LICHENS AND THE ROCK FLORA. 25 1 



Other animals. Later on, the grasses or heather around, 

 gradually creep up the sides, or fix themselves on the 

 wall top. 



In ordinary lowland districts, the process is a little 

 different. The grey Parmelias are common enough ; 

 but very often little moss tufts take their place, and 

 occupy the surface already worked over by Lecanora- 

 Lecidea. These mosses belong to many different 

 genera, Ceratodon^ Tortula^ Bjyum, etc. As described 

 on p. 258 they are followed by other mosses, and 

 eventually colonised by flowering plants such as Sagina 

 procumbens. Very soon grasses begin to grow between 

 the stones or crevices ; they spread downwards from 

 above, and also creep up from below until the stones 

 are almost wholly covered. 



In very wet places stones and rocks have a different 

 flora, composed generally of algae of one sort or another. 

 A green stain is very common on damp buildings 

 especially at the base of the wall. On certain building 

 stones it always occurs before the house is dry ; but its 

 presence is an infallible sign of damp in old-established 

 houses. The algae which produce this stain are not 

 always the same ; but Pleurococcus vulgaris, Hormiscia 

 and RMzoclonium seem to be the most common. 

 Occasionally one finds on damp walls beautifully rusty- 

 red tufts about a quarter of an inch in height ; they are 

 often passed over as old and dead mosses, but are really 

 composed of an alga, CJii^oolepus aureus, which is exceed- 

 ingly beautiful under the microscope. 



Although, in the preceding examples, the flora of 

 rocks and stones has been taken chiefly from walls and 

 dykes, rocks in old quarries, boulders and cliffs show a 

 very similar series. Waterfalls in wooded places show 

 a bewildering variety in vegetation. Lichens, Liverworts, 



