THALLOPHYTA. 



693 



and Coleochfeteas of the class Gamophyceae of Green Algae. Thus the same Alga 

 serves for many different Lichens. 



Classifying Lichens according to the characters of the fungal constituent, we 

 find membei's of the following families: Discomycetes, Pyrenomycetes, Hymeno- 

 mycetes, and Gasteromycetes. We may therefore speak of 4 families of Lichens: 



Discolichenes, Pyrenolichenes, Hyvienolichenes, and Gasterolichenes. 



Biscolichenes. — All produce asci in apothecia after the manner of Discomycetes 

 (cf. p. 682). The apothecia arise in numbers on the surface of the Lichen-thallus 

 (c/. figs. 393 ", 394, and 387 "), and the spores are formed in the asci in the usual 

 way. In many genera the spores are multicellular. In not a few cases there are 

 arrangements for the simultaneous distribution of the Alga-cells along with the 



Fig. 392.— Lichens. 



I Hyphaj of Stereocaulon ramulosum enveloping filaments of the blue-green Alga Scytomma; x650. 2 Cladonia /urcata with 

 Frotococcu^ ; x 950. s Coccocarpia molybdUBa, section of thallus ; x 650. (After Bornet. ) 



ascospores, so that it shall not be left to chance for the spores to alight upon a suit- 

 able Alga. In these cases the algal cells bud actively below the layer of asci, and 

 some are pushed up between the asci at the time when the latter reach maturity. 

 Many Lichens possess also conidial fructifications, known as p^'cnidia (cf. p. 678). 

 These are flask-like excavations, into which tiny conidia are budded by the lining 

 cells. These receptacles were formerly termed spermogonia and the conidia sper- 

 matia; but the evidence that they have any such function as the terminology sug- 

 gests is of the most slender description. On the other hand, these conidia have been 

 caused to germinate and produce characteristic Lichen-thalluses in a number of 

 cases. Pycnidia may be easily seen in the Iceland Moss {Cetraria islandica); they 

 occur one in each of the tiny teeth on the margins of the ribbon-like thallus. Very 

 common is vegetative propagation by means of brood-bodies known as soredia. 

 These arise as little buds below the surface, and consist of an algal cell or two and 

 a weft of fungal hyphae. Being formed in quantities together they burst through 

 to the surface as a dust-like powder and constitute the " soredia-heaps ". They are 

 distributed by the wind or washed away by rain. Both constituents of the Lichen 



