THE LICHEXS 369 



esculenta, a Lichen living in limestone deserts, it has been shown that these 

 crystals constitute about 60 per cent, of the dry weight of the thallus. Another 



Inner cells with 

 chlorella 



Fig. 354. — Hydra viridis. Transverse section 

 coelenteron showing Chlorella cells in 

 these tissues. 



important group of substances derived from the I>ichens are what are termed 

 the lichen acids. These substances are sometimes formed in large quantities 

 and are probably directly associated with the peculiar metabolism. One of 

 the most important, chrysophanic acid, gives the bright orange colour to 

 Lichens like Xanthoria parietina (Fig. 355), which is so commonly seen on 

 old roofs. Another valuable series of these acids yield the litmus of 

 commerce and the closely similar orchil (orseille). These purple dyes are 

 chiefly extracted from Roccella tinctoria, which is common on the rocky coasts 

 of southern Europe (see Volume IV, " Economic Botany "). 



As an example of a well-developed Lichen we shall consider the common 

 species Peltigera canina. 



Peltigera canina 



This Lichen is found growing in large masses on the surface of mossy 

 banks and rocks (Fig. 356). The thallus is dark olive-green above and 

 alm.ost pure white below. It is dichotomously branched, and down the 

 middle of each branch runs a thickened ridge or vein. The thallus is foliaceous 

 and may be several inches long and about an inch across. 



The thallus is made up of a continuous cortex on the upper surface, 

 which in mature parts of the thallus consists of hyphae forming fairly large, 

 broad cells (Fig. 357). These hyphae may be composed of from two to six 

 cells and may continue beyond the surface of the cortex in the form of simple 

 or branched hairs. Apart from these hairs the surface of the thallus is 



13 



