PAPERS ON BOTANY 127 



the more mesophytic species of Mithlcnbergia and Agropyron. 

 Among the more abundant grasses are several species of Poa, 

 Agropyron, Agrostis, Deschampsia, Phleum and Bromns, the 

 last mentioned genus being particularly abundant wherever 

 there is a contact between the forest and grassland. 



THE XERARCH SUCCESSION 



The porous condition of the glacial gravels, which form 

 the greater portion of the soils of Boulder Park and similar 

 montane areas, provide such excellent natural drainage that 

 with the recession of the mountain glaciers large areas of 

 these gravels must have been left without vegetation and with 

 surface but little more inviting to plants than the bare rock. 

 At present these gravels are seen upon the mountain slopes, the 

 ridges representing lateral and medial moraines and in the ir- 

 regularly placed terminal moraines. More level areas probably 

 represent the gravel bars of glacial streams. Upon the dry 

 suface of these soils lichens appear to have played a compara- 

 tively small part in the establishment of vegetation, probably 

 on account of the instability of the surface particles during 

 high winds. Still crustose and foliose lichens are fairly abun- 

 dant upon the larger boulders. The most important pioneer 

 plant appears to be Selaginella densa forming mats over and 

 between the coarse soil particles. It is closely followed by the 

 succulent Sedum stenopetalum and several xerophytic grasses 

 growing as crevice plants. The mat forming habits of the 

 Selaginella prepares the soil for the other pioneers, among 

 which Antennaria parvifolia, Arenaria Fendleri, Chrysopsis 

 villosa, Orthocarpus Inteus, Carex stenophylla and Commandra 

 pallida are conspicuous. The constant presence and predom- 

 inance of the first mentioned xerophyte would make it appro- 

 priate to term this pioneer vegetation the "Selaginella asso- 

 ciation." It is characterized by low growing perennial plants, 

 a large percentage of bare ground and very slow advance to- 

 wards a less xerophytic condition. This slow advance is large- 

 ly due to the low water content of the soil, due to its coarse 

 texture and to the extreme slowness of the humus accumula- 

 tion. High winds dry the dead vegetation, break it into frag- 

 ments and carrying it off, leave almost nothing to form 

 humus, while the little that may be formed is still liable to be 

 removed by the same agency in the form of dust. The finer 

 soil particles, resulting from the disintegration of the gravel, 

 are aften lost in the same manner. The mat forming ten- 

 dency most evident in the Selaginella and the habits of vegeta- 

 tive reproduction in such plants as the Carex and Commandra 



