The families of flowering plants.
Excluding currently numerous genera formerly referred to Flacourtiaceae (q.v.)
Habit and leaf form. Trees and shrubs (including some procumbent and ‘almost herbaceous’ Salix species); leptocaul, or pachycaul (e.g., Salix alaxensis, S. richardsonii). Helophytic (often on riversides, in fens and bogs, dune slacks, etc.), or mesophytic. Heterophyllous (in some Populus species), or not heterophyllous. Leaves deciduous; small to large; alternate (mostly), or alternate to opposite; spiral, or spiral to distichous; flat; petiolate (the distal end of the petiole characterised by one or more closed rings of xylem and phloem, which in Populus are often vertically superimposed); non-sheathing; simple; epulvinate. Lamina entire (mainly), or dissected (in some Populus species); when dissected, somewhat pinnatifid, or palmatifid (e.g., in summer leaf blades and on suckers of P. alba); pinnately veined (mostly), or parallel-veined (some arctic species being campylodromous or parallelodromous); cross-venulate. Leaves more or less exstipulate (e.g., Chosenia, some Salix species), or stipulate (usually, at least on vigorous shoots). Stipules intrapetiolar (lateral to the petioles); free of one another; caducous (often), or persistent. Lamina margins entire (to undulate or crispate), or dentate (occasionally, more or less, or shallowly lobed), or crenate (or crenulate), or serrate (or serrulate, or spinulose-serrulate, the margins sometimes furnished with vesicular glands). Vegetative buds scaly. Leaves without a persistent basal meristem. Domatia occurring in the family (Populus); manifested as hair tufts.
Leaf anatomy. Mucilaginous epidermis present (commonly), or absent. Stomata present; on both surfaces (frequently), or mainly confined to one surface; often paracytic (sometimes with two subsidiaries at either side). Hairs usually (?) present; eglandular; unicellular. Unicellular hairs unbranched. Complex hairs absent.
Adaxial hypodermis present, or absent. Lamina dorsiventral (usually), or isobilateral (e.g., recorded in Salix purpurea, Populus nigra). The mesophyll containing calcium oxalate crystals. The mesophyll crystals druses and solitary-prismatic. Minor leaf veins without phloem transfer cells (Salix).
Stem anatomy. Young stems cylindrical. Secretory cavities absent. Cork cambium present; initially superficial. Nodes tri-lacunar. Cortical bundles absent. Medullary bundles absent. Internal phloem absent. Secondary thickening developing from a conventional cambial ring. The secondary phloem stratified into hard (fibrous) and soft (parenchymatous) zones, or not stratified. ‘Included’ phloem absent. Xylem with libriform fibres; with vessels. Vessel end-walls simple. Vessels without vestured pits. Primary medullary rays narrow. Wood parenchyma apotracheal (exclusively terminal). Sieve-tube plastids S-type.
Reproductive type, pollination. Plantsdioecious. Pollination anemophilous, or entomophilous.
Inflorescence, floral, fruit and seed morphology. Flowers aggregated in ‘inflorescences’; in catkins (these pendulous in Populus, usually erect in Salix). The ultimate inflorescence unit (i.e., the catkin) racemose. Inflorescences terminal (and appearing after the leaves, e.g. in Salix herbacea, S. reticulata), or axillary (usually, then lateral on the previous year’s wood, and maturing with or more commonly before the leaves); catkins, the males and females on different plants; pseudanthial (perhaps, somewhat), or not pseudanthial. Flowers individually bracteate (with each member of the catkin subtended by a bract, which is generally entire in Salix, and lobed or cut in Populus); ebracteolate (according to the usual interpretations of catkin structures); minute to small. Free hypanthium absent.
Perianthabsent, or vestigial (a supposed vestigial calyx being represented in Populus by a cupular disk, or in Salix by one or two to several small, sometimes fringed, often unequal and sometimes united nectariferous scales).
