VEGETATION OF CONGIiOMERATE ROCKS 387 



most xerophytic of the mosses and is really a relict of an earlier 

 stage; Anomodon minor seems to be the most mesophytic of the 

 mosses here represented, and indicates an advance to more 

 mesophytic conditions. It is usually found in more sheltered 

 spots or on gentler slopes as a constituent of the ever thickening 

 moss mat. 



On flat ledges and in sheltered and shaded spots, the mosses 

 of the first stage are replaced by more mesophytic kinds. 



A second moss stage, much more mesophytic than the first, 

 becomes e\ddent. This is characterized by a very dense and 

 deep moss mat, in which Mnium cuspidatum is the facies, and 

 Rhodohryum roseum, Catharinea crispa, Entodon seductrix, Brachy- 

 thecium salebrosum, Fissidens incurvus and the mosses of the pre- 

 vious stage, except Grimmia, are secondary species. The foliose 

 lichen, Peltigera apthosa, and the liverwort, Conocephalum coni- 

 cum, are important locally. The mosses of this stage vary in 

 importance at different periods of development and in different 

 parts. Mnium — always characteristic of this community — 

 becomes increasingly important with the development of the 

 succession. The series from Anomodon as the dominant plant, 

 to Mnium as the dominant is well marked in a number of places. 



Beneath the moss mat of this stage there is being accumulated 

 h thick layer of humus, preparing the way for the advent of the 

 herbaceous plants which follow. 



3. Herhaceous stages. Herbaceous plants begin to appear as 

 soon as there is some soil accumulated in the irregularities of 

 the surface. The usual distinction between crevice and sur- 

 face plants is scarcely applicable here. The first herbaceous 

 plants grow in the shallow soil of the pockets, which unlike 

 crevices, do not permit of deep rooting. 



With the accumulation of soil in the pockets, and the growth 

 of the moss mat, there begins an open herbaceous stage, which 

 on sunny south rock slopes may telescope the second moss 

 stage. The first herbaceous plants of the pockets appear in the 

 foliose lichen stage (fig. 2). The pioneer herbs are Aquilegia 

 canadensis (wild columbine), Cystopteris fragilis, Woodsia obtusa, 

 and Poa compressa. As the mosses spread over the surface and 



