LICHENS 



117 



-p 



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 hyphse, 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 - ^.-VERTICAL SECTION 

 patches, or balls of a powdery 

 appearance are formed; they 

 consist of a few Algal cells and 

 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 



OF AN APOTHECIUM OF PHY- 

 SCIA PARIETINA, X 250. 



a, Asci, two of them contain- 

 ing eight ascospores (sp.) 

 each; c, (below) lower cor- 

 tical layer; c, (above) hypo- 

 thecium; A, A, strata of 

 algal captives; m, medullary 

 layer; p, paraphyses. 



