284 PLANT LIFE. 



dry conditions, began to grow. It was only after 

 some twenty years that a few woody shrubs could 

 obtain a foothold on the lava. Finally, trees, and 

 even true woods, were able to occupy the older 

 flows. In the island of Krakatoa, it was also found 

 that algae were the first to develop on the ashes 

 and pumice stone, and so prepare the way for Ferns 

 and Flowering Plants. 



It has been shown in a previous chapter, that the 

 succession of vegetations von bare rock or walls, whether 

 by the seaside, along roads or at the summit of High- 

 land hills, is almost always (i) Bacteria,^ (2) either 

 Algae or more usually Lichens, (3) Mosses, and (4) 

 Dicotyledons or Monocotyledons. Usually the Grasses 

 {Monocotyledons) precede the Dicotyledons (see p. 25 i, 9). 

 One can, however, in the screes of rough angular stones 

 often found in corries, occasionally trace precisely the 

 succession described for Vesuvian lavas : First Lichens ; 

 then Moss ; and then Ferns. Grasses or perhaps Saxi- 

 frages come next, and it is possible that careful observa- 

 tion might show Birches growing on old grass-covered 

 screes. 



It would be interesting if we could find traces of a 

 Moss vegetation preceding the Devonian clubmosses 

 and ferns. This might have been similar to the present 

 flora of the tundras of" Siberia. Of such a moss period, 

 there does not seem to be any record ; but, whatever 

 the precise affinities of the curious Devonian plants may 

 have been. Ferns and their allies were certainly pre- 

 dominant at that time, and preceded all the flowering 

 plants. 



From the Devonian period the succession of vegetation 



^ It is, at least, probable that the ubiquitous Bacteria begin the 

 process (p. 198). 



