248 STRUCTURAL BOTANY 



such cell or group isolate themselves from the rest of 

 the medullary tissue. Ultimately the cortex is broken 

 through, and the mass of algal cells, with their envelop- 

 ing hyphse, is exposed as a powdery layer, which is easily 

 dispersed by the wind, or carried away by trickling 

 raindrops. Each grain of the powder is called a 

 soredium, and consists of one or more algal cells enveloped 

 in a little web of fungal filaments. The soredia may 

 grow and break up into new groups, or may develop 

 directly into a Lichen-thallus, if they come to rest in a 

 suitable place. The great point is that each soredium, 

 however small, contains both the Alga and the Fungus, 

 and so provides the two constituent elements of which a 

 Lichen is built up. 



The ascus-fruits of Physcia parietina are conspicuous 



to the naked eye as flat, shield-like discs on the upper 



surface of the thallus, generally of a rather deeper 



orange colour than the rest of the plant (see Fig. 98). 



These open fruits of the Discomycetes bear the name of 



apothecia. In the mature condition there is a rim of 



sterile tissue at the edge of the apothecium, the whole 



disc within this rim being covered by the hymenium. 



Below the hymenium is a dense mass of closely 



interwoven hyphae forming the hypothecium. The 



hymenium itself consists of elements of two kinds, the 



asci and the paraphyses. The asci, of which a great 



mmber are present in each fruit, are stout, club-shaped 



cells set vertically to the surface of the apothecium, 



each ascus whei ripe containing eight ascospores. The 



paraphyses are ste'ilJe hairs rising to the same height as 



the asci, both being llosely packed together, so that the 



hymenium has a smooth, cdL tmuous surface (see Figs. 98 



and 101). 



