24 UNIVERSITY OF MISSOURI STUDIES [ 172 



7,000 feet. Below 6,500 feet there occur still many species be- 

 longing to the Great Plains ; above 7,000 feet there is a rapid 

 thinning out of species, and subalpine species become occas- 

 ional, although it is not rare for such species in cold situations 

 to go down to the 6,000 foot level. Yet at the summit of 

 Green Mountain (8,100 feet) I found the flora still consisting in 

 the main of the genuine foot-hill species. The Foot-hill Flora 

 may be gathered into four main societies : a. The wooded 

 slope (Sylvestres). b. The foot-hill meadow (Pratenses). 

 c. The foot-hill caiion (Vallicolae), d. The crevice and 

 cranny vegetation of the rocks (Rimosae). 



a. Sylvestres.* The wooded slope society consists quite 

 purely of bull pine and Douglas spruce, with now and then 

 a few trees of other species of pine, and spruce, and fir. The 

 trees stand usually at wide intervals, oftenest in rows, where 

 some fault in the rock enables them to get a secure foothold. 

 Occasionally on the north slopes, which are moister than any 

 other, the trees stand in such close formation that it is almost 

 impossible to make one's way through them. Ordinarily it is 

 the Douglas spruce that behaves in this way, since the bull 

 pine prefers a more open formation. Often, two rather dis- 



*Young (Bot. Gaz. 44. 321-352) finds the following forest associa- 

 tions about Boulder: i. Populus occidentalis — Salix fluviatilis, riparian 

 upon the plains, but extending somewhat up the canons. 2. Populus 

 angustifolia — Salix Nuttallii, riparian in the foothills. 3. Pinus scopu 

 lorum, sylvan on the dry slopes of the foothills. 4. Pinus Murrayana, 

 sylvan on the dry mountain sides. 5. Apinus flexilis, dry mountain 

 slopes up to timber line. 6. Pseudotsuga — Picea Engelmanni, lower 

 canons (submontane and montane). 7. Picea Engelmanni — Abies 

 lasiocarpa, upper canons (high montane and subalpine to timber line). 

 S. Aspen society, throughout (north slopes at low altitudes, all slopes 

 higher altitudes). 



