A difficult genus of variable plants. Inflorescence have an outer spathe-like or sometimes leaf-like bract, concealing either 2 (In Sisyrinchium albidum) or 1 inner spathes. Some species have the spathes (and bracts) sessile and solitary on the stem. In others, the spathes are 2 or more per stem, and on a peduncle; the stem thus appearing branched. Unfortunately, occasional specimens with peduncled spathes occur in species in which spathes are usually sessile; similarly, sessile spathes may occur in usually peduncled species. One should always collect a fair sample of a population and be sure to include mature fruit. The flowers of our species range from violet to blue or white.
Measurements of stem width in the key always refer to the approximate middle of the stem—the widest portion. Measurements of the fused portion of the outer spathe bracts are from the end of the central portion of the stem (not the wing, which when broad may extend over the base of the spathe) to the end of the fused portion. The fused portion may tear open with rough handling and needs to be sampled carefully. Measurements of the length of the spathe are also from the end of the central or proper stem, not the wing.
1. Spathes on peduncles arising from a leaf-like bract, usually more than one, the upper portion of the stem thus appearing branched.
2. Base of plant with very prominent dense tufts of light brown, rather straight fibers, sometimes as long as 6–7 cm but very fragile and breaking back to shorter tufts resembling a worn broom; corolla less than 7 mm long; spathes 15–21 mm long, the bracts subequal, the outer one with margins fused ca. 2.5–4 mm; capsule pale brown or tan, 3–5 mm long.
2. Base of plant without dense fibers, at most with a few rather lax fibers prominent only on lower 1–3 cm of plant; corolla often over 7 mm long; spathes various; capsule dark or pale, 2.5–5 mm long.
3. Stems 1.5–2.5 mm wide; spathe 16–22 mm long, the inner spathe and outer bract subequal; pedicels (at least some of them) wing-margined basally more than half their length; capsule pale whitish tan (sometimes flushed with purple apically).
3. Stems 0.5–5.5 mm wide, if less than 2 mm then the inner spathe less than 15 mm long; pedicels merely 2-edged or slightly winged basally; capsules dark.
4. Stems (2–) 2.5–3.5 (–5.5) mm wide, with broad wings, each wider than the central portion; inner spathe 13–22 mm long, the outer bract ± longer or even leaf-like; margins of outer bract fused for (2.5–) 3.5–6 mm; stems usually very minutely denticulate on margins [30–40×]; plants nearly always turning dark green in drying.
4. Stems 0.5–2 mm wide, the wings distinctly narrower than the central portion; inner spathe 10–13 mm long, the outer scarcely longer; margins of outer bract fused for 2.5–3.5 (–4.5) mm; stems completely smooth-margined; plants drying pale green.
1. Spathes sessile or nearly so at the end of a simple stem.
5. Spathes 2, surrounded at base by an outer leaf-like involucral bract with margins not fused beyond the wing (if any) of stem.
5. Spathe 1, the outer (usually ± leaf-like) bract with the margins slightly to moderately fused at the base beyond wing of stem (except in S. campestre).
6. Margins of outer bract distinct nearly to the base, spathes greenish, very little purple tinged; stems less than 2 mm wide.
6. Margins of outer bract fused for (0.8–) 1–5 (–6) mm, spathes often strongly purple tinged or stems often more than 2 mm wide (or both).
7. Plant very slender, the stem 0.5–1.7 mm wide (usually 1 mm or less) and barely margined or narrowly winged; largest leaves at most 1.5 mm wide; capsule ca. 2.5–4 mm long.
7. Plants stout, the stem usually 1.7–3 (–3.5) mm wide (most often 2–2.5 mm), winged; largest leaves usually 2–3 mm wide; mature capsule ca. 5–7 mm long.