Flora of North America species comparison

Sometimes Salix interior is treated as a subspecies of S. exigua (R. D. Dorn 1998). Salix exigua and S. interior hybridize and apparently intergrade in the western Great Plains; because the area of overlap is relatively small and distinctiveness of the two taxa is not compromised by hybridization and introgression, it is best to treat them as separate species.

Leaves on sylleptic shoots are usually very densely silky. Salix interior sometimes has shoots that arise from buds on either side of the normal axillary bud. They do not seem to be directly related to the stipules because they are enclosed by the petiole. Catkins with both staminate and pistillate flowers are rare in S. interior, but a Quebec specimen had some catkins predominantly pistillate and others staminate; most were a mixture. The flowers were not teratological, but a mature capsule contained aborted ovules.

Hybrids:

Salix interior forms natural hybrids with S. exigua var. exigua. Controlled pollinations using S. interior (as S. exigua) from southern Ontario (A. Mosseler 1990) successfully produced F1 hybrids with S. bebbiana, S. discolor, S. eriocephala, and S. petiolaris. Seed production was usually relatively low, except in crosses with S. discolor. In general, F1 viability was relatively low in crosses with these members of subg. Vetrix. No seeds were produced in crosses with members of subgenera Protitea or Salix. Morphology of the hybrids usually was intermediate between the two parents, but when S. petiolaris was used as the maternal parent, the F1s more closely resembled that species. J. Salick and E. Pfeffer (1999) extended these findings to show that, although crosses between S. interior (as S. exigua) and S. eriocephala are partially sterile, their clonal growth parameters (sprouting, shoot length, and biomass production) are strong and thus permit these partially sterile hybrids to exist as successful individuals and perhaps to “... make a contribution to interspecific gene flow over time.” Of particular taxonomic interest is that, in this cross, the staminate parent has a significant influence on leaf shape, whereas in the cross S. eriocephala × S. petiolaris it is the pistillate parent that is significant for leaf shape. Relatively few hybrids resembling those produced by Mosseler have been recognized in nature, but it is possible that the unusually broadly leaved plants named S. interior var. exterior and var. wheeleri, from northern Maine, Nebraska, New York, and West Virginia, and probably elsewhere, may be hybrids. Phenological isolation may be strong enough to prevent crosses in nature (A. Mosseler and C. S. Papadopol 1989) with the earlier flowering S. eriocephala and S. petiolaris, a barrier that even an occasional period of overlap cannot breach.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Genera 50+, species ca. 1000 (4 genera, 123 species in the flora).

Taxonomic placement of the Salicaceae and the genera included in it have varied greatly. Some botanists (H. G. A. Engler and K. Prantl 1887–1915) treated it as a primitive member of the Dicotyledoneae and grouped it with other families having simple, apetalous, unisexual flowers arranged in catkins, the “Amentiferae.” At about the same time, others (C. E. Bessey 1915) took a different view, regarding the simple flowers as the result of reduction, and placed the taxa in Caryophyllales. As early as 1905, H. Hallier could see that there were similarities between Salicaceae and Flacourtiaceae; at the time, he was vigorously challenged by E. Gilg (1915). A. D. J. Meeuse (1975) summarized evidence for a close relationship between these families, including wood anatomy, phytochemistry, host-parasite relationships (including rust fungi), and morphology. He concluded that the Salicaceae could be combined with the Flacourtiaceae, “perhaps as a tribe.” A. Cronquist (1988) and R. F. Thorne (1992b) placed the Salicaceae, in a narrow sense, in Violales near Flacourtiaceae.

Molecular studies support a close relationship between Salicaceae and Flacourtiaceae in Malpighiales and show that Flacourtiaceae, in a broad sense, is paraphyletic. Based on a study of plastid rbcL DNA sequences, Salix and Populus were nested within a subset of 52 genera of Flacourtiaceae (M. W. Chase et al. 2002). Chase et al. proposed moving some genera from broadly circumscribed Flacourtiaceae to Salicaceae. Other studies, based on different gene sequences, came to the same conclusion (O. I. Nandi et al. 1998; V. Savolainen et al. 2000; K. W. Hilu et al. 2003; Angiosperm Phylogeny Group 2003). The discovery of the extinct fossil genus Pseudosalix (L. D. Boucher et al. 2003), from the Eocene Green River Formation of Utah, provided further support for placing some members of Flacourtiaceae in Salicaceae. The well-preserved Pseudosalix fossils, in which reproductive structures are directly associated with the leaves, occur intermixed with Populus fossils. The leaves are slender and have salicoid teeth, inflorescences are cymose, flowers are unisexual, pedicellate, tetrasepalous, and 3- or 4-carpellate, and seeds are comose, i.e., having characteristics intermediate between Salicaceae and Flacourtiaceae.

The presence, in both families, of salicoid teeth is often cited in support of their close relationship (W. S. Judd 1997b; O. Nandi et al. 1998; M. W. Chase et al. 2002; H. P. Wilkinson 2007). Salicoid teeth were first recognized and defined as having the tip of the medial vein (seta) of the tooth retained as a dark, but not opaque, non-deciduous spherical callosity fused to the tooth apex and were reported to occur in Salicaceae and Idesia of the Flacourtiaceae (L. J. Hickey and J. A. Wolfe 1975). Nandi et al. reported that a broad survey of angiosperm leaves showed that salicoid teeth occur outside of Flacourtiaceae and Salicaceae only in Tetracentraceae.

Isozyme and cytological evidence show that Populus and Salix are ancient polyploids (D. E. Soltis and P. S. Soltis 1990; Wang R. and Wang J. 1991). All Salix and Populus species contain salicin (R. T. Palo 1984).

The genera often included in Salicaceae, in the narrow sense, are Chosenia, Populus, Salix (A. K. Skvortsov 1999), and, sometimes, Toisusu. Molecular studies (E. Leskinen and C. Alström-Rapaport 1999; T. Azuma et al. 2000) show that Chosenia is nested within Salix. H. Ohashi (2001) treated Toisusu as Salix subg. Pleuradinea Kimura and Chosenia as Salix subg. Chosenia (Nakai) H. Ohashi.

(Discussion copyrighted by Flora of North America; reprinted with permission.)