136 HANDBOOK 148, U.S. DEPARTMENT OF AGRICULTURE 



10. "Berries" (drupes) whitish, shining, appearing ribbed 

 or lined, their papery outer coats finally shed and 

 exposing waxy, fibrous inner coats surrounding hard, 

 ridged, single seeds, borne in dense erect or ascending 

 clusters (racemes) or in lax, spreading or drooping 

 clusters (panicles) in leaf-scar axils; true end buds 

 lacking; buds without bud scales, the tiny, hairy, 

 undeveloped leaves exposed; leaf scars crescent- 

 shaped; bundle traces mostly 5, arranged in a single 

 curved line; sap resinous, milky, poisonous to the 

 touch for humans; shrubs or vines often browsed by 

 cattle and sheep. 



poison-ivy and poison-oak (Toxicodendron spp.). 

 15. Stems simple or erect-branched, sometimes clam- 

 bering but not vinelike, oft en connected underground ; 

 leafstalks persisting awhile after their 3 leaflets have 

 fallen; "berries" in dense, erect or ascending clus- 

 ters; shrubs to ,3 or 4 feet high ; in sunny but rather 

 moist sites, often common locally, e. Oreg., e. Wash. 

 western poison-ivy (Toxicodendron rydbergii). 31 

 15. Stems often vinelike, climbing by aerial rootlets; 

 leafstalks not persisting after leaflets fall; "berries" 

 in loose, drooping clusters; slender-branched, often 

 tall shrubs; dry woods, roadsides, mostly w. of Cas- 

 cades but coming through Columbia River Gorge to 

 e. of the Dalles on both the Oregon and Washington 

 sides of the river. 



Pacific poison-oak (Toxicodendron diver silobum). 

 2. Seed-head or fruit clusters not persistent and distinctive as 

 above. [Alternate 2, p. 126.] 

 16. Buds undeveloped or tiny and hard to see. 



17. Twigs densely white- woolly-hairy, at least in intercon- 

 nected ridges or lines and around buds; seed-head bracts 

 4, in a single row, a few persisting until late fall; leaf- 

 cluster scars resembling very short spurs; low, strong- 

 scented shrubs of dry open sites; poisonous to sheep, 

 especially in spring just before flowering and when browsed 

 together with sagebrush. 



horsebrushes (Tetradymia spp.). 

 18. Branching rather rigid, broomlike; woolly-hairiness 

 close, evenly matted, thinning with age especially below 

 the tiny, spurlike leaf-cluster scars; shrubs with rough, 

 shreddy bark, not so poisonous as littleleaf horsebrush; 

 dry scablands or sagebrush areas, e. Oreg., e. Wash.; 

 type locality along the Columbia River. 



gray horsebrush (Tetradymia canescens). 

 18. Branching slender, spreading; woolly-hairiness in fluffy, 

 interconnected lines or ridges, persistent, twigs bare or 

 nearly so between them; more poisonous than is gray 

 horsebrush; bark gray, shreddy; dry open sites, often 

 with sagebrush, central and se. Oreg., not reported from 

 Wash_ .littleleaf horsebrush (Tetradymia glabrata). 



31 Some authors reduce Toxicodendron rydbergii to synonymy under T. radicans. 