Androecium 1, or 2 (most Salix species have two while some (e.g., S. sitchensis) genuinely have only one, and others such as S. purpurea have a pair of coalesced filaments but two anthers), or 3–30(–60) (with Populus usually having ‘several to many’). Androecial members branched (?), or unbranched (‘additional’ stamens in Salix having been variously interpreted as representing dedoublement of the medians or ‘development of the inner series’); free of one another, or coherent (by coalescence of filaments); when coherent, 1 adelphous. Androecium exclusively of fertile stamens. Stamens (1–)2–30(–60). Filaments not appendiculate. Anthers basifixed; dehiscing via longitudinal slits; tetrasporangiate. Endothecium developing fibrous thickenings. Anther epidermis persistent. Microsporogenesis simultaneous. The initial microspore tetrads tetrahedral, or isobilateral, or decussate. Anther wall initially with one middle layer, or initially with more than one middle layer; of the ‘monocot’ type. Tapetum glandular. Pollen grains aperturate, or nonaperturate (Populus); when aperturate, (2–)3(–6) aperturate; colporate (colporoidate); 2-celled.
Gynoecium 2(–4) carpelled. The pistil 1 celled. Gynoecium syncarpous; synovarious, or synovarious to synstylovarious (with styles partially fused), or synstylovarious; superior. Ovary 1 locular. Gynoecium when 2-carpelled (i.e. usually), transverse. Stigmas 2–4; dorsal to the carpels, or commissural. Placentation basal, or parietal. Ovules in the single cavity 4–40(–50) (generally described as ‘(2-) several to many’, but most Salix species have fewer than 20 per carpel, some have ‘few’, and some Salix species and Chosenia arbutifolia consistently exhibit only 2 ovules per carpel); funicled; ascending; arillate (the aril becoming silky-hairy and surrounding the seed); anatropous; unitegmic, or bitegmic (when the inner integument is weakly developed); crassinucellate. Embryo-sac development Polygonum-type. Polar nuclei fusing prior to fertilization. Antipodal cells formed; 3; not proliferating; ephemeral. Synergids with filiform apparatus. Endosperm formation nuclear. Embryogeny onagrad, or asterad.
Fruit non-fleshy; dehiscent; a capsule. Capsules 2–4 valvular. Seeds scantily endospermic, or non-endospermic. Endosperm when present, oily. Cotyledons 2; plano-convex. Embryo chlorophyllous (2/8); straight.
Physiology, biochemistry. Cyanogenic (?), or not cyanogenic. Alkaloids absent (usually), or present. Iridoids not detected. Proanthocyanidins present, or absent; when present, cyanidin, or cyanidin and delphinidin. Flavonols present; kaempferol and quercetin. Ellagic acid absent (2 genera, 8 species). Arbutin absent. Saponins/sapogenins absent. Aluminium accumulation not found. Sugars transported as sucrose, or as oligosaccharides + sucrose, or as sugar alcohols + oligosaccharides + sucrose (but sucrose always predominating, in 12 Populus and Salix species). C3. C3 physiology recorded directly in Populus, Salix. Anatomy non-C4 type (Populus, Salix).
Geography, cytology. Frigid zone and temperate. Absent from Australasia and New Guinea, otherwise cosmopolitan, but mostly temperate Northern hemisphere. X = 11, 12, 19.
Taxonomy.Subclass Dicotyledonae; Crassinucelli. Dahlgren’s Superorder Violiflorae; Salicales. Cronquist’s Subclass Dilleniidae; Salicales. APG 3 core angiosperms; core eudicot; Superorder Rosanae; fabid; Order Malpighiales.
Species 350. Genera 4; Salix, Populus, Chosenia.
General remarks. This description corrected and extended via comments from George Argus (1998). Rendle (1959) discusses ‘classical’ interpretations of catkin and floral morphology.
Economic uses, etc. Many are cultivated as ornamental trees and shrubs, and Salix contributes timber (notably for cricket bats) and withy twigs for basketry. Willows are widely planted for erosion control, land reclamation and as biomass for energy.