250 PSYCHROPHYTES 



the day is long from fourteen to sixteen hours on the alps of temperate 

 countries, and much longer in Polar lands. This prolonged illumination 

 decreases growth in length. 



Intensity of sunlight. The intensity of direct sunlight increases 

 with altitude,^ and is very great on high mountain-tops, owing to rare- 

 faction of the air, to its absolute dryness, and to the thinness of the 

 intervening atmospheric stratum. It is calculated that the intensity 

 of the sun's rays falling on the summit of Mont Blanc is 26 per cent, 

 greater than at Paris. On high mountain-tops the temperature in the 

 sunlight may be thirty-four centigrade degrees higher than in the shade ; 

 moreover, the temperature of the soil is much higher than that of the air. 

 On mountains the fluctuations of the temperature of the air are much 

 less than in the plains, but those of the soil are much greater ; for the 

 maximal soil-temperatures greatly exceed the maximal air-temperatures, 

 though the minimal temperatures of the soil are not much lower than 

 those of the atmosphere. Thus the excess of the mean temperature 

 of the soil over that of the air increases considerably with the altitude. 

 The sun's rays awaken shoots into activity and growth at a time when 

 the soil is still very cold. In Polar countries light is less intense and 

 temperature lower, but their action on growth is subject to less inter- 

 mission ; the distinction between the temperature by day and by night 

 becomes obscured. Intense light by day and cold by night co-operate 

 at high altitudes in retarding growth ; prolonged, though weaker, hght 

 and lower temperatures in Polar lands have the same effect. These 

 circumstances bring nanism in their train. 



The vegetative season. In regard to the vegetative season there 

 are two extreme types the arctic-temperate, and the tropical. 



The arctic-temperate type. Within the Arctic zone, as well as on 

 high mountains within the temperate zone, the vegetative season is 

 short, lasting, as a rule, only a few weeks. In Franz Josef Land only 

 one or two months enjoy a mean temperature exceeding C. In the 

 Eastern Alps at the extreme altitude reached by Spermophyta (about 

 3,300 metres) and in unfavourable spots the vegetative season lasts only 

 for one month. As regards humidity and illumination there are greater 

 differences between the arctic lowlands and the alps in temperate zones. 

 Nevertheless the general character of the vegetation and many of the 

 species of plants are identical in both these places. Common to both 

 are strong winds, the very brief vegetative season, and above all the 

 frequency of frosts during the vegetative season. 



The tropical type. This differs from the arctic-temperate in that the 

 vegetative season endures throughout the whole year. 



The subtropical intermediate type with a long vegetative season 

 stands between the two extremes. 



On tropical mountains the insolation is more intense than in higher' 

 latitudes, and consequently the maxima of the soil-temperature arej 

 much higher; and this gives to the vegetation an entirely different cha- 

 racter. 



* Cp. Wiesner, 1905 a, and other papers. 



