Perideridia howellii (Coult. & Rose) Mathias

Perideridia howellii (Coult. & Rose) Mathias, Brittonia 2 : 244. 1936.
Carum Howellii Coult. & Rose, Rev. N. Am. Umbell. 129. 1888. Taenia plemum Howellii Coult. & Rose, Bot. Gaz. 14: 284. 1889. Alaenia Howellii Greene, Pittonia 1: 274. 1889.
Stout, from a fascicle of fibrous or slightly thickened roots, 6-12 dm. high; leaves oblong to ovate in general outline, excluding the petioles 1-3 dm. long, 1-2-pinnate, the leaflets ovate to ovate-lanceolate, 2-4 cm. long, 5-25 mm. broad, entire to serrate-lobed ; petioles 6-25 cm. long; peduncles slender, 5-15.5 cm. long; involucre of several linear to linear-spatulate, acuminate or acute, scarious-margined, reflexed bracts, 10-20 mm. long; involucel of several bractlets like the bracts, 3-6 mm. long, often longer than the flowers but shorter than the fruit; rays usually 20-25, 3-6 cm. long; pedicels 4-8 mm. long; styles divergent, less than 0.5 mm. long; stylopodium conic; fruit oblong, 3-6 mm. long, 2.5-4 mm. broad; oil-tubes solitary in the intervals, 2 on the commissure; seed sulcate under the tubes, the face plane to concave.
Type locality: Wet places. Grants Pass. Josephine County, Oregon, Thomas Howell 710. Distribution: Southern Oregon and northern California (Cusick 2957, Heller 11,678).

Perideridia howellii is a species of flowering plant in the family Apiaceae known by the common name Howell's yampah. It is native to the mountains of Oregon and northern California, where it grows in moist soils, often near streams and rivers. It is a perennial herb which may exceed 1.5 metres in maximum height, its slender, erect stem growing from a cluster of many narrow fibrous roots measuring up to 15 centimetres long. Leaves near the base of the plant have lance-shaped blades up to half a metre long which are divided into many leaflets made up of toothed, oval segments. Leaves higher on the plant are similar, but generally smaller. The inflorescence is a compound umbel of many spherical clusters of small white flowers. These yield ribbed, oblong-shaped fruits each about half a centimetre long.

Perideridia howellii là một loài thực vật có hoa trong họ Hoa tán. Loài này được (J.M. Coult. & Rose) Mathias mô tả khoa học đầu tiên năm 1936.