Pseudotrillium is a monotypic genus of flowering plant in the family Melanthiaceae. Its sole species, Pseudotrillium rivale, is commonly known as the brook wakerobin.[5] It is endemic to the Siskiyou Mountains of southern Oregon and northern California. The Latin specific epithet rivale means “growing by streams”, with reference to a preferred habitat.[6]
Pseudotrillium rivale is a perennial, herbaceous, flowering plant that persists by means of an underground rhizome. In general appearance, it is similar to a Trillium (and at one time, it belonged to that genus). It has a whorl of three bracts (leaves) and a single trimerous flower with three sepals, three petals, two whorls of three stamens each, and three carpels (fused into a single ovary with three stigmas). It differs from Trillium in that it has spotted petals, leathery leaves with a cordate base, and a continuously elongating pedicel. At the onset of anthesis, the pedicel rises above the leaves, but once the flower is pollinated, the pedicel elongates and declines below the leaves.[7]
Pseudotrillium rivale grows up to 20 cm (7.9 in) tall. The lance-shaped leaves are up to 11 cm (4.3 in) long with leaf stalks (called petioles) 1 to 3 cm (0.4 to 1.2 in) in length. The leaves are glossy blue-green with conspicuous silvery veins. The plant has a nodding, non-fragrant flower on a pedicel 2.5 to 11 cm (1.0 to 4.3 in) long. The flower has green sepals and pink-blushed white petals up to 3 cm (1.2 in) long and 2 cm (0.8 in) wide.[8][7]
Pseudotrillium rivale was first described as Trillium rivale by American botanist Sereno Watson in 1885.[9] Its type specimen was collected in 1880 at Big Flat in the Siskiyou Mountains, thirty miles east of Crescent City, California.[10] Watson compared the new species to the eastern Trillium nivale, "which it much resembles in habit." Presumably he was referring to the tendency of the pedicel to decline below the leaves after pollination, a common habit of both species.
Based on morphology and molecular phylogenetic evidence, Trillium rivale was segregated into a monotypic genus by Susan B. Farmer in 2002.[11] For this purpose, Farmer simultaneously described the taxa Pseudotrillium and Pseudotrillium rivale.[3][2] As of March 2023[update], the name Pseudotrillium rivale (S.Watson) S.B.Farmer is widely recognized.[1][4][8][12][13]
Pseudotrillium is a member of tribe Parideae. It is sister to the remainder of Parideae, a clade that includes Paris and Trillium.[11] Based on molecular phylogenetic studies, Pseudotrillium rivale is the first diverging (basal) branch of Parideae, a result that is well supported. Historically, studies that omit this taxon have given strikingly different results, suggesting that "Pseudotrillium could hold the key to phylogenetic studies" of Parideae.[14]