526 INFLUENCE OF HEAT ON CONFIGURATION AND DISTUIBUTION OF PLANTS. 



3517 metres, a blackened thonuoineter in vacuo rose to even 10r7°, that is 

 almost 1-i' higher than the boiling point o£ water, which at that height is only 

 88° C. 



It is readily intelligible that under such conditions growing plants which require 

 heat should nestle to the ground in high mountain regions, or more correctly, that 

 only such plants are capable of living at these heights which make the best possible 

 use of the most abundant of all sources of heat; which, so to speak, seek a warm 

 situation and settle themselves against the sunny stones and the black humus, 

 occupj'ing and covering the rocky crevices. Plants whose nature is to grow erect 

 with their woody stems in the air would not succeed well in Alpine regions, and 

 ultimately would be crowded out by species which thrive better by clinging to the 

 relatively warm soil. 



The increase in the excess of the ground temperature above that of the air with 

 the increasing altitude is also manifested in another phenomenon which, though it 

 has been frequently observed and discussed, has not always been correctly inter- 

 preted. The Ling (Calluna vulgaris), which extends from the lowlands at the foot 

 of the Alps up to high Alpine regions, blossoms on the sea-coast in Istria usually at 

 the end of July; in Alpine valleys, which lie 1000 metres above the sea-level, it 

 opens its first flowers at the end of August, and therefore the retardation of flower- 

 ing at 1000 metres amounts to something over a month. From this it might be 

 expected that the Ling would first blossom at an altitude of 2000 metres at the 

 end of September, but this is not so, for on mountains of the Central Alps at 2000 

 metres the Ling nestling on the ground is seen to be in full bloom before the 

 middle of September. By comparing the time of blossoming of high Alpine plants 

 cultivated in the botanic gardens at Lmsbruck, with the time at which the same 

 species open their flowers at various altitudes on the neighbouring mountains, it 

 was shown that the retardation of the blossoming amounted to a mean of 25 days 

 at an altitude of 500-1000 metres; an average of 18 days at 1500-2000 metres; 

 and 14 days at 2500-3000 metres; and this can only be explained by the much 

 greater intensity of the sun's raj's in the high regions, and the consequent elevation 

 of the temperature of the ground above that of the air. It must yet be mentioned, 

 for the completion of the observations here detailed, that all plants in the valleys 

 develop larger leaves and taller stems than those on lofty mountain sites. While 

 the Ling forms considerable bushes with erect branches on the coast of Istria, 

 plants of the same species on the slopes of high mountains 2000 metres above the 

 sea, appear as dwarf shrubs, whose woody stems lie on the ground and are par- 

 tially imbedded in the dark humus. 



The great contrast which vegetation on a mountain exhibits in different parts of 

 the world may be explained by the action of the sun's rays. On slopes illuminated 

 directly by the sun, the temperature of the soil, and indirectly that of the layer of 

 air in contact with it, rises far higher than on shady declivities, and in consequence 

 of this very remarkable differences may occur even in the closest proximity. Ob- 

 servations of the temperature of the soil at a depth of 80 centimetres, spread over 



