134 
PLANTS OF YELLOWSTONE NATIONAL PARK 
4. Leaves more or less hairy. Arnica pumila. 
5. Stems and leaves somewhat hairy. Arnica cordifolia. 
5. Stems and leaves smooth or nearly so. Arnica ventorum. 
6. Stems leafy below but nearly leafless upward. Arnica fulgens. 
6. Stems leafy throughout. 7. 
7. Plants usually growing single, not in clumps or mats. 8. 
7. Plants growing in clumps or mats. 9. 
8. Leaves more or less toothed. Heads rather large. Arnica mollis. 
8. Leaves entire. Heads smaller. Arnica foliosa. 
9. Leaves more than 2 inches long, 5 or 6 pairs on the stem. Arnica longi- 
folia. 
9. Leaves 1 or 2 inches long, 3 or 4 pairs on the stem. Arnica rydbergii. 
Arnica suibplumosa has also been reported in the park. 
Tetradymia inermis .—A low shrub with a much branched, woody 
base and numerous herbaceous 
stems that are 3 to 6 inches 
long. These young stems, as 
well as the leaves, are clothed 
Figure 111.— Arnica. Yellow. Photo- Figure 112. —Groundsel. Yellow. Copy- 
graph by Joseph S. Dixon, National right, J. E. Haynes. 
Park Service. 
with dense whitish woolly hairs. The numerous leaves are narrowly 
oblong with tapering ends, nearly sessile, and an inch or less in 
length. The heads are very compact terminal clusters of 10 to 20 
each with bractlike leaves between them. The heads have only 4 disk 
flowers and 4 bracts in the involucre. There are no ray flowers. The 
pappus is abundant and consists of fine, soft, whitish hairs. 
The genus is commonly known as horsebrush. 
