Cascara False Buckthorn
Frangula purshiana (DC.) J. G. Cooper

Collected near Kamiah in Idaho Co., Idaho, on 29 May 1806. (Source)

The Cascara false buckthorn is an spreading shrub or tree growing up to 33 feet.  It is in leaf all year with prominently veined, 2-4 inches,  long leaves.  The flowers are greenish yellow, with 8-40 flowered staled flower clusters.  The fruits are reddish to bluish-black berry like drupes 1.4 to 3/8 inch long.It is flowers from May to June, and the seeds ripen in October. The plant prefers light (sandy), medium (loamy) and heavy (clay) soils. The plant prefers acid, neutral and basic (alkaline) soils. It can grow in semi-shade (light woodland) or no shade. It requires moist soil.  It is found in wooded foothills of British Columbia, Idaho, and Montana. 

Food Uses
The fruit may by used raw or cooked. A thin, rather juicy flesh. It is sometimes eaten.. There is some debate as to whether the fruit is edible or slightly toxic[226]. The fruit is about 10mm in diameter and contains 2 - 3 small seeds. An extract of the bark, with the bitterness removed (by drying?) is a common flavoring for soft drinks, baked goods and ice cream.

Medicinal Uses
 Cascara Sagrada, also called Sacred Bark and Chittem Bark, is an herb derived from the year old bark of Rhamnus purshiana. Native American Indians commonly used Cascara Sagrada to treat constipation and upset stomachs. Cascara sagrada is widely used as a gentle laxative that restores tone to the bowel muscles and thus makes repeated doses unnecessary,. It is often sold in chemists etc.. The bark is used, this is harvested on a commercial basis from wild trees and plantations in western N. America. It should be harvested in the autumn or spring at least 12 months before it is used medicinally, in order to allow the more violent purgative effect to be mollified with age.. Three year old bark is considered to be the best age. It is considered suitable for delicate and elderly persons and is very useful in cases of chronic constipation..

The bark also has tonic properties, promoting gastric digestion and appetite. As well as its uses as a laxative, it is taken internally in the treatment of digestive complaints, hemorrhoids, liver problems and jaundice. This remedy should be used with caution since in excess it causes vomiting and diarrhoea. It should not be prescribed for pregnant or lactating women, or patients with intestinal obstruction. An infusion of the bark is sometimes painted over finger nails in the hope that the bitter taste will deter the person from biting their nails. 
(Source)