252 PLANT LIFE. 



Mosses, Algae, like Rivularia or Diatomaceae, entirely 

 cover every particle of the stone-surface ; not a square 

 inch is without vegetation which is at work assimilating 

 and accumulating material. 



All the organic matter so formed is utilised in some 

 way. It may be carried off by the brooks and trickles 

 of water, and assist in enriching the silt brought to the 

 low grounds by the rivers ; or it may be used as food 

 by the minute animals which are devoured by fishes 

 and birds. The importance of the rock and stone flora, 

 however, lies in its having been an essential preliminary 

 to other forms of vegetation. In all probability nothing 

 could ever have grown on lava-flows, rock-falls and 

 the like, if it had not been for lichens, mosses, 

 algae, and other minute and little known forms of 

 vegetation. 



