136 THE BOOK OF NATURE STUDY 



orange cups make this lichen distinguishable from a great dis- 

 tance. 



Next, we have a large number of lichens whose thin, flat 

 thallus is attached to, and often actually embedded in, rocks 

 and the stones or bricks of walls, or clinging to the bark of trees, 

 and attached so firmly that one has to chip off bits of the stone 

 or bark in order to remove the lichen itself. These crust-lichens 

 form very thin films or scales, or appear to be mere stains on 

 the stone or bark. More than three-fourths of the total number 

 of British lichens belong to this class, which provides a delightful 

 study for those who will take the trouble to examine with a lens 

 the surface of old walls and tree trunks. On walls these crust- 

 lichens have varied and beautiful tints, though the wall itself 

 looks dull and monotonous from a distance owing to the blending 

 of the colours. 



The most interesting bark-inhabiting kind of crust-lichen 

 is Gr aphis script a, the " letter-wort " or " scripture-wort. " The 

 thallus forms a thin grey or whitish film on smooth-barked trees, 

 while the spore fruits form black or brown lines, either straight, 

 curved, or branched, having a strange resemblance to ancient 

 hieroglyphics. In Gr aphis itself these fruit lines are sunk in 

 the thallus, but in some allied kinds they project from the surface. 

 The commonest crust-lichens on rocks and walls are species 

 of Lecanora and Lecidea ; some Lecanoras and Lecideas also 

 grow on trees, however. The Lecanoras, in which the fruit cup 

 has a distinct rim of the same colour and texture as the thallus, 

 are generally grey or whitish. In L. atra, on stones, the thallus 

 is white above and black below, and the fruits are jet black. In 

 L. subfusca, chiefly on trees, the thallus is whitish on both sides 

 and the fruits brown. In other species the fruits are white, 

 yellow, orange, or red. The Lecideas, which form the great 

 majority of rock-encrusting lichens, have a dark coloured thallus, 

 usually black and charcoal-like, and often sunk into the actual 

 substance of the rock. They have very small but neat fruit 

 cups which have no distinct rim. 



In the higher lichens the thallus is either shrub-like and 

 tufted, attached at only one point to the substratum, or bears 

 erect branches in addition to a flat, creeping portion. To the 



