APPENDAGES OF THE AXOPHYTE. 45 



more fragments will roll down, and vegetation will cover the 

 slope. Vegetation appears on no soil but what is in a state of 

 rest, and when it is once established in any place, ii is not only 

 a sure indication that the soil is at rest, but a me^ns of keeping 

 it so. It is by operations of this kind, not performed in a day, 

 but in ages, that rugged peaks and abrupt precipices are 

 gradually transformed into rounded summits, gentle slopes, 

 and habitable surfaces. On precisely the same principle, the 

 sloping sides of railways are secured from disintegration and 

 destruction, by being sown with grass seeds or covered with 

 grass sods. 



The lower orders of the Cryptogamia, or flowerless plants, 

 such as lichens and mosses, appear to derive their nutriment 

 mainly from the atmosphere. Mosses appear to take in their 

 nutriment from the air by their whole expanded surface, 

 although doubtless the delicate root hairs below that surface 

 perform their part in absorption. Hence some species are only 

 found growing on the bark of trees, others on rocks and 

 boulders, whilst numerous genera cover the surface of the 

 ground. The roots of lichens, when they have any, are mere 

 holdfasts, the plant being developed wholly from the atmo- 

 sphere. Some species, however, would seem to attach them- 

 selves to stones of a calcareous, whilst others form a beautiful 

 plaiting on the surface of whins, sandstones and granites. 

 These atmospheric cryptogamia, are the first plants which 

 clothe the surface of barren rocks, and by their decay form 

 a humus or foothold for a more highly organised vegetation. 



