THE SILVA OF COLORADO 59 
Crataegus colorado Ashe. THORN APPLE 
A shrub or small tree with thick, rough leaves; forming thickets in open gulches 
of mesas and lower foothills. 
Altitude: 5,500to 6,500 ft. Collections: near Boulder; Gregory Canyon. 
Crataegus coloradensis A. Nelson.t THORN APPLE 
Similar in appearance to Crataegus colorado and occurring in the same situations. 
Altitude: 5,500 to 6,500 ft. Collections: Bluebell Canyon; near Boulder; “lower 
Boulder Canyon” (Rydberg). 
Sorbus scopulina Greene. Mountain ASH 
Distribution in Boulder County is not known to the writer. A single specimen col- 
lected by Mr. W. W. Robbins near Tolland came from near the line between Boulder 
and Gilpin counties. 
DRUPACEAE, Pium FAmity 
Prunus americana Marsh. WILD PLUM 
Scattered along streams at canyon mouths. 
Altitude 5,900 to 8,000 ft. Collections: Bluebell Canyon; Sunshine Canyon; Flag- 
staff Mountain; ‘“‘ Boulder” (Rydberg). 
Prunus besseyi Bailey. SAND CHERRY 
On rock ridges and slopes. Apparently rare in the county. 
Altitude: about 5,000 ft. A specimen was photographed, but not collected, at 
White Rocks. 
Prunus melanocarpa (A. Nels.) Rydb. CHOKE CHERRY 
A shrub usually growing in clumps. Common in slightly moist places in foothill 
region. 
Altitude: 5,000 to 9,000 ft. Collections: Valmont Butte; near Boulder; South 
Boulder Canyon; Bluebell Canyon; lower Gregory Canyon; north of Boulder; head of 
Gregory Canyon; Sugarloaf Mountain; near Long’s Peak. 
Prunus pennsylvanica Linn. f. WILD CHERRY 
Moist hillsides and open woods, not common. 
Altitude: 6,000 to 9,000 ft. Collections: Eldora; “Eldora to Baltimore” (Rydberg). 
PAPILIONACEAE, PuLsE FAMILY 
Amorpha angustifolia (Pursh) Boynton 
Altitude: 4,900 to 5,500 ft. Collection: near Boulder. 
SPONDIACEAE, Sumac FAMILy 
Rhus glabra Linn. Sumac 
A medium-sized shrub generally growing in masses on mesa tops, canyon sides, and 
slopes. The leaves turn a handsome red in the autumn. 
Altitude: 5,500 to 7,500 ft. Collections: Bear Canyon; near Magnolia; “lower 
Boulder Canyon” (Rydberg). 
Rhus rydbergii Small. Porson Ivy 
A shrub or trailing vine common on moist hillsides and in gulches. Attaining greatest 
size at canyon mouths. Easily recognized by the three-parted dark green leaves which 
t This is probably the Crataegus occidentalis of Rydberg’s Flora of Colorado. 
