394 ECOLOGY 



but mosses were more in evidence, and the next stage consisted almost 

 exclusively of mosses and hepatics with Peltigera canina. A thick layer of 

 humus was gradually built up by these plants on which Phanerogamous 

 plants were able to flourish. 



In tropical countries the first vegetation to settle on bare rocks would 

 seem to be blue-green gelatinous algae. Three years after the eruption of 

 Krakatoa, dark-green layers of these plants were found by Treub 1 on the 

 surface of the pumice and ash, and on the loose stones in the ravines of the 

 mountain. It was only at a later stage that lichens appeared. 



b. OUTPOSTS OF VEGETATION. Lichens are the only plants that can 

 survive extreme conditions of cold or of heat. They grow in Polar regions 

 where no other vegetation could obtain sustenance ; they are to be found 

 at great heights on mountains all over the globe ; and, on arid desert rocks 

 they persist through long dry seasons, depending almost entirely on night 

 clews for the supply of moisture. Here we have true lichen formations in 

 the sense of modern ecology. 



1 Treub 1888. 



