INFLUENCE OF HEAT ON CONFIGURATION AND DISTRIBUTION OF PLANTS. 525 



from the mountain ridge deeper into the valley. The Fohn-wind in the Alps often 

 appears on the heights as only a slight breeze, but accelerates its velocity as it enters 

 the valley, and when it arrives there may be as destructive as a hurricane. There- 

 fore if the woody plants on the slopes of high mountains were unable to exhibit 

 erect growth on account of storms, then the neighbouring valleys must also be 

 deprived of upright trees, which, however, is known not to be the case. 



The clinging of woody plants to the ground in high Alpine regions must not be 

 regarded either as an adaptation to snow pressure or to storms; it is due rather to 

 the fact that in the high Alps the ground is relatively much warmer than the air, 

 and that plants lying on the soil profit by this higher temperature. I have ascer- 

 tained through numerous observations at different heights in the Central Tyrolese 

 Alps that the mean temperature of the soil exceeds that of the air by the following 

 amounts : — 



At a height of 1000 metres, about 1-5°C. 



„ 1300 „ 1-7° C. 



„ 1600 „ 2-4° C. 



„ 1900 „ 3-0° C. 



„ 2200 „ 3-6° C. 



Thus the soil, in comparison with the air, becomes warmer the higher one 

 ascends the mountain. Everywhere the earth absorbs the sun's rays to a much 

 greater degree than the air does; but that the excess of the heat of the soil above 

 that of the air increases so remarkably with the increasing altitude, is due to the 

 fact that the intensity of the sun's rays increases as we ascend. 



This is further explained by the fact that the layers of air which absorb the 

 sun's rays are less dense the greater the elevation above the sea-level, or, to use a 

 current expression, that the air is thinner on the heights than in the valleys. As is 

 well known, the water vapour of the air also absorbs the sun's rays, and since this 

 aqueous vapour diminishes rapidly with the height, as might be concluded from the 

 lessening of the pressure, the intensity of the sun's rays consequently increases with 

 the increasing altitude. It has been estimated that the force of the sun's rays on 

 the top of Mont Blanc (4810 metres) is 26 per cent greater than at the level of 

 Paris, and that at an altitude of 2600 metres the chemical activity of the sun's rays 

 is 11 per cent greater than at the sea-level. Everything which is benefited by the 

 sun has, in consequence, a relatively striking appearance in the higher regions of 

 the mountains, and the illuminated soil especially exhibits a temperature of sur- 

 prising height. On the Pic du Midi in the Pyrenees (2885 metres) the temperature 

 of the illuminated soil rose on a clear September day to 33'8° C, while the air onlj 

 registered 10"1° C, and in point of fact the temperature of the soil on this summit 

 was almost twice as great as at the Bagneres situated 2326 metres below. On the 

 Diavoiezza (Switzerland) the black bulb thermometer registered 59'6° in the sun, 

 and at the same time in the shade a temperature of 6'0°. In the Himalayas the 

 blackened thermometer at a height of over 3000 metres showed in the sun 40°- 50° 

 above the temperature of the shade, and once stood at 55'5° while the temperature 

 on the snow, in the shade close by, amounted to only — 5 "6°. In Leh (Kashmir) at 



