352 STRUGGLE BETWEEN PLANT-COMMUNITIES SECT, xvn 



Stereocaulon ; and among the very few mosses that occur Rhacomitrium 

 lanuginosum deserves special mention. Not until lava is weathered does 

 it become a soil suitable for plants. Comes 1 has described the colonization 

 and decomposition of the lava of Vesuvius by algae and lichens, the later 

 immigration of mosses and ferns, and the final appearance of xerophytic 

 Phanerogamia, including perennial herbs, shrubs, and trees, which 

 eventually give rise to forest. 



The devastation of Krakatoa in 1883 provides another example. The 

 immigration of plants was investigated by Treub, 2 who came to the con- 

 clusion that the ashes and pumice first became clothed with a thin film 

 of Cyanophyceae, and particularly of Lyngbya Verbeekiana and L. minu- 

 tissima, which prepared the way for the germination of the abundant 

 fern-spores. ' Three years after the eruption the new flora of Krakatoa 

 consisted almost entirely of ferns. Spermophyta occurred only singly, 

 dotted here and there near the shore or on the mountain.' These were 

 largely brought by water and birds. The later development of this flora 

 was studied by Penzig and Ernst. 3 Beccari found that Tamboro volcano 

 on Sambava, which in 1815 had been denuded of vegetation, in 1874 was 

 clothed from base to summit with virgin forest. 



Landslips. 



Rocky soil is laid bare by landslips or by human activity. In the 

 Alps and many other mountainous countries one sees huge masses of 

 stones surrounding the base of a mountain at a definite angle of inclina- 

 tion : these are masses of rubble or talus.' 1 The developmental succession 

 of the plants thereon is as follows : First there are lithophytes lichens, 

 algae, and mosses 5 whose rhizoids penetrate the stone more or less 

 deeply according to its hardness and porosity, and cause it to crumble. 

 Between and on these plants masses of dust, brought by rain and wind, 

 together with the mouldering remains of the plants themselves, provide 

 a scanty stock of humus, in which larger plants can gain footing. 6 The 

 richness of the vegetation depends upon the steepness and the liability 

 to weathering of the substratum. In steep places the vegetation remains 

 open and low essentially composed of Thallophyta and mosses (rock- 

 vegetation 7 ) ; on less-steep ground, where the stones soon become covered 

 with plants and humus, forest often arises eventually. 8 Near Eisenach 

 torrents of rain have produced deep chines and stony terraces. On these, 

 according to Senft, 9 the vegetation shows the following developmental 

 succession. First, the bare stony heaps were clothed with mosses, in- 

 cluding Hypnum sericeum and Barbula muralis ; then followed some 

 xerophytic grasses, such as Festuca ovina and Koeleria cristata, also 

 shallow-rooted perennial herbs (a type of vegetation belonging to 

 dry places). Thereafter succeeded other xerophytic herbs, such as 



1 Comes, 1887 ; see Engler, 1899, p. 179. 2 Treub, 1888. 



3 Penzig, 1902 ; A. Ernst, 1907. 4 See p. 246. 6 See p. 241. 



6 The majority of rubble-heaps cannot be described as entirely new soil, because 

 their descent is slow and is accompanied by humus, seeds, and the like, but when 

 a mountain-slope simultaneously gives way to landslip, this becomes gradually 

 clothed with common species derived from the neighbouring plant-communities 

 (Blytt). 



7 See p. 241. * See p. 329. Senft, 1888. 



