SUBGLACIAL CLIMATIC CONDITIONS 249 



in Polar countries, and on high mountains are not lower than in parts of 

 Siberia and Canada where forest occurs. Of far greater significance 

 to vegetation are the temperatures during summer and the length of the 

 vegetative season. 



Wind. Movements of the air are much stronger on mountains than 

 in plains, and especially above altitudes at which a mountain begins 

 to divide into its separate peaks. The wind is also very strong on arctic 

 coasts during summer. Wind has an intense drying action on plants, 

 and even when these grow with their roots in water the coldness of this 

 renders it difficult to replace the water lost by transpiration : the plants 

 must guard themselves against desiccation. 



Moisture. A sufficiency of moisture is found periodically during the 

 vegetative season both in air and soil. Rain and mist may abound, 

 and much snow may fall ; and water derived from melting snow may 

 moisten the soil perhaps throughout the whole vegetative season. Never- 

 theless the soil is cold and the activity of the roots is depressed a circum- 

 stance that is of the most profound oecological import to the plant. In 

 Polar countries the relative humidity of the air during summer is high 

 in most places, the number of days on which atmospheric precipitations 

 fall is very small, but mists are frequent. On the Polaris Expedition it 

 was found that the mean relative humidity during summer was 75 per 

 cent., but such a low figure is exceptional : in Lady Franklin Bay, 

 however, it was 81 per cent. In winter in high latitudes great dryness 

 of air prevails. 



On mountains the rainfall increases with the altitude up to a certain 

 height, and decreases above this ; the height in question varies according 

 to situation and season. The zone of greatest rainfall is the inferior 

 limit of the cloudy belt. Higher up, the rainfall decreases because as 

 the temperature sinks there is a diminution in the absolute amount of 

 aqueous vapour in the atmosphere ; yet the frequency of rainfall is not 

 lessened. Where the summits of high mountains extend into the region 

 of the upper air-currents, there is above the cloudy belt a dry region 

 where the air is very dry, and only a slight rainfall occurs for example, 

 on Teyde Peak (Teneriffe). According to Meyen, on the Andes the wind 

 is sometimes so dry that one's skin is ruptured, blood exudes, and one 

 can travel only in woollen clothes. Evaporation on mountains is always 

 greater than in the plains also because of the lower atmospheric pressure. 

 Vertical currents of air among mountains are always of great import. 

 Every ascending current of air brings with it a high atmospheric humidity, 

 mist or rain ; every descending current brings with it great atmospheric 

 aridity. Accordingly, aridity and saturation of the air may alternate 

 very rapidly. The air on high mountains may become suddenly very 

 dry, and even extremely so, on account of rarefaction of the air and 

 intense sunlight. Thus, both air and soil may be very dry periodically, 

 and for this reason the vegetation must be xerophytic, even where the 

 dry periods last for only a few hours. Vegetation on high alps, according 

 to Kerner, may at times be so soaking with moisture that water can be 

 squeezed out of the tufts of moss, but a few hours later, after an east 

 or south wind, it may be so dry as to crackle as one steps on it. 



Duration of daylight. Vital activity commences at a time when 



