Salix ×fragilis
Salix irrorata
brittle willow, crack willow, hybrid white willow
blue-stem willow, dewy-stem willow
branches (highly brittle or not at base), red-brown to violet, usually glaucous, glabrous;
branchlets yellow-brown, (strongly glaucous or not), glabrous, sparsely velvety, or tomentose.
stipules absent or rudimentary on early ones, foliaceous or rudimentary on late ones, apex acute;
petiole convex to flat adaxially, 4–14 mm, velvety adaxially;
largest medial blade lorate, narrowly oblong, narrowly elliptic, or narrowly oblanceolate, 47–115 × 8–22 mm, 3.5–7.7 times as long as wide, base cuneate or convex, margins flat to slightly revolute, entire and gland-dotted, serrulate or crenate, apex acuminate, acute, or convex, abaxial surface glaucous, glabrous, sparsely tomentose, or short-silky, hairs wavy, adaxial slightly to highly glossy, glabrous or pilose;
proximal blade margins entire or serrulate;
juvenile blade yellowish green or reddish, glabrous or sparsely villous abaxially, hairs white.
adaxial nectary narrowly oblong to oblong, 0.3–0.8 mm;
filaments connate less than to more than 1/2 their lengths;
anthers yellow or purple turning yellow, ellipsoid or shortly cylindrical, 0.4–0.7 mm.
adaxial nectary narrowly oblong to oblong, 0.3–0.7 mm;
stipe 0.4–1 mm;
ovary pyriform, beak slightly bulged below styles;
ovules 9–12 per ovary;
styles 0.3–0.9 mm;
stigmas flat, abaxially non-papillate with rounded tip, 0.15–0.28–0.6 mm.
3.5–4 mm.
×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.
flowering before or just before leaves emerge;
staminate stout or subglobose, 15–34 × 8–22 mm, flowering branchlet 0(–2) mm;
pistillate densely flowered, stout or slender, 14–43 × 7–12 mm, flowering branchlet 0–4 mm;
floral bract brown or black, 1.3–2.5 mm, apex rounded, abaxially hairy, hairs straight or wavy.
= 38.
Salix ×fragilis
Salix irrorata
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.)
Salix irrorata is very closely related to S. lasiolepis. The two are here maintained as separate species primarily because S. irrorata is a diploid and S. lasiolepis a tetraploid, and also because of their largely allopatric ranges (G. W. Argus 2007). Studies of sawflies (Euura Newman) by P. W. Price (pers. comm.) show that the same species of Euura can successfully reproduce on either willow. The only morphological character that separates the two is that in S. irrorata branchlets and branches are very strongly glaucous, whereas in S. lasiolepis they are not glaucous. Some plants have weakly glaucous stems (wax not visible except by polishing or only as sparkling crystals); this may be infraspecific variability or evidence of hybridization.
Hybrids:
Salix irrorata forms natural hybrids with S. drummondiana, S. geyeriana, and S. lasiolepis var. lasiolepis.
Salix irrorata × S. lasiolepis var. lasiolepis: This putative hybrid occurs in Arizona and New Mexico. It is characterized mainly by weakly glaucous branches.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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