Betulaceae S.F. Gray
Including Nuculaceae Dulac
Excluding Corylaceae, Carpinaceae
Habit and leaf form. Trees, or shrubs; non-laticiferous and without coloured juice; 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. Leaves without a persistent basal meristem. Domatia occurring in the family (from both genera); manifested as pits, or pockets, or hair tufts (mostly).
Leaf anatomy. Mucilaginous epidermis present, or absent. Stomata anomocytic.
Minor leaf veins without phloem transfer cells (Alnus, Betula).
Stem anatomy. Cork cambium present; initially superficial. Nodes tri-lacunar. Primary vascular tissue in a cylinder, without separate bundles. Internal phloem absent. Secondary thickening developing from a conventional cambial ring. ‘Included’ phloem absent. Xylem with tracheids, or without tracheids; with fibre tracheids, or without fibre tracheids; with vessels. Vessel end-walls simple, or scalariform and simple. Vessels without vestured pits. Wood parenchyma apotracheal. Sieve-tube plastids S-type.
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 unit 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, biochemistry. Not cyanogenic. Alkaloids absent (13 species). 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). Arbutin absent.
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 (1998) Eudicot; core Eudicot; Rosid; Eurosid I. APG 3 (2009) Order: Fagales.
Species 95. Genera 2; Betula, Alnus.
Economic uses, etc. Hardwood timber (especially for plywood) from Betula.
• Technical details: Betula, Alnus.
• Alnus glutinosa (B. Ent.).
• 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.).
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’)