CHAP. LXII LITHOPHYTES 241 



far north, as well as in Scandinavia and on the Alps one sees black stripes 

 running down rocks and indicating vegetation composed of Cyanophyceae 

 (species of Stigonema) which follow the course of the trickling water. 

 The algae, Trentepohlia iolithus and T. aurea, colour rocks red and yellow. 

 The ' Black Rocks ' in Angola earn their name from investing algae, 

 and the conical tops of the granite mountains near Rio de Janeiro owe 

 their brown colour to a small kind of alga. In many tropical places 

 where a considerable amount of moisture is combined with a high tem- 

 perature there is a rich sub-aerial vegetation of algae, which are mainly 

 Cyanophyceae. 1 Algae fix themselves in most cases simply by aid of a 

 mucilaginous layer of the cell-wall. 



The lichens concerned include crustaceous lichens (Lecanora, Lecidea, 

 Biatora, and others), and foliose lichens (Parmelia, Xanthoria, the black 

 species of Gyrophora, and others). On less precipitous spots, where the 

 vegetation is older, fruticose lichens are added. Here, attention may 

 be directed to the fact that on the littoral rocks of northern Europe 

 halophilous lichens display a zonal distribution, as already mentioned on 

 page 224. 



The mosses, including species of Hypnum, the blackish - brown 

 Andreaea, and the grey species of Grimmia, may form dense cushions 

 on the rock, over which their protonemata spread as flat incrustations. 



Though these plants on rocks are often dull in hue, being black or 

 grey, yet some species are bright-coloured for example, the lichens Buellia 

 geographica and Xanthoria elegans are greenish-yellow and yellowish-red. 



ADAPTATIONS. 



As the substratum is absolutely physically dry lithophytes belong to 

 growth-forms that are capable over their whole surface of absorbing 

 water derived from rain, dew, melting snow, or water running down the 

 rocks. And the plants enumerated have this power. 2 Devices for 

 collecting water are possessed by many mosses, in the form of felted 

 rhizoids, and by certain liverworts, in the form of peculiar, concave leaves. 3 



Lithophytes require haptera by which they can attach themselves to 

 rock, unless the thallus itself adhere closely to this. 



Rock is to many plants (for instance, marine algae) only a basis of 

 support, but in other cases (for example, lichens) it is likewise a nutritive 

 substratum into which the plants delve more or less deeply ; the rhizoids 

 of calcicolous and silicicolous lichens, according to Bachmann, 4 penetrate 

 the mica of granite. In the case of these plants the nature of the rock plays 

 an important part ; the harder and freer from clefts it is, the more diffi- 

 culty will plants find in attaching themselves. On Etna, according to 

 J. F. Schouw, there are pre-historic streams of lava which up to the 

 present sustain no vegetation ; apart from this, the lichen Pterocaulon 

 vesuvianum may be mentioned as the first plant to settle on lava. On 

 the other hand, soft rocks, such as many limestones, are readily colonized ; 

 rhizoids of mosses and lichens, and filaments of algae perforate and erode 

 them ; indeed, the whole thallus of certain endolithic lichens lies buried 

 to a depth of several millimetres in the stone, the apothecia alone becoming 



1 Fritsch, 1907. 



Regarding lichens, see Zukal, 1895 ; and regarding mosses, see Oltmanns, 

 1885. 3 Seep. 119; Gobel, 1898-1901. * Bachmann, 1904. 



WARMING R 



