188 BOTANICAL GAZETTE. 



which will be found to increase or diminish as the rain-fall is more 

 or less abundant and more or less equally distributed. 



In the territory betw^een the -ilstand 37th parallels of latitude, and 

 extending from the eastern base of the Rocky Mts. to the foot of the 

 western slope of the Sierra Nevada are three distinct belts of vegeta- 

 tion. Bep:inning at the east there is : 1. The Rocky Mountain Re- 

 gion, including, besides the main range, the Uinta and the Wahsatch, 

 and embracing Colorado and the eastern half of Utah; 2. The Ne- 

 vada Regions, extending from the western base of the Wahsatch, to 

 the eastern base of the Sierra Nevada, and embracing the western 

 half of Utah and the whole of Nevada with the exception of the 

 extreme northern and southern portions of the State; 3. The Sierra 

 Nevada Region. 



In the Rocky Mountain Region, to which in spite of its mid-con- 

 tinental position considerable moisture is attracted by the high 

 peaks which everywhere dominate it, there are 25 trees and 48 

 shrubs, in all 73 species. In the Nevada Region, where, owing to 

 its isolated position between high mountain ranges, the rain-fall is 

 small and very unequally distributed the number of species is re- 

 duced nearly one-half — to thirty-eight; ten trees and twenty-eight 

 shrubs. In the Sierra Nevada Region, to which the Pacific con- 

 tributes a large although unequally distributed, snow and rain-fall, 

 the number of species is increased to 89 ; of these 35 are trees, or 3.V 

 times more than occur in the adjoining Nevada Region, and a third 

 more than are found in the Rocky Mountain Region ; and 54 are 

 shrubs, or double the number of the Nevada Regions. 



The absence of arborescent and frutescent Lcgiuainosse from the 

 three regions, when herbaceous genera of this order are so largely 

 represented, is remarkable, especially as they abound farther south 

 in New Mexico and Arizona. In the Rocky Mt. Region there is a 

 single representative of this order, a Rohinia nearly allied to those of 

 the Eastern States; in the Nevada Region there is not a single frut- 

 escent Lcguminosa, and in the Sierra Nevada but one species, a large 

 shrub Cercis. On the contrary the number of genera of frutescent 

 Rosacese, many of them endemic and monotypic, is very large in pro- 

 portion to other Angiospernifr. In the Rocky Mt. Region there are 

 13 genera with 15 species; in the Nevada Region 7 genera with 10 

 species; in the Sierra Nevada Region 11 genera with 13 species; in 

 all. 14 genera with 28 species. In all the United States east of the 

 Mississippi River there are but 10 woody Rosaceous genera, all rep- 

 resented in our three Regions with the exception of the Southern 

 Chrysobalanus and Neviusia. 



