Establishment of Vegetation at the Surface of the Globe. 67 



habitations. We observe the walls covered with greenish spots, 

 which grow from humidity, and which the light and heat reduce 

 to black and tenacious spots ; these are so many byssi which have 

 essayed to establish vegetation there, as well as upon the most 

 polished statues and marbles ; it is they which impress the seal 

 of age upon our old castles and gothic edifices. Elsewhere, par- 

 ticularly upon- rough stones, we see spreading out into broad 

 plats those hchens of various colours, like the ulcerous crusts 

 which corrode the skin of animals ; they scoop out and corrode 

 the surface of rocks, and deposit in the vacuities which they have 

 formed, the portion of earth produced by their destruction. Al- 

 though in very small quantity, this earth suffices to administer 

 to the development of lichens of a higher order. Their debris, 

 added to those of the former, furnish a small layer of earth suf- 

 ficient for the existence of mosses of an inferior order, to which, 

 in like manner, succeed more vigorous species *. 



Already a turfy layer invests the tops of walls and the surface 

 of rocks ; it increases from year to year by the remains of- the 

 vegetables which it nourishes ; its pulverulent particles are re- 

 tained by the dense and tufted roots, and stems of mosses ; the 

 moisture is long preserved in it ; the layer of earth grows thick- 

 er : gramineae, and other herbaceous plants, with low stems, be- 

 gin to establish themselves, such as semperviva, drabae, saxifra- 

 ges, dandelions, some gerania, &c. The soil increases in propor- 

 tion as the generations succeed each other ; it is converted, 

 through time, into a meadow, visited by a great number of ani- 

 mals. Plants, with ligneous stems, announce that this newly 

 formed soil will quickly receive larger vegetables, the multipli- 



* Those who have not directed their attention to the study of nature, will, per- 

 haps, be very much astonished to be told, that all those black or greenish spots 

 which invest the surface of statues and walls exposed to humidity, are true plants. 

 These plats are formed by a bys&us, to which Linnaeus has given the name of Bys. 

 eus antiquitatis. Stones which are constantly shaded and moist are covered with 

 another byssus, of a beautiful deep green ; it is the Byssus velutina^ L. 



The lichens, which ordinarily occur upon walls and rocks, are the Lichen calea- 

 reus^ perttcsus, tartareus^ candelarius, parellui^ saxatilis, centrifugus, crispus, om- 

 phalodeSf parietinus^ pustulatus^ &c. 



The mosses which occur upon old walls are the Mnium setaceum, capillare, &c. ; 

 Bryum apocarpum^ striatum^ rurale^ truncaiulum, murale, ccespititium ,* Hypnum 

 •ericeum., serpens ^ myosuroidesj &c. 



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