LICHENS AND THE ROCK FLORA. 249 



general crust. Lecanora is generally grey or whitish 

 though sometimes it is yellow, tawny, orange red or 

 even vermilion. The cups of Lecanora are in most 

 cases reddish brown but they vary also. Sometimes 

 the cups are a ghostly pale colour clearly brought out 

 against the grey green background of the frond ; or 

 they may show rich deep crimson on pure white. The 

 tiny bright yellow cups of Lecanora vitellina especially, 

 have quite a startling effect when one suddenly notices 

 them on monotonous grey rocks or concrete walls. 

 The colour is no doubt of the same importance as it is 

 in the larger Parmelias. It is easy to observe very 

 minute insects wandering about or flying over these 

 lichens but it is difficult not to disturb them ; still 

 harder to catch them ; and to get them named when 

 caught is quite impossible. These Lecanoras and 

 Lecideas are of great importance in the flora of rocks 

 and stones. Another crust-lichen, Pertusaria, forms an 

 uninteresting leaden-grey crust very common on tree 

 bark. It has generally no cups, but is recognised by 

 the small pure white discs, really soredia, scattered 

 over the frond. Another group, Graphidei^ can be 

 recognised by their forming greyish or white stains on 

 bark. They have no cups but have instead, black or 

 brownish lines very like short scratches with a pen. 

 Graphis and Opegrapha are the two common forms. 

 Very old trees are sometimes attacked by another 

 group, Calicieae. The lichen looks like a yellow or grey 

 dust covering deep furrows in the bark. On very close 

 observation minute black stalks ending in pin's heads 

 may be seen. These are the fructification of the 

 lichen. 



This extraordinary situation is interesting as an 

 example of the struggle for existence which has resulted 



