~ Betulaceae
Habit and leaf form. Trees, or shrubs; leptocaul. Mesophytic. Leaves deciduous; alternate; spiral to distichous; flat; petiolate; non-sheathing; simple; epulvinate. Lamina entire; pinnately veined; cross-venulate. Leaves stipulate. Stipules intrapetiolar; free of one another; caducous. Lamina margins serrate and dentate; flat. Vegetative buds scaly. Leaves without a persistent basal meristem. Vernation conduplicate. Domatia recorded (from Corylus); represented by hair tufts.
Leaf anatomy. Lamina dorsiventral. Minor leaf veins without phloem transfer cells (Corylus).
Stem anatomy. Nodes tri-lacunar. Internal phloem absent. Secondary thickening developing from a conventional cambial ring. The secondary phloem stratified into hard (fibrous) and soft (parenchymatous) zones. `Included' phloem absent. Xylem with fibre tracheids, or without fibre tracheids; with vessels. Vessels without vestured pits. Wood ring porous; parenchyma rather sparse apotracheal. Sieve-tube plastids S-type.
Reproductive type, pollination. Monoecious. Anemophilous.
Inflorescence, floral, fruit and seed morphology. Flowers aggregated in `inflorescences'; in catkins (the male catkins long and pendulous, the females short and erect). The terminal inflorescence unit racemose (i.e. the male flower solitary in the bract axil, though supposed to represent the central member of a former cymule), or cymose (the paired female flowers representing cymules with the central flower missing). Flowers bracteate; bracteolate (the bracteoles united with the bract); small.
Perianth absent (male flowers), or sepaline (female flowers); 1 whorled.
Androecium 4-8 (each member split almost to the base). Androecial members branched (or split); borne on the bract. Androecium exclusively of fertile stamens. Stamens 4-8. Anthers dehiscing via longitudinal slits; bilocular (the locules separated); tetrasporangiate. Microsporogenesis simultaneous. Tapetum glandular. Pollen grains aperturate; 3 - aperturate; porate; 2-celled.
Gynoecium 2; syncarpous; synovarious, or synstylovarious; inferior. Ovary 2 locular. Gynoecium median. Epigynous disk absent. Gynoecium stylate. Styles 2; free; apical. Stigmas dry type; non-papillate; Group II type. Placentation axile to apical. Ovules 1 per locule; funicled; pendulous; anatropous; unitegmic; crassinucellate. Embryo-sac development Polygonum-type. Endosperm formation nuclear.
Fruit non-fleshy; indehiscent; a nut (samaroid, shed with the accrescent involucre of bract plus bracteoles); 1 seeded. Seeds non-endospermic. Embryo well differentiated. Cotyledons 2. Embryo achlorophyllous (1/1); straight. Micropyle not zigzag.
Seedling. Germination cryptocotylar.
Physiology, biochemistry. Flavonols present; kaempferol, quercetin, and myricetin. Sugars transported as oligosaccharides + sucrose. C3. C3 recorded in Corylus.
Geography, cytology. Holarctic. Temperate. Widespread North temperate.
Taxonomy. Subclass Dicotyledonae; Crassinucelli. Dahlgren's Superorder Rosiflorae; Fagales. Cronquist's Subclass Hamamelidae; Fagales. Takhtajan's Subclass Hamamelidae; Hamamelidanae; Fagales. Species 15. Genera 1; Corylus.
Economic uses, etc. Sources of hazelnuts and filberts.
Illustrations. coryl717.gif
