154 MORPHOLOGY 



In Corella, the mature lichen is squamulose or consists of small lobes; in 

 Dictyonema there is a rather flat dimidiate expansion; in both the alga is 

 Scytonema,\h.e.\x\c!aovciQ?, of which largely retain their form and are surrounded 

 by parallel growths of branching hyphae. The whole tissue is loose and 

 spongy. 



Corella spreads over soil on a white hypothallus without rhizinae. In 

 the other two genera which live on soil, or more frequently on trees, there 

 is a rather extensive formation of hold-fast tissue. When the dimidiate 

 thallus grows on a rough bark, rhizoidal strands of hyphae travel over it 

 and penetrate between the cracks ; if the bark is smooth, there is a more 

 continuous weft of hyphae. In both cases a spongy cushion of filamentous 

 tissue develops at the base of the lichen between the tree and the bracket 

 thallus. There is also in both genera an encrusting form which Johow 

 regarded as representing a distinct genus Laudatea, but which M5ller found 

 to be merely a growth stage. Moller' judged from that and from other 

 characteristics that the same fungus enters into the composition of both 

 Cora and Dictyonema and that only the algal constituents are different. 



C. Sporiferous Tissues 



As in Hymenomycetes, the spores of Hymenolichens are exogenous, 

 and are borne at the tips of basidia which in these lichens are produced on 

 the under surface of the thallus. In Cora the fertile filaments may form a 

 continuous series of basidia over the surface, but generally they grow out 

 in separate though crowded tufts. As these tufts broaden outwards, they 

 tend to unite at the free edges, and may finally present a continuous 

 hymenial layer. Each basidium bears four sterigmata and spores (Fig. 8/^); 

 paraphyses exactly similar to the basidia are abundant in the hymenium. 

 In Dictyonema the hymenium is less regular, but otherwise it resembles that 

 of Cora. No hymenium has as yet been observed in Corella; it includes, so 

 far as known, one species, C. brasiliensis , which spreads over soil or rocks. 



1 MoUer 1893. 



