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The families of flowering plants

L. Watson and M.J. Dallwitz

Betulaceae S.F. Gray

Including Nuculaceae Dulac; excluding Corylaceae and Carpinaceae (q.v.).

Habit and leaf form. Trees, or shrubs; leptocaul. Helophytic, or mesophytic. Leaves deciduous; small to medium-sized; alternate; spiral to distichous; flat; ‘herbaceous’; petiolate; non-sheathing; simple. Lamina entire; pinnately veined; cross-venulate. Leaves stipulate. Stipules intrapetiolar; free of one another; scaly; caducous. Lamina margins serrate, or dentate. Vegetative buds scaly. Leaf development not ‘graminaceous’. Domatia occurring in the family (found in both genera); manifested as pits, or pockets, or hair tufts (mostly).

Leaf anatomy. The leaf lamina dorsiventral (usually), or dorsiventral to bifacial (in that the mesophyll largely consists if palisade cells in A. glutinosa). Mucilaginous epidermis present, or absent. Stomata mainly confined to one surface (all abaxial, or relatively few adaxially); anomocytic. The mesophyll containing crystals. The crystals druses and solitary-prismatic (the former predominating). Minor leaf veins without phloem transfer cells (Alnus, Betula).

Axial (stem, wood) anatomy. Pith almost or quite homogeneous. Cork cambium present; initially superficial (in the outer part of the cortex). Nodes tri-lacunar. Primary vascular tissues in a cylinder, without separate bundles; collateral. Internal phloem absent. Cortical bundles absent. Medullary bundles absent. Secondary thickening developing from a conventional cambial ring.

The wood usually ring porous (?). The vessels small, or small to medium; radially paired, in radial multiples, and in tangential arcs (with numerous multiples of 2–3 cells, usually with some of 4 or more, and sometimes patterned obliquely). The vessel end-walls scalariform, or reticulately perforated and scalariform. The vessels without vestured pits; without spiral thickening. The axial xylem with tracheids, or without tracheids; with fibre tracheids to without fibre tracheids (the fibres with pits rather few, with small but distinct borders); with libriform fibres to without libriform fibres; without septate fibres. The fibres without spiral thickening. The parenchyma apotracheal. ‘Included’ phloem absent. The wood not storied. Tyloses commonly present.

Reproductive type, pollination. Plants monoecious (the males and females in separate inflorescences). Pollination anemophilous.

Inflorescence, floral, fruit and seed morphology. Flowers aggregated in ‘inflorescences’; in catkins, or in heads and in catkins. The fruiting inflorescences conelike (notably Alnus), or not conelike. The ultimate inflorescence units cymose (typically a 3-flowered cymule in each catkin bract or cone-scale axil). Inflorescences terminal (the stems sympodial); more or less elongate, pendulous catkins (male), or erect, short heads or woody cones (female), with few-flowered dichasia in the bract axils. Flowers bracteate; bracteolate; minute, or small. Free hypanthium absent.

Perianth sepaline (male), or vestigial, or absent (female); 0, or 2–4. Calyx (in male flowers) 1–6 (minute, scale-like).

Androecium 2–4 (but ostensibly more by congestion of the three members of the cymule). Androecial members unbranched; free of the perianth, or adnate (to the base of the perianth); free of one another, or coherent; when coherent, 1 adelphous (the filaments united at the base). Androecium exclusively of fertile stamens. Stamens 2–4; isomerous with the perianth; oppositisepalous. Anthers basifixed; non-versatile; dehiscing via longitudinal slits; bilocular (the cells juxtaposed in Alnus, on the separate branches of the bifurcated filament in Betula); tetrasporangiate. Pollen grains aperturate; porate; 2-celled.

Gynoecium 2 carpelled. The pistil 1 celled (above), or 2 celled. Gynoecium syncarpous; synovarious; superior. Ovary incompletely 2 locular (unilocular above). Gynoecium median; stylate. Styles 2; free; apical. Stigmas 2; dry type; non-papillate; Group II type. Placentation axile to apical. Ovules 1 per locule; pendulous; non-arillate; anatropous; unitegmic; crassinucellate. Embryo-sac development Polygonum-type. Polar nuclei fusing prior to fertilization. Antipodal cells formed; 3; not proliferating; ephemeral to persistent. Synergids pear-shaped. Endosperm formation nuclear. Embryogeny asterad.

Fruit non-fleshy; indehiscent; a nut, or a samara (then two-winged, and the fruit sometimes remaining attached to the scalelike organ representing accrescent bracts and bracteoles). Dispersal unit the fruit. Dispersal by wind. Fruit 1 seeded. Seeds non-endospermic. Cotyledons 2 (oily); flat. Embryo achlorophyllous (2/3); straight.

Seedling. Germination phanerocotylar, or cryptocotylar.

Physiology, phytochemistry. Not cyanogenic. Alkaloids absent (13 species). Arbutin absent. Iridoids not detected. Proanthocyanidins present; cyanidin, or delphinidin, or cyanidin and delphinidin. Flavonols present; kaempferol and quercetin, or kaempferol, quercetin, and myricetin, or quercetin and myricetin. Ellagic acid present, or absent (2 Alnus species listed). Sieve-tube plastids S-type.

Geography, cytology. Temperate. North temperate, tropical mountains, Andes, Argentina. X = 8, 14.

Taxonomy. Subclass Dicotyledonae; Crassinucelli. Dahlgren’s Superorder Rosiflorae; Fagales. Cronquist’s Subclass Hamamelidae; Fagales. APG III core angiosperms; core eudicot; Superorder Rosanae; fabid. APG IV Order Fagales.

Species 95. Genera 2; Betula, Alnus.

Economic uses, etc. Hardwood timber (especially for plywood) from Betula.


Fond fathers,
Having bound up the threatening twigs of birch,
Only to stick it in their children’s sight
For terror, not to use; in time the rod
Becomes more mock’d than fear’d.
(‘Measure for Measure’, i.,4)

The birch begins to crack its outer sheath
Of baby green and show the white beneath,
As whosoever likes the young and slight
May well have noticed. Soon entirely white
To double day and cut in half the dark
It will stand forth, entirely white in bark
(Robert Frost, ‘A Young Birch’)

Illustrations. • Le Maout and Decaisne: Betula, Alnus. • Betula papyrifera: Hutchinson. • Alnus glutinosa (B. Ent., 1830). • Alnus glutinosa: Eng. Bot. 1294, 1868. • Betula nana, Betula pendula (as B. verrucosa) and Betula cf. pubescens (as B. glutinosa): Eng. Bot. 1295–1297, 1865. • Betula pendula (B. Ent.).

We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or the interactive key, which allows access to the character list, illustrations, full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, distributions of character states within any set of taxa, geographical distribution, genera included in each family, and classifications (Dahlgren; Dahlgren, Clifford, and Yeo; Cronquist; APG). See also Guidelines for using data taken from Web publications.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 1992 onwards. The families of flowering plants: descriptions, illustrations, identification, and information retrieval. Version: 15th April 2018.’.