157] FLORA OF BOULDER, COLORADO 9 



the Plains, CAMPESTRES; B. The Zone of the Mesas, 

 MENSALES; C. The Zone of the Foot-hills and Mountain 

 Plateau, SUBMONTANAE; fourth, The Zone of the Lower 

 Mountain Slopes, MONTANAE; fifth, The Zone of the Sub- 

 alpine Mountain Slopes, SUBALPESTRES; sixth, The 

 Zone of the Alpine Summits, ALPESTRES. Of these the 

 Plains Flora, the Foot-hill Flora, the Montane Flora, the Sub- 

 alpine Flora, and the Alpine Flora are primary, while that of 

 the Mesas is a transition from the Flora of the Plains to the 

 Flora of the Foot-hills. The Alpine corresponds to the Arctic 

 Circumpolar vegetation, the Subalpine to the Hudsonian, the 

 Montane to the Canadian, the Foot-hill and the Mesa to the 

 Upper Transition, and that of the Plains to the Lower Transi- 

 tion with some Upper Sonoran forms. 



A. CAMFESTBES 



The plains are not so arid about Boulder as they are far- 

 ther east. In fact after riding for hundreds of miles through 

 a desert of dried up grass, it is with a feeling of inutterable 

 joy that one sees this narrow ribbon of green from six to 

 twelve miles wide at the foot of the mountains. This green- 

 ness and freshness is due mainly to two causes : First, this 

 strip receives more rain than does the rest of the Great Plains. 

 The clouds do not quite rain out before reaching the plains. 

 These rains are, however, capricious. The clouds are narrow. 

 The southern part of Boulder may receive a thorough drench- 

 ing, the northern part may not have a drop. One Sunday 

 there was a cloud-burst in Sunshine Cafion, farms and bridges 

 were washed away ; from three to five feet of water came dash- 

 ing through the main street of Boulder, while it scarcely 

 sprinkled where I was a half mile to the south. The second 

 cause is the abundant irrigation. 



