165] FLORA OF BOULDER, COLORADO 1 7 



Monolepis Nuttalliana Iva axillaris 



Atriplex carnosa Chrysothamnus graveolens 



A. argentea C. pulcherrimus 



Dondia depressa Solidago gilvocanescens 

 Sophora sericea 



B. MENSALES* 



The Flora of the Mesas is a transitional flora ; the mesas 

 have most of the plants of the plains and in addition many of 

 the plants of the foot-hills. There are, however, a considerable 

 number of species, which are peculiar to the mesas. These 

 mesas are flat tablelands rising abruptly a hundred feet or so 

 above the plains in successive terraces. The altitude of the 

 plains in Boulder County is from 5,000 to 5,500 feet. The 

 lowest mesa, at an altitude of about 5,600 feet, has the 

 flora of the plains, but at the next mesa, at an altitude of 5,700 

 feet, the flora begins to change, and from then on to the foot 

 of the crags, 6,000 feet, the plains plants gradually tend to 

 disappear and the foot-hill flora to come in. The highest 

 mesas are so filled with waste from landslips from the crags, 

 that they may be said to be an integral part of the foot-hills. 

 And so, too, the streams have made deep canons through the 

 mesas, the flora of which is not so very unlike that of the 

 canons of the foot-hills. West of Marshall there is a high bog 

 on the mesa, but as its plants differ in no wise from the bog 

 plants of the plains, it will be dismissed with this notice. 



Six plant-societies are to be found upon the mesas : a. The 

 meadow (Pratenses), which differs little from the plains 

 meadow, although certain mountain species, such as the Mari- 



*For a detailed account of the vegetation of the mesas, see the pa- 

 pers by Dodds, Ramaley, and Robbing, Univ. of Colo. Studies, 6, 11-49. 



