LICHENS 



115 



is placed on a flat or saucer-shaped arrangement, and 

 is always exposed at maturity. But the structure of 

 Lichens will be better understood if we study a particular 

 example with the assistance of some drawings. 



The Lichen Physcia parietina (Fig. 38) is abundant 

 on rocks, palings, old roofs and walls in the neigh- 

 bourhood of the sea. It forms flat, reddish-yellow 

 rosettes, which evidently grow adpressed to their sub- 

 strata in order to avoid being torn by the wind. Being 



rtsnmrOTmjBn 



Fia. 38. — The Lichen Physcia pabib- 

 TiNA Thallus. As seen i^om 

 Above, Natxtral Size. 



a, Apothecia; h, vertical section of 

 apothecium, showing hymenium. 

 X about 5. 



Fig. 39. — ^Vbeticai, Section op 

 Thallus of Physcia paeie- 



TINA. X 500. 



I and u, Upper and lower cor- 

 tical layers; a, captive algae. 



very conspicuous, it is easily seen by insects, and they 

 may be observed crawling over the surface and be- 

 coming dusted with the spores: doubtless insects are 

 instrumental in spore- dispersal. The growth of this 

 and allied species is far from rapid. It is calculated at 

 J inch per annum, so a specimen 6 inches in diameter 

 will probably be about twelve years old. 



The structure of the thallus of Physcia is shown in 

 the transverse section, which in Fig. 39 is magnified 



