1921] Smiley: Flora of the Sierra Nevada of California 17 



as passes, as Donner Pass, Tioga Pass. These places where the con- 

 tinuity of the summit region is interrupted are numerous but to only 

 a few can much significance he reasonably ascribed as barriers to plant 

 invasion from the north. The first depression which seems significant 

 is that through which the railroad passes from Sacramento to Truckee 

 (Donner Pass, 7,000 feet) ; a number of forms present in the northern 

 Sierra do not appear to the southward. The second of these possibly 

 significant depressions occurs about 120 miles to the southeast, inter- 

 secting the summit east of Yosemite Valley; Tioga Pass, 9,941 feet, 

 breaks the continuity' of the arctic-alpine life-zone for a distance 

 of about three miles. The last gap reasonably to be considered as 

 effective in this connection is some 25 miles southeast of Tioga Pass. 

 This last pass has not been visited by me but Professor J. N. Le Conte 

 describes^^ the High Sierra breaking down completely at Mammoth 

 Pass (9,350 feet), where the crest consists of rolling hills and the 

 forest belt crosses the range for a space of 20 miles. 



CLIMATOLOGY 



CLIMATE OF THE SIEREA NEVADA 



The data bearing upon the climate of the higher Sierra, and 

 especially of the region included within the limits of this report, are 

 still so fragmentary that only general statements are warranted. The 

 section across the range, through which the Central Pacific Railroad 

 passes, has been longest studied and its central position permits cer- 

 tain general conclusions to be drawn concerning the climate of the 

 Sierra as a whole. In very recent years there has been an increasing 

 interest in the climate of the California mountains and numerous 

 stations of record have been established. 



The climate of the Sierra is conditioned by its northwest-southeast 

 trend across the track of the winds blowing from the Pacific. All 

 elements of its climate are effected by this geographic position. 



TEMPERATURE 



The area whose vegetation is here considered is surrounded on all 

 sides by districts of much lower altitude and quite different tempera- 

 tures. Within the high mountain region of the Sierra the similarities 

 and contrasts in temperature follow as a consequence of its position 



