314 LICHENS. 



otherwise resemble Collema; (6) the plant is loosely 

 attached to the substratum by rhizines consisting chiefly 

 of single rows of cells. The apothecium has the same 

 general structure as in Physcia, but the spores show 

 several cross-walls and also usually longitudinal divisions, 

 and have thin walls. 



457. Apothecium of Discomycetes. Since the Fungus 

 in the great majority of Lichens is of the Ascomycetous 

 type, it is advisable to study some Ascomycete, like Peziza 

 or Ascobolus, with special reference to the structure of the 

 ascocarp. 



In Eurotium and Sphaerotheca, which have already been studied, 

 the ascocarp is a closed case or cleistothecium, but in many other 

 Ascomycetes it is either a cup-like apothecium (Discomycetes), or a 

 flask-like perithecium with a pore (Pyrenomycetes). In a fair num- 

 ber of Ascomycetes, a process of fertilisation has been found to 

 precede the formation of the ascocarp, and in some cases male cells 

 (spermatia) are produced in spermogonia, like those of Puccinia 

 in form, while the ascogonium has a filamentous outgrowth (tri- 

 chogyne) which receives the male cell. 



In many Ascomycetes the ascocarp is developed without a fertili- 

 sation process ; either an oogonium is formed which produces the 

 ascogenous hyphae without being fertilised, or there may be no 

 trace of an oogonium at all. 



Exactly the same applies to Lichens. In a few cases, an oogonium 

 consisting of a coiled lower portion embedded in the thallus, and 

 a straight upper portion (trichogyne) which protrudes from the 

 surface is fertilised by a spermatium, which is carried by rain-water 

 to the trichogyne and adheres to it. This has been seen, for in- 

 stance, in species of Collema and Physcia. In other cases there 

 are oogonia with projecting trichogyne, but fertilisation has not 

 been observed ; in others there are oogonia without a trichogyne 

 spermatia may or may not be produced, but no fertilisation 

 occurs, the ascocarp arising directly from the unfertilised oogonium 

 in others ; again, no oogonium has been found, and the ascogenous 

 hyphae apparently arise from ordinary vegetative hyphae. 



In any case, the ascocarp of Lichens resembles that of the higher 

 Ascomycetes, the ascogenous hyphae ending in a palisade-like 

 layer of asci, between which there grow up sterile hyphae (para- 

 physes), the two together forming the hymenium. The paraphyses 

 are usually gelatinous, and serve to keep the asci moist, besides 

 assisting in the dispersal of the ascopores. The spores, usually 

 eight in each ascus (but sometimes six, four, two, or one), are 



