70 



JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



At a height of 1000 metres about 1°.5 G. 



1300 1°.7 0. 



1600 2°.4 C. 



„ 1900 „ 3°.0 0. 



2200 3°. 6 



The intensity of the sun's rays, combined with the rarefaction of 

 the atmosphere at high altitudes, causes the soil to absorb heat to a 

 much greater degree than does the air. Moreover, aqueous vapour in 

 the atmosphere also absorbs the sun's rays, and, as we have seen, 

 aqueous vapour in the air diminishes rapidly as we ascend, the intensity 

 of the sun's rays being consequently increased. 



Another point in connexion with the question of soil temperature 

 in the Alps is the effect which the great depth of snow has in main- 

 taining the warmth of the soil. Keener has again made several inter- 

 esting observations on this subject, and has noted some instances at 

 altitudes approximating to 6000 feet, where the snow varied from four 

 to ten feet in depth, and the soil at one foot below the surface was not 

 during the whole winter cooled below freezing point. On the other 

 hand, where at similar altitudes the snow was not more than 1 foot to 18 

 inches deep the soil was cooled to —5°. 3 0. (that is, the thermometer 

 stood at about 22°. 5 Fahrenheit, or 9°. 5 below freezing point). f Not 

 only as regards the radiation of heat is there a difference between the 

 lowland and the mountain climate, but also in the radiation of light. In 

 addition to the increased intensity of the alpine light there is also a 

 difference in quality owing to its greater richness in the blue, violet and 

 ultra-violet rays, and to its greater chemical activity. This increased 

 intensity has been observed by everyone who has any alpine experience, 

 and on the snow fields at altitudes of ten and eleven thousand feet 

 emphasizes itself to the extent of enforcing the employment of smoked 

 glass spectacles as an alternative to snow blindness. The photographic 

 plate is also greatly affected by the alpine light. In addition to the 

 reduced exposure which (relatively to sea level) has to be halved for 5000 

 feet, and again reduced by one-third for 7500 feet, the employment of 

 ortho-chromatic plates and colour screens becomes a much greater essen- 

 tial of successful photography at high altitudes on account of the in- 

 creased activity of the blue violet rays. I merely mention this in order 

 to emphasize both the intensity and quality of the alpine light. 



I come now to the consideration of the important question as to 

 how far the peculiar characteristics of alpine plants bear relation to 

 those conditions of their environment which I have endeavoured to 

 describe. 



I suppose that everyone will admit that the first feature of alpine 

 vegetation to arrest our attention is its dwarf or procumbent habit. Low- 

 growing creeping plants, such as Salix herhacea, and Dryas octopetala, 

 and cushion plants like Silene acaulis, various Saxifrages, Drabas, and 



* Kerner, Natural History of Plants, English Edition, vol. i. p. 525. 

 t Kerner, cp. cit. vol. i. p. 548. 



