306 



PART IV. CLASSIFICATION. 



The reproductive organs of the Ascolichenes are sterigmata, producing 

 spermatia, contained in spermogonia (Fig. 185) ; archicarps (in the order 

 Collemacese), differentiated into a coiled ascogonium and a multicellular 



projecting trichogyne ; and 

 ascocarps, which are either 

 apothecia (discomycetous) or 

 perithecia (pyrenomycetous) ; 

 the archicarp, apparently 

 after fertilisation by means 

 of spermatia, gives rise to 

 filaments which form the 

 hymenial layer (consisting 

 of asci and paraphyses) of 

 the apothecium, and out- 

 growths from the adjacent 

 FIG. 186. A-D Soredia of Usnea barbata. A A. 



simple soredium , consisting of an algal cell covered vegetative hyph 

 with a web of hyphse. B A soredium, in which the 

 algal cell has multiplied by division. C A group 

 of simple soredia, resulting from the penetration of 

 the hyphffi between the algal cells. D K Germin- 

 ating soredia : the hyphae are forming a growing- 

 point, and the algal cells are multiplying. (Alter 

 Bachs.) 



wall of the apothecium. 

 In the fructification of the 

 Basidiolichenes there is a 

 hymenial layer consisting of 

 paraphyses and basidia, the 

 latter bearing apical sterig- 

 mata, on each of which a 

 basidiospore is produced by terminal abstriction. 



Lichens are also reproduced by gemmse, termed soredia, which consist 

 of one or more algal cells invested by hyphse ; they are budded off from 

 the surface of the thallus, and grow into new plants (Fig. 186). 



The Lichen- Algse belong either to the Cyanophycese or to the Chloro- 

 phycese. The algal cells or filaments may be distributed throughout the 

 thallus, when it is said to be homoiomerous ; this is usually the case in 

 gelatinous Liphens (such as the Collemacese), in which the Alga belongs 

 to the Cyanophyceae, but also in some non-gelatinous forms in which the 

 Alga belongs to the Chlorophycese (such as Coenogonium, Eacodium, and 

 others, in which the Alga is Trentepohlia) : or they may be arranged in 

 a definite layer near the surface of the thallus, when it is said to be 

 heleromerous (Fig. 188), as in the case of nearly all those Lichens of which 

 the Algae belong to the Chlorophycese, and 

 some in which the Algse belong to the 

 Cyanophyceee (e.g. Peltigera, Pannaria). 

 In some heteromerous forms (e.g. Theli- 

 dium) the Algse are quite on the surface. 

 Occasionally (e.g. Endocarpon) algal cells 

 are present in the hymenium. 



It may be generally stated that the form 

 of the thallus is determined in the homoio- 

 merous Lichens by the Alga, in the hetero- 

 merous Lichens by the Fungus. In the latter 

 case three main forms are distinguished: 



FIG. 18". A gelatinous Lichen, 

 Cnllema jiulposum, slightly magni- 

 fied. It is homoiomerous, and the 

 Alga is Xostoc. (After Sachs.) 



