ios THE FACTORS [Part I 



fact that even when the physical properties of the substratum are identical, 

 the flora varies with its chemical properties. On the rocks of a stream 

 in granite mountains that are poor in lime, according to Boulay, may 

 be observed, for instance, Hypnum dilatatum, H. ochraceum, Brachy- 

 thecium plumosum, Amblystegium irriguum, Fontinalis squamosa, Rhaco- 

 mitrium aciculare, Pterigophyllum lucens. One would look in vain for 

 these species on calcareous mountains. On the other hand, in the waters 

 of such calcareous mountains many species absent from silicious strata 

 may be found, such as those of Cinclidotus. Not less essentially distinct 

 is the algal flora, even the surface flora (e.g. the Desmidiaceae), of water 

 rich in lime and of water poor in lime. In all such cases the difference 

 must depend on the amount of lime in the water, for the physical nature 

 of the substratum is the same in the case of the superficially attached 

 mosses and can have no significance at all in the case of the floating 

 Algae. 



The difference between the flora of sphagnum-moors and of grass-moors 

 is also very instructive. In both cases the substratum is peat ; in the 

 former, however, it is saturated with water that contains but little lime, 

 in the latter with water rich in lime. The flora of the two moors is quite 

 dissimilar. Only on the sphagnum-moors are found Sphagnum, Viola 

 palustris, Spergula pentandra, Drosera, Vaccinium uliginosum and V. 

 Vitis-Idaea, Calluna vulgaris, Rhododendron ferrugineum, Pedicularis sylva- 

 tica, Carex dioica, Aira flexuosa, Pteris aquilina ; on calcareous moors, 

 on the other hand, Spergula nodosa, Pedicularis palustris, Erica carnea, 

 Primula Auricula, Carex Davalliana, Sesleria coerulea. 



A great dissimilarity is also seen between the mosses and lichens that grow on the 

 surfaces of rocks, according as the latter are poor or rich in lime, whilst to most 

 species, if not to all, the physical nature of the rock is irrelevant. Thus the species 

 of Andreaea are all calciphobous, also many species of Rhacomitrium, Grimmia, 

 Dicranum ; whereas the presence of certain other species, especially those of Barbula, 

 Pottia, Desmatodon, Encalypta, Gymnostomum, entitles one to infer with certainty 

 the existence of a limestone substratum. Many lichens on rocks show a similar 

 dependence on the chemical nature of the substratum, whilst in regard to others, 

 especially those developing very slowly, great durability of the substratum, a physical 

 property in fact, is said to be the determining factor. Lichens of the latter kind are 

 found chiefly on granite or porphyry, but also on very hard crystalline limestone. 



A quartz sand possesses physical properties that are quite similar to those of 

 a crystalline calcareous sand, and yet both have their thoroughly characteristic mosses, 

 the former for instance Brachythecium albicans, the latter Barbula inclinata. 

 Not less dissimilar is the moss-flora of clay, according as it is poor or rich in 

 lime, although the physical properties of the two are very slightly dissimilar. 

 Thus Sendtner learned to consider Ephemerum serratum, Phascum crispum, Pleuri- 

 dium subulatum as so calciphobous, that he believed their presence to indicate a clay 

 suitable for a brick-kiln. 



