34 University of California Publications in Botany [Vol. 9 



The end of the vegetative season in autumn is less easy to define 

 since not only at that time of the year is temperature falling but, in 

 the Sierra, the water available to plants is less, due to several causes 

 — seasonal distribution of the rainfall, drainage from the slopes, and 

 lowered soil temperature with increasing difficulty of root absorption 

 (physiological dryness).** Little has yet been done to satisfactorily 

 determine when the vegetative season may be considered to close ; the 

 appearance of the snow-cover marks the appearance of winter but 

 before this, the vigor of plant life, as interpreted by growth, has 

 lessened. At present the most satisfactory date to regard as closing 

 the vegetative season is in October for, just as the spring resumption 

 of growth in the higher mountains follows a large increase in the 

 monthly mean temperature (Tahoe, May, 42.6°, June, 50.6°; Tama- 

 rack, May, 36.4°, June, 46°, July, 57.2°), so in the autumn, the marked 

 fall in temperature in October to November (Tahoe, 45.0°-36.5° ; 

 Tamarack, 43.6°-34.8°) indicates the time of change from active 

 metabolism to the nearlj^ static plant life of winter. In this connec- 

 tion it is interesting to note the concentration of effective tempera- 

 tures at the higher stations; at Tahoe, considering the vegetative 

 season to last from June to October and that the effective temperatures 

 may be gauged by the sum of the monthly mean temperatures with 

 sufficient accuracy for comparison, we find that July and August have 

 44.3 per cent of the total heat, but that in the shorter season of the 

 upper Canadian life-zone, as represented by the climate of Tamarack, 

 in the same two months is concentrated 55.3 per cent of the total. 



Within the vegetative season the temperature extremes vary con- 

 siderably; at all stations and in all months frost occurs. Tahoe, in 

 the period 1914-17, was subject to minima in July of 35, 30, 31, and 35 

 degrees, and in August of 33, 37, 30 and 35 degrees. Fordyce, though 

 the next coldest station in July and the coldest in August, has mini- 

 mum temperatures little lower than Tahoe : in the quadrennium the 

 low for July and August were 38, 34, 28 and 32, and 35, 38, 28 and 32 

 respectively. The data show that the higher stations Summit and 

 Tamarack had in these warmest months of the vegetative season dur- 

 ing these four years, minimum temperatures as follows : Summit, July, 

 35, 27, 34, 41; August, 30, 29, 33, 33; Tamarack, July, 32 (1915 not 

 given) , 37, 34 ; August, 32, 30, 40, 36. The east slope station of Bridge- 

 port in the same two months had lows of 35, 27, 34, 41, and 30, 29, 33, 

 33. Maximum temperatures vary through wider limits than minimum 

 temperatures; at Tahoe, the maximum recorded in the four years, 



