Flora of North America species comparison

Plants of Saxifraga mertensiana bear bulbils in the axils of basal leaves.

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

Antiphylla Haworth; Heterisia Rafinesque ex Small; Hirculus Haworth; Leptasea Haworth; Lobaria Haworth; Muscaria Haworth

Species ca. 390 (25 in the flora).

Saxifraga has had differing treatments (e.g., J. K. Small and P. A. Rydberg 1905b; H. G. A. Engler and E. Irmscher 1916; P. E. Elvander 1984; D. A. Webb and R. J. Gornall 1989). Most have been based on morphology and cytology. Relationships among species are difficult to assess. The broad Linnaean concept of the genus had been accepted by most recent authors. Molecular phylogenetic data (summarized by D. E. Soltis et al. 2001) show that Saxifraga as traditionally understood is polyphyletic and should be divided into two main groups, Saxifraga in the narrow sense and Micranthes Haworth, the treatment followed here. Other taxa, notably the North American Cascadia, must also be recognized. Cascadia differs from both Saxifraga and Micranthes in its lax, trailing stems and only cauline leaves, its carpels connate only proximally and each independently adnate to the hypanthium, and its spiny seeds; its ovules are bitegmic as in Saxifraga; ovules are unitegmic (except for sect. Merkianae) in Micranthes. In addition, Saxifraga and Micranthes differ by the following traits: leaves basal and cauline (these sometimes reduced and bractlike) in the former versus basal only (or proximally crowded) in the latter (the flowering stems being leafless); carpels usually connate more than one-half versus less than one-half, although there is variation in both genera for this trait; seeds smooth, tuberculate, or papillate versus longitudinally ribbed; and pollen striate or granular versus reticulate (L. Brouillet and R. J. Gornall 2007).

Saxifraga in the narrow sense, as adopted here, includes most sections of the traditional genus as well as a majority of the species. In North America fewer species belong to Saxifraga than to Micranthes. Eight species of Saxifraga are endemic to North America. A single introduced species has established itself in the flora area; other garden species have usually not escaped or were ephemeral. The following species are excluded from this treatment: S. cymbalaria Linnaeus (misidentification of S. sibthorpii Boissier, a garden plant, not established), S. ×geum Linnaeus and S. umbrosa Linnaeus (both misidentifications of S. hirsuta specimens), and S. hirsuta Linnaeus (not established). The report of S. stolonifera Meerburgh (syn. S. sarmentosa Linnaeus f.) from California is old; there has been no recent collection, and that species also is excluded here.

There is extensive occurrence of polyploidy and dysploidy throughout Saxifraga (P. E. Elvander 1984; D. A. Webb and R. J. Gornall 1989).

The stigmas and styles are typically two; occasional flowers on plants in some species have three or, rarely, four stigmas and styles. In addition, ovary position in some taxa can change from mostly inferior at flowering to superior in fruit. Ovary position is described at flowering. Capsules usually have two connate carpels that are distally distinct; in rare cases, carpels are distinct nearly to the base and fruits are folliclelike. Only such exceptions are indicated in descriptions, the usual case being assumed.

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