LICHENS 



117 



numerous, and the total spore production is great. In 

 the same section we observe two layers of Algal cells, 

 A, A, the medullary zone, m, and the upper and lower 

 cortical layers of densely packed hyphae, c, c, the upper 

 cortical layer in the fruit forming the hypothecium. 



The spores, when fully ripe and other conditions are 

 favourable, are expelled from the asci with much force, 

 and become dispersed; they are 

 pure Fungus spores, and, there- 

 fore, cannot form a Lichen un- 

 less they meet cells of the Alga 

 Cystococcus humicola. Once 

 such a meeting is effected, the 

 partnership becomes estab- 

 lished, and the Fungus-partner, 

 now furnished with food-sup- 

 plies, takes the lead in the 

 formation of a new Physcia 

 parietina thallus. 



In some Lichens tiny heaps, FlG - 40.-Ver T ical Section 



J .. x OF AN APOTHECITJM OF PHY- 



patches, or balls of a powdery SCIA PARIETINA , x 250. 

 appearance are formed; they tt> Asci, two of them contain, 

 consist of a few Algal cells and in g ei § ht ascospores (s P .) 

 fragments of hyphse, the latter 

 enclosing the former. The term 

 soredia (Gr. sows, a heap) is 

 applied to these patches. They 

 are formed below the surface of the thallus, but are 

 finally pushed above it, and become dispersed by 

 insects, wind, and rain; they serve to reproduce the 

 Lichen vegetatively, and must be very useful to the 

 species, because, in their case, hunting for partners is 





» 



each; c, (below) lower cor- 

 tical layer; c, (above) hypo- 

 thecium; A, A, strata of 

 algal captives ; m, medullary 

 layer; p, paraphyses. 



