''Rhamnus purshiana'' (cascara buckthorn, cascara, bearberry, and in the Chinook Jargon, chittem and chitticum; syn. ''Frangula purshiana'', ''Rhamnus purshianus'') is a species of buckthorn native to western North America from southern British Columbia south to central California, and eastward to northwestern Montana.
The dried bark of cascara has been used for centuries as a laxative, first by Native Americans in the Pacific Northwest, and then by European/U.S. colonizers. The chemicals primarily responsible for the laxative action are the hydroxyanthracene glycosides (particularly cascarosides A, B, C and D), and emodin. These act as stimulant laxatives, with the hydroanthracene glycosides stimulating peristalsis, and emodin exciting smooth muscle cells in the large intestine.
Cascara is a large shrub or small tree 4.5–10 m tall, with a trunk 20–50 cm in diameter.
The outer bark is brownish to silver-grey with light splotching (often, in part, from lichens) and the inner surface of the bark is smooth and yellowish (turning dark brown with age and/or exposure to sunlight). Cascara bark has an intensely bitter flavor that will remain in the mouth for hours, overpowering the taste buds.
The leaves are simple, deciduous, alternate, clustered near the ends of twigs. They are oval, 5–15 cm long and 2–5 cm broad with a 0.6–2 cm petiole, shiny and green on top, and a dull, paler green below; and have tiny teeth on the margins, and parallel veins.
The flowers are tiny, 4–5 mm diameter, with five greenish yellow petals, forming a cup shape. The flowers bloom in umbel-shaped clusters, on the ends of distinctive peduncles that are attached to the leaf axils. The flowering season is brief, from early to mid- spring, disappearing by early summer. The fruit is a drupe 6–10 mm diameter, bright red at first, quickly maturing deep purple or black, and containing a yellow pulp, and two or three hard, smooth, olive-green or black seeds.
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