Trilliaceae is a family of petaloid, lilioid monocots. To paraphrase what Steven Elliott wrote of the genus Trillium in 1817, "this family is an interesting one. A whorl of leaves at the summit of a stem, supporting a single flower, it contains and conceals many species." In its most recent treatment (Farmer and Schilling 2002) the family, which exhibits an arcto-tertiary distribution, is comprised of 6 genera. Three genera have a wide distribution: Paris from Iceland to Japan, Daiswa from eastern Asia, and Trillium from North America and eastern Asia. Three remaining genera are endemic and each of them has only one species: Trillidium govanianum Kunth, with a tepaloid inflorescence, from the Himalayan Mountains; Kinugasa japonica (Franch. & Sav.) Tatew. & Suto, with petaloid sepals, from Japan; and the newly described genus Pseudotrillium with one species, Pseudotrillium rivale (S. Wats.) S. Farmer that has spotted petals, from the Pacific Northwest.
Pseudotrillium rivale, formerly Trillium rivale, was first recognized as distinct based on DNA sequence data, being neither a Trillium nor a member of the genus Paris but separate from both. Morphological characters that supported the distinct and basal position of Pseudotrillium rivale are:
Pollen was examined to see if it was Trillium-like (spherical and omniaperturate) or Paris-like (ellipsoidal, and monosulcate); it was Trillium-like. Other characters were contributed by members of Trillium-L, an internet mailing list devoted to Trillium and other woodland plants. These characters included a leaf-like cotyledon rather than the normal strap-like cotyledon of Trillium. It was also reported that once a plant reached the 3-leaf stage, there was always a flower unless the plant had been damaged by predation.
Molecular evidence also supported the recognition of the segregate genera within Paris s.l. Surprisingly, Kinugasa is more closely related to Daiswa than it is to Paris. The placement of Trillidium is still problematic. It is very closely related to the eastern Trillium undulatum, but morphologically it is more closely related to Paris, Daiswa, and Kinugasa than to Trillium. Recent work published by Dr. Fukuda indicated that Trillium undulatum, or its progenitor, may be the Trillium parent of the polyploid Trillidium (Fukuda 2001a, 2001b) . Work is ongoing to investigate the relationships between these taxa.
Current studies concern the Delostylis group of pedicellate Trillium. The name was applied by Rafinesque (1819) to refer to species with a common style and three slender stigmas. In all other species of Trillium, the stigmas are sessile upon the ovary. As defined by Rafinesque, this group is comprised of four species: Trillium persistens, T. catesbaei, T. nivale, and Trillium pusillum. Other than northern US T. nivale, all of these are species of the southern Appalachians and southeastern United States.
The new genus Pseudotrillium S.B. Farmer, a monotypic genus, with the type species Pseudotrillium rivale (S.Wats.) S.B. Farmer based on Trillium rivale S.Wats. was published in Systematic Botany 27(4): 687.
Key to the genera of Trilliaceae 1. Inflorescence composed of tepals (if outer perianth segments are green, shape and size of inner and outer segments similar); phyllotaxy trimerous ....................................... Trillidium (1 sp.) 1. Inflorescence composed of sepals and petals (shape and size of inner and outer segments dissimilar); phyllotaxy trimerous to numerous. 2. Sepals showy, white; petals filiform (to 1[-2] mm wide) or absent ...................................... Kinugasa (1 sp.) 2. Sepals green or purplish; petals filiform to broad (0.1-6 cm wide), or absent. 3. Phyllotaxy mostly 4- to 11-merous; leaves (0.8-) 2-5 (- 7) cm wide (rarely to 60 cm with fewer leaves and height to 1m or more); petals filiform 1-2 (-3) mm wide (rarely 5-6 mm). 4. Placentation axile; seeds with partial green aril or aril absent .................................. Paris (14 sp.) 4. Placentation parietal; seeds with enclosing red or orange sarcotesta ................................. Daiswa (10 sp.) 3. Phyllotaxy mostly trimerous with leaves (0.8-) 5–15 (- 25) cm wide; petals (2-) 7–15 (-60) cm wide (if nar- rower, petals either white or pink, or plants sessile- flowered). 5. Petals generally spotted, ovate, frequently appear- ing clawed; leaves cordate to rounded, coriaceous .......................... Pseudotrillium (1 sp.) 5. Petals not spotted, from ovate to obovate; leaves ovate to obovate, herbaceous, or not coriaceous ............................... Trillium (41 sp.)