250 PLANT LIFE. 



in the occupation by a specially adapted plant of a 

 most unusual and exceptional habitat; only one tree in 

 five hundred will possess such fissures, and on this one 

 trunk only a square foot or so is occupied by the 

 lichen. 



The lichens are of some importance in woods and 

 forests, but they are especially interesting in the flora 

 of rocks and stones. In a new wall or dyke, the 

 stones show their natural colour, but this only remains 

 for a very short time. Weathering begins, and a series 

 of successive vegetations soon covers the entire surface. 



The first stage of weathering is largely assisted by 

 Bacteria^ which seem able to disintegrate some of the 

 salts of which the rock is composed. Next come the 

 Lecidea lichens. Some of these are almost indis- 

 tinguishable to the naked eye, and consist of scattered 

 tiny stains of lichen substance. Others form a nearly 

 continuous powdery crust not close enough to hide the 

 stone colour. The best developed kinds entirely hide 

 the surface, and form circular patches broken up into 

 little flat-topped pieces amongst which are the cups. 

 The second, or Lecidea stage, is very soon followed 

 either by Parmelias or by Mosses. The Parmelias 

 extend over the surface already partially worked over 

 by Bacteria and Lecideas, and form the grey circular 

 rosettes already described. 



On dry basalt dykes, Rhacomitrium and other mosses 

 soon begin to grow amongst the Parmelias. Other lichens 

 also, Cetraria aculeata and Cladonias fix themselves be- 

 tween the stones where debris and moisture accumulate. 

 Thus even at this point the wall is by no means a 

 barren useless desert, but is almost entirely covered by 

 an actively assimilating vegetation, which, moreover, 

 affords shelter and livelihood to a variety of insects and 



