PERIODIC PHENOMENA OF VEGETABLE LIFE. 



69 



mencement of winter, undergoes considerable changes according 

 to altitude. The comparison of this period in similar climates 

 will afford interesting results as to the degree of development 

 which vegetation may reach during this period. In the north, the 

 duration of vegetation is also very short ; at St. Petersburg, for 

 instance, it comprises, according to Giisebach, only four mouths 

 and ten days.* Yet the high maxima of temperature during this 

 time, and the increased excitement, from the long duration of 

 daylight, occasions a much more active development and a greater 

 richness of vegetation than might have been expected from the 

 short duration of the period of vegetation, when comparing these 

 relations with what takes place in the Alps. 



DURATION OF THE PERIODS OF VEGETATION. 



The mean difference is twenty-nine days for every thousand 

 feet, but it would appear from the above figures that the diminu- 

 tion is less rapid at lower elevations than in higher localities. 

 Above the line of snow, and especially towards the furthest limits 

 of phanerogamic vegetation (above 10,000 feet), the period of 

 vegetation is still further shortened ; this period is, on an average, 

 not much more than a month in localities not much exposed to 

 the sun, yet still accessible to plants, and is chiefly confined to 

 the month of August. During this short period, there occur, not 

 unfrequently, very considerable changes of temperature and great 

 falls to which the last phanerogamic plants appear to be but 

 little sensitive. These plants continue to flower when the noctur- 

 nal temperature sinks very low, even below the freezing point, 

 and when the surrounding rocks, as well as the leaves and flowers 

 of the plants, are covered with rime. After very snowy winters 

 and duriug cool summers, it happens sometimes that the last 

 phanerogamic plants remain completely covered with snow the 



* On the influence of climate on the limitation of natural floras. 

 Linntm, vol. xii., p. 194. 



