894 PLANT COMMUNITIES AND FLORAS. 



lastly, Montias. Of the Cryptogams almost all are Mosses, the chief species being 

 the Sphagnums, Polytrichums, the species of Dicranum and Gymnostomum which 

 grow in dense aggi-egates, and the Hypnums which cling together in loose mesh- 

 work (e.g. Hypnv/m Schreberi, Hylocomium splendens, and Uylocomium tri- 

 quetrum). 



Only Lichens, FloridesB, and a few Stoneworts take part in the formation of 

 incrustments. Extreme variety is exhibited by Lichens which combine to form 

 crusts closely appressed to the substratum. The chief dominant plants are various 

 species of Acarospora, Amphiloma, Lecanora, Lecidella, Pertusaria, and Verrn- 

 caria. The best-known and most widely-distributed incrustment is that which is 

 formed by Lecidea geographica or Rhizocarpon geographicuTn, and imparts a 

 characteristic colour to the blocks of stone on slaty mountains. A looser type of 

 incrustment is composed of various fruticose Lichens, principally of Cladonias and 

 Cetrarias, and are most striking in high mountains and in the Arctic tundra. A 

 less important form is one composed of submerged Characese and Floridese incrusted 

 with lime, e.g. by species of Corallina. The lime-incrusted species of Lithothamnium, 

 and Lithophyllum which combine to form solid bank-like incrustations known as 

 nullipore-banks only occur in the sea. 



Felts are formed by filamentous Algaj. The dominant plants are Scytonemaceae, 

 ConjugatsB, Ulotrichaceas, CEdogoniaceae, Vaucheriacese, and a few others. The 

 Conjugatae, such as the various species of the genus Spirogyra, fill stagnant ponds 

 with their green filaments; several of the Vaucheriaceas grow in closely felted masses 

 in water-courses or on damp earth, and several Scytonemaceae and Confervoideae 

 weave themselves into thin mantles over stones. Felts are but seldom of very con- 

 spicuous appearance. The best known is the thin felt formed by Trentepohlia 

 lolithus, to which is due the red coloration of blocks of stone (" Violet-stone ") seen 

 in mountain regions. 



It is only in rare cases that a plant-community is composed of a single species 

 alone. For the most part two, three, or even more species of similar aspect are 

 jointly predominant. At the same time it may happen that at one spot one species, 

 at another spot another species is kept in check without any material alteration 

 being thereby incurred in the general appearance of the particular commimity. 

 Thus, for instance, the slopes of the schistose mountains of the Central Alps are 

 clothed with carpets in which Carex curvula, Juncus trifidus, and Oreochloa 

 disticha are dominant species. Here and there these three species share equally 

 in the composition of the carpet, but in many places one is paramount, whilst the 

 others sink into the backgroimd, so as to be scarcely noticeable at first sight. No 

 further explanation is requisite to show that in such cases the community must not 

 be named after one only of the dominant species. 



As regards the subordinate species of a commimity it would be a great mistake 

 to suppose that their occurrence is subject to no sort of rule. Though they seldom 

 have any striking influence on the tout ensemble of the community their importance 

 in it must not be under-estimated. Many of them are so constantly associated witli 



