Flora Europaea. 2nd ed., vol. 1. Cambridge:
Cambridge University Press; 1993:68–69.
Wichtl M, ed. Herbal Drugs and
Phytopharmaceuticals, 3rd English
ed. Stuttgart: medpharm Scientific Publishers and Boca Raton, FL: CRC Press;
2004:87–90.
Figure 13: a, Betula pendula leaf; b, B. pubescens leaf.
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