﻿igi6] SHAW— THE S ELK IRKS 493 



The alpine desert.— Here low temperatures, snow, and the 

 avalanches due to these are evidently the controlling ecological 

 factors. It is necessary to distinguish between the deserts due 

 to cold and snow, and those due to rock avalanches. The latter are 

 either absolute deserts because recurring avalanches have covered 

 the soil of the slopes with rock to such a depth that vegetation is 

 impossible, or they show isolated plants or cushions of plants 

 where rock covering chances to be shallow. These comprise very 

 few species, Gentiana prostrata, Erigeron aureus, Silene acaulis, 

 Potentilla {emarginata?), Epilobium latifolium, and Cryptogramma 

 acrostichoides being the only species found very commonly in these 

 locations. All of these rock desert plants are, of course, extremely 

 small, tough, and dry; their blossoms often impress one as being of 

 unusual size and brilliancy in contrast with the dwarfed and dull 

 green foliage and stems. Since these plants secure moisture only 

 with difficulty, and endure the strong insolation and the desiccating 

 winds of the exposed rock slides, they are to be designated without 

 question as xerophytes, a designation usually supported by such 

 structural characteristics as have just been suggested. 



The snow and cold deserts of the high altitudes are peculiarly 

 interesting. At the surface and immediately beneath the surface 

 of the snow, especially on the more extensive snow fields, many 

 thousands of plants of Sphaerella nivalis give to the snow exquisite 

 rose and lavender tints. In the Selkirks the alga never has the 

 green color which Schroeter reports that it shows on the alpine 

 snows. With Sphaerella there is frequently associated a fungus 

 which spreads its dark brown, branched mycelium over the surface 

 of the snow. Except for these two plants, the Selkirk snows seem 

 to be truly bare of plant life. 



On those high slopes where snow patches remain practically 

 throughout the summer, and where the ground between the patches 

 is free of snow during a few weeks only, a very definite succession is 

 always to be discerned. Very near the snow, surrounding the 

 melting snow patches, there is a black mud with knobby surface 

 evidently covered with fungus mycelia; farther back from the edge 

 of the snow, tiny patches of Polytrichum appear. As the clumps 

 of moss increase in size, we find it associated with small bunches 



