is in imminent danger of being dried up. To this danger 

 the plants of high altitudes respond in various ways. In 

 the first place, their stunted character not only diminishes 

 the area of leaf surface for the exhalation of watery vapour 

 and protects them to some extent from strong winds, but it 

 also places them in the lowest stratum of air which is most 

 likely to assume the temperature of the soil. The felted, 

 cushion-like habit (Plate 9) of so many Alpine plants, and 

 the rosette-like arrangement of their leaves (Plate 19), are 

 also of advantage to the plants by exposing less leaf surface 

 directly to the air. The succulent leaves of plants like the 

 house-leeks (Plates 18 and 19), and the hairy covering of 

 the Cat's-foot (Plate 23), Edelweiss, and others, seem also 

 designed to counteract the tendency to too rapid desicca- 

 tion, and the tough leathery character of many leaves 

 (Plate 7), and the thickened cuticle of others (Plate 

 30), are clearly of like value. The relatively increased 

 thickness of the leaves of many Alpine plants is also, 

 no doubt, due to the intensity of the sunlight, for direct 

 experiments have shown that increased illumination has this 

 effect. Their usual dark green colour is thought to result 

 from an excess of the green pigment by which plants are able 

 to extract the carbon required for their growth from the air. 

 The rarefaction of the air and shortness of the season of 

 growth at high altitudes, as well as the low stature of most 

 Alpine plants, make it essential that assimilation should take 

 place as rapidly as possible when conditions are favourable for it. 



xviii 



