igoS] COOPER— ALPINE VEGETATION IN COLORADO 329 



principally of the decomposed rock material in the cracks, the other 

 developing later, but usually on new ground, in basins and very wet 

 places, and profiting from the organic material brought down from 



the snowdrifts. 



In cracks and joints of the floor we find a pioneer group consisting 

 of the following species, all but one or two of which are perennials. 



Deschampsia alpicola Rydb., Agrostis tenuiculmis Nash, Trisetum 

 subspicatum (L.) Beauv., Carex atrata L., Juncoides spicatum (L.) Kuntze, 

 Sagina Linnaei Presl, Draba crassifolia Graham, Sibbaldia procumbens L., 

 Androsace subumbellata (A. Nels.) Small, Artemisia scopulorum Gray. 



2.— Salix petrophila Rydb., Ox>Tia digyna (L.) Camptd., Saxifraga debilis 

 Engelm., Senecio Fremontii T. & G. 



The species in the first group, to which further exploration would 

 doubtless add many more, are commonly found in the dry meadows. 

 Since these meadows are all above the level of the canon floors, this 

 group of pioneers must have come from seeds blown or washed down 

 from them. There is one other possibiHty— that these plants have 

 followed the glaciers as they retreated. It seems Ukely, however, 

 that pioneers in the invasion of the lower portions of the canons were 

 plants belonging to lower altitudes and more favorable conditions; 

 since they were as well sheltered from the wind as are the pioneers of 

 today, and the water supply was far more abundant. The plants m 

 the second group are commonly found in clefts and among bare 

 talus in the higher altitudes. These species, with those of the first list, 

 constitute a group of invaders of xerophytic character from higher 

 altitudes. Thev do not occupy a ' ''--''^- "'^"" "*^ ^^^ ^^ 



defmite 



permit 



them to exist. In some measure they prepare the way for the group 



of invaders that comes from the opposite direction, but hke the hchens 



they are relativelv unimportant, and, as will be shown, the plants ttia 



come in from below to fill the depressions may be entirely independeni 



of any aid from earlier arrivals. Sometimes, however, the cracks 



containing these plants may become the starting-point for quite large 



. °- ^ ^ , . , .„j„„f oT^^iicrh to make pos- 



pos 

 and 



mats of turf, 



sible the establishment lu a Lia^r. v^x ...v- ^ -, j +>,«/> mav 



grasses, such as grow in the basins soon to be descnbed these . 

 spread out over the surface of the rock by means of rootstocks, 



