296 George E. Nichols, 



Crevice association-types. — Contemporaneously with the rock 

 surface "subsuccession" (Cooper '13, p. 118) occurs the crevice 

 "subsuccession." In the crevices, and also, to some extent, in 

 hollows of the rock surface, a soil is usually present, and this 

 enables plants to grow which are unable to secure a foothold on 

 a rock surface or to maintain themselves in such an environment. 

 The pioneer crevice vegetation may include the fruticose lichens 

 already mentioned as growing on rock surfaces. It may also 

 include various mosses, such as Ceratodon purpureus, Leuco- 

 bryum glaucum, Dicranum scopariiim and D. Bonjeanii, and 

 Polytrickiim piliferum. But more important than these, in the 

 light of subsequent events, are the ferns and seed plants. Of 

 the ferns, Pteris aquilina is the most frequent crevice form, 

 although Polypodium viilgarc often grows here, in sheltered 

 situations. Among the more important herbaceous seed plants 

 which inhabit crevices may be cited Potentilla tridentata, which 

 seldom grows anywhere else, Deschampsia flexuosa and Dan- 

 thonia spicata, Cornus canadensis, and Solidago hicolor. Of the 

 shrubby and semi-shrubby seed plants, Vaccinium pennsyl- 

 vanicnm, V. canadense, and Gatdtheria procumbens are rarely 

 absent, while Vaccinium Vitis-Idaea is especially characteristic 

 of such habitats. Almost any of the trees to be mentioned 

 presently as occurring on the heath mat may be found in 

 crevices. In a sense there may appear to be a succession of 

 growth forms in crevices, herbs preceding shrubs, etc., but suc- 

 cession of this sort, on the whole, is probably more apparent than 

 real. 



The heath association-type. — Up to a certain point, the rock 

 surface and the crevice "subsuccessions" are distinct from one 

 another. But with the formation of the lichen-moss mat over 

 the rock surface, and the gradual accumulation of soil which 

 accompanies the process, the two tend to merge into one. The 

 various seed plants, particularly the shrubs, which hitherto have 

 been largely confined to the crevices, become increasingly 

 abundant over the rock surface, and ultimately there may arise 

 what Cooper has aptly termed a "heath mat" ('13, p. 125). Here 

 the ground is still covered by a mat of fruticose lichens and 

 mosses, but these are no longer the dominant plants. As such 

 they have been superseded by ferns and seed plants, whose roots 

 tend to bind together the hitherto loose mat and to consolidate it 



