GREENLAND BY THE POLAR SEA 



The thermometer with its ball of mercury : 



1. On a sunny, flowering tuft of saxifrage : plus 21'1° C. 



2. 2 cms. down in dry, sandy soil : plus 14' 2° C. 



3. 6 cms. down in moist, sandy soil : plus 12'5° C. 



4. 12 cms. down in moist, sandy soil : plus 7'3° C. 



July 15th, 1 a.m. — A slope 100 metres above sea-level. Plentiful vegeta- 

 tion. Calm, clear sunshine (Summer Valley). 



Temperature of the air in the shade plus 12° C. 

 The thermometer with its ball of mercury : 



1. On a sunny cluster of poppies : plus 24' 4° C. 



2. 3 cms. down in a vigorous green clump of silene : plus 24'3° C. 



3. 10 cms. down in the same clump (near its bottom) : plus H'8 C. 



4. 1 cm. down in moist soil : plus 18'5° C. 



5. 13 cms. down in moist soil : plus 14' 8° C. 



These examples show quite plainly that the plants, fortunately for them, 

 get considerably more heat than one might expect, judging from the tempera- 

 ture of the air alone. But one must not overestimate the significance of these 

 figures, as they hold good only when the air is calm. The wind naturally cools 

 considerably the surface of the soil and the vegetation, so that on a windy 

 day there will be no appreciable difference between the temperatures of the air 

 and the soil. Further, sun is necessary, so that again on dull days conditions 

 will be different. Thus the limitations must not be underestimated, though 

 it is of importance to note that a sunny slope, w T ell sheltered, always exhibits 

 the most vigorous and the earliest development of vegetation. 



In Arctic regions, where snow and ice abound, one would expect there to 

 be always sufficient moisture for the plants ; but this is not the case under all 

 circumstances. The ability of the plants to absorb water is relative to the 

 degree of warmth. Below zero the roots of the plants naturally are unable to 

 absorb water ; but also at low positive temperatures the absorption of water 

 takes place very slowly. Thus a disproportion between absorption and evapora- 

 tion from the parts above ground might easily arise when the latter are exposed 

 to strong sunshine. In this comparison one must remember that the soil in 

 the high Arctic countries is permanently frozen at a certain depth ; the summer 

 heat is able to thaw merely the upper layers. To satisfy their need of water 

 the plants are thus restricted to the absorption of moisture from this layer, 

 and from the water liberated by the melting of the snow. There may, of 

 course, be cases where this is insufficient, or where, at any rate, water can 

 merely be absorbed to so slight a degree that only certain varieties of plants 

 can manage. Because of the evaporation due to very dry air and strong sun- 

 shine, the soil, which first is laid bare when the snow evaporates, often becomes 

 very dry, as the frozen subsoil only to a slight degree permits the water to 

 rise to the surface. Wulff's diary contained many notes about this ; for 

 instance, on the 9th of June he wrote: "The snow is melting rapidly, but 

 the water evaporates quickly, so that it does not moisten the soil at all except 

 round the patches of snow "; and not until the 15th of June did he notice 

 that the melting took place to such a degree that the water could run along 

 the ground. He therefore thought it probable that, under conditions like 

 these, the plant roots must be able to absorb the water which presumably rises 

 from the frozen subsoil because of the capillary action between the particles 

 of the dry upper layers. Not until the 10th of July does he write in his 

 294 



