52 University of California Publications in Botany [VOL. 9 



melting in summer till the season is well advanced, with consequent 

 local cooling of air and soil in their neighborhood. In the Sierra this 

 effect is much less significant than in mountains having perennial 

 snow fields of large extent, such as the Cascades or the Rockies of 

 British Columbia. Such residual drifts in the Sierra do, however, 

 cause a characteristic assemblage of plants to grow about their 

 margins comparable to the "Schneefleckflora" of European plant 

 geographers. 59 



Summarizing what has been stated with regard to the factors 

 modifying zonal limits, we note : 



(1) That life-zones are not to be considered as vegetative belts 

 bounded above and below by planes passed through the range. 



(2) That zone boundaries are extremely sinuous or even disrupted 

 and are determined by topography. 



(3) That at the same altitude on opposite flanks unlike zones will 

 be found in the lower mountains, the degree of unlikeness diminishing 

 as approach is made to the summit region 



(4) That within small areas in the higher mountains the vegeta- 

 tion will show, on the average, greater commingling of distinct forms 

 in the valleys, with purer types of plant communities upon the ridges 

 and summits. 



(5) That the temperature difference existing between localities 

 with distinct assemblages of plants are often so small that the only 

 conclusion possible, if temperature difference be admitted as the 

 controlling factor, is that plants are subject to temperature control 

 within much narrower limits than ordinarily supposed. 



The lower boundary of the Canadian life-zone has been stated to 

 conform in the central Sierra approximately to the 6,500-foot contour 

 for the west slope, descending at the north end of the range and higher 

 in the mountains of Tulare County; also that it is uniformly higher 

 on the east slope of the range, where subject to the influence of the 

 desert, than on the seaward flank. At the north end of the range in 

 Plumas County on the western crest (Bucks Mountain to Sierra 

 Buttes), the elevation of the line separating the Transition and the 

 Canadian zones may not exceed 6,000 feet and in places may become 

 depressed by as much as 300 to 400 feet below that altitude. The 

 boundary rises to the southward attaining an average altitude of 

 approximately 6,300 feet in the country about Sierra Buttes. On the 

 middle and eastern north Sierran ridges, the line is higher, owing to the 

 diminished rainfall ; east of Mohawk Valley only the highest summits, 



