388 REPRODUCTIVE ORGANS OF LICHENES. 
2. LIcHENES or LicHens.—Of late years Lichens have been 
regarded by many botanists as, in reality, Ascomycetous Fungi, 
parasitic upon Algz. But as the recent researches of Crombie 
and others have shown that this view of their nature is highly 
improbable, and as moreover Lichens present so many charac- 
teristics peculiar to themselves, we shall describe them and 
their modes of reproduction under a separate head. According 
to the view that Lichens are species of Fungi, parasitic upon 
Algze, the chlorophyll-containing cells or gonidia ( Sigs. 853, 
Fig. &51. Fic. 858, 
Wun al iu 
Ey maa 
Fig. 851. Thallus of Opegrapha atra, showing linear apothecia, termed 
lirelle. Fig, 852. Portion of the thallus of Parmelia parietina, with 
young round «apothecia, ap, and spermogonia, sp. After Henfrey.—Fig. 
853. Section of the thallus through an apothecium of Cetravia islundica. 
as. Asci, three of which contain ascospores. par. Paraphyses. gon. 
Gonidia. After Berg and Schmidt. 
gon, and 855, gon), found within the substance of their thal- 
lus, and which used formerly to be regarded as special asexual 
reproductive organs, are in reality Algz upon which the 
Fungus is parasitic. Thus the thallus of a Lichen is a com- 
pound structure, consisting of two elements, the fungal and the 
algal. 
The reproductive organs of Lichens are of three kinds :—(1) 
Apothecia ; (2) Spermogonia ; and (3) Pyenidia. 
The apothecia are of various forms, and have received different 
names accordingly; the more usual are the round (fig. 852, ap) 
and linear (fig. 851); the latter are commonly termed lirelle. 
