LICHENS AND THE ROCK FLORA. 243 



cavities on the surface of the frond. These spores will 

 germinate and form a new lichen if they are carried to 

 a favourable position. 



Lichens are very difficult to name, because, un- 

 fortunately, the number of species has been absurdly 

 multiplied. Very many are only known by one 

 solitary specimen, and are described in perhaps eight 

 lines of diagnosis. Thus for beginners it is very difficult 

 to make a start, and it is therefore necessary to give a 

 more detailed account than in the cases of other 

 groups. 



We will divide the lichens into the following roughly 

 characterised groups. 



Erect-branched lichens, with stems more or less 

 flattened, such as Evernia and Ramalina. These are 

 found on the bark of old trees or on rocks by 

 the seaside. They are small branching shrub-like 

 lichens, generally of a greyish or yellowish-green, 

 and are distinguished by their flattened or irregular 

 rounded stems. The flattening will enable them to 

 intercept more light than Usnea and Alectoria (see 

 below) ; whilst they are still sufficiently flexible to turn 

 about, and so to resist being torn off by the wind. Most 

 of these are found on trees, and especially upon the 

 trunk. Evernia prunastri^ which is very common, is 

 white underneath, whilst in Evernia furfur acea the lower 

 surface is black or dark- coloured. Ramalina has both 

 upper and under side of the same yellowish grey-green 

 colour. The seaside Ramalinas grow on exposed rocks, 

 and have rather large swollen yellow cups. The others, 

 R. farinacea and R. fraxinea, grow on trees often in 

 fairly dense woods. The first is whitish, and has 

 narrower much-branched pointed lobes. The second 

 has a broad, green frond, and is only slightly branched. 



