Salix hastata
Salix ×fragilis
halberd willow
brittle willow, crack willow, hybrid white willow
stipules foliaceous, apex acute to acuminate;
petiole convex to flat, or shallowly grooved adaxially, 2–6(–9) mm, pilose or villous adaxially;
largest medial blade narrowly to broadly elliptic or narrowly ovate to ovate, 25–92 × 10–45 mm, 1.5–2.6(–3.4) times as long as wide, base convex, cuneate, or rounded, margins slightly revolute or flat, shallowly serrulate or entire, apex acuminate, acute, or convex, abaxial surface glaucous, sparsely pubescent, hairs wavy, adaxial dull to slightly glossy, pilose, sparsely pubescent or glabrous, midrib hairy, (hairs white and ferruginous);
proximal blade margins entire or finely serrulate;
juvenile blade sometimes reddish, sparsely pubescent abaxially, hairs white, sometimes also ferruginous.
adaxial nectary oblong or square, 0.3–0.7 mm;
filaments distinct or basally connate, glabrous;
anthers purple turning yellow, 0.4–0.6 mm.
adaxial nectary square or obovate, 0.3–0.6 mm, usually shorter than stipe;
stipe 0.4–1.2 mm;
ovary pyriform, glabrous, beak gradually tapering to styles;
ovules 12–22 per ovary;
styles connate (sometimes distinct 1/2 their lengths), 0.2–0.5 mm;
stigmas flat, abaxially non-papillate with rounded tip, or broadly cylindrical, or 2 plump lobes, 0.2–0.32–0.44 mm.
3.2–8 mm.
flowering as leaves emerge; staminate slender, stout, or subglobose, 14.5–34.5 × 8–12 mm, flowering branchlet 1–7 mm; pistillate moderately densely or loosely flowered, slender or stout, 21–59 × 6–16 mm, flowering branchlet 1.5–9 mm;
floral bract brown or bicolor, 1.2–1.8 mm, apex acute to rounded, abaxially glabrate to hairy, hairs straight to wavy.
×fragilis Linnaeus: The hybrid white willow, S. alba Linnaeus × S. euxina I.
, a European introduction, is the most commonly cultivated and naturalized tree-willow in the flora area.
is characterized by: trees, 3–20 m, stems erect or drooping;
branches highly brittle at base;
petioles with spherical or foliaceous glands distally, pilose or villous adaxially;
largest medial leaf blade amphistomatous, very narrowly elliptic or narrowly elliptic, margins uniformly serrate or serrulate, abaxial surface glaucous, both surfaces sparsely long-silky to glabrescent, adaxial surface slightly glossy or dull;
juvenile leaves at first densely long-silky soon glabrous; pistillate bract deciduous after flowering;
stamens 2;
anthers yellow; pistillate adaxial nectary shorter than or equal to stipe;
stipe 0.3–0.5 mm;
ovary pyriform, glabrous;
ovules 6–12 per ovary;
styles 0.4–1 mm;
capsules 4.5–6 mm; 2n = 57, 76.
= 38.
Salix hastata
Salix ×fragilis
Salix hastata is characterized by branches pilose with short, curved hairs; juvenile and mature leaves with white or ferruginous hairs adaxially, glaucous or not abaxially; and ovaries often reddish. See 61. S. barclayi and 64. S. farriae.
Subspecies subintegrifolia, characterized by entire or subentire leaf margins, is recognized in Flora Nordica as the major northern Eurasian race of Salix hastata, and is described as occurring across Russia into northwestern North America (B. Jonsell and T. Karlsson 2000+, vol. 1). Because leaf marginal toothing is highly variable throughout the species range (E. Hultén 1967), the subspecies is not recognized here or in Russia (A. K. Skvortsov 1999).
Hybrids:
Salix hastata forms natural hybrids with S. barclayi.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Individual trees can persist for years by trunk suckering and spread vegetatively by shoot fragmentation along stream margins, shingle and sand beaches, sedge meadows, hardwood forests, and sand pits.
A study of Salix ×fragilis in Colorado (as S. ×rubens) showed that 2172 of 2175 trees were pistillate. Occasionally seed was set, possibly fertilized by S. alba (P. B. Shafroth et al. 1994). There are at least five clones of S. ×fragilis (as S. ×rubens) in cultivation (T. Berg in B. Jonsell and T. Karlsson 2000+, vol. 1); the pistillate are sterile but the staminate produce viable pollen. The hybrid plants are often misidentified as S. “fragilis” or as S. nigra. In the flora area, reproduction of the hybrid seems to be mainly by stem fragmentation.
Prior to the lectotypification of Salix fragilis Linnaeus and the description of S. euxina (I. V. Belyaeva 2009), the name S. “fragilis” was often inadvertently used for both the pure species and for its hybrids with S. alba. Thus all herbarium specimens under the names “fragilis” and “×rubens” need to be revised.
Salix ×fragilis can be separated from S. euxina by having branches and branchlets hairy or glabrescent in age versus glabrous; leaf blades not glaucous abaxially versus glaucous; leaves amphistomatous versus hypostomatous or with stomata only along veins and at apex; and pistillate catkins slender and loosely flowered versus stout and moderately densely flowered.
Several molecular studies have been designed to understand the nature of this hybrid. H. Beissmann et al. (1997), using AFLP markers, were able to recognize three clusters: Salix alba, S. euxina (as S. fragilis), and S. ×fragilis (as S. ×rubens); but a study by K. De Cock et al. (2003), also using AFLP markers, was unable to resolve S. alba and S. ×fragilis (as S. ×rubens). They recommended the use of experimental hybridization to study the genesis of this hybrid. Molecular and genetic studies by L. L. Triest (2001) and coworkers concluded that in modern open agricultural situations in Belgium, hybridization was of low occurrence, and that morphologically intermediate plants were not necessarily genetically intermediate. These studies saw different facets of the question. Clearly there are three entities, S. alba, S. euxina, and their hybrid but, because S. euxina may be rare outside of cultivation, natural hybridization may not occur and the question of whether S. ×fragilis can be backcrossed with S. alba remains to be studied. The specimens used in these molecular studies require reidentification.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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