416 



BOTAXY 



branched thallus consisting of algal filaments interwoven with fungal 

 hyphae. An example of such a filamentous form is presented by 

 Ephebe pubescens, which is found growing on damp rocks, forming a 

 blackish layer. 



Another group Is formed by the GELATINOUS Lichens, whose 

 thallus, xisually foliaceous, is of a gelatinous nature. The algae 

 inhabiting the thalli of the gelatinous Lichens belong to the families 

 of the Chroococcaceae and Xostocaceae, whose cell walls are swollen, 

 forming a gelatinous mass traversed by the hyphae of the fungus. 

 The genus Collema is a European example of this group. 



FIG. 365. Xantlioriu ^mriitinu. 1, Germinating ascdsjHUc (.<//) with branching germ-tube ]i|>lii-<l 

 to the Chlorococcum cells (a) ; S, thallus in process of formation : sp, two ascospores ; p, Chloro- 

 coccum cells. By the fusion of the hyphaa in the middle of the mycelium, a pwado-ptNBChj - 

 matous, cortical layT has br^vm to form. (After BONNIER, from v. TAVEL, x 500.) 



In both the filamentous and gelatinous Lichens the algae and 

 the fungal hyphae are uniformly distributed through the thallus, 

 which is then said to be unstratified or HOMoiOMEROUS. 



The other Lichens have stratified or HETEROMEROUS thalli. The 

 enclosed algae are usually termed GONIDIA. They are arranged in a 

 definite GONIDIAL LAYER, covered, externally, by a CORTICAL LAYER, 

 devoid of algal cells and consisting of a pseudo-parenchyma of closely 

 woven hyphae (Fig. 366). It is customary to distinguish the three 

 following forms of heteromerous Lichens. CRUSTACEOUS LICHENS, 

 in which the thallus has the form of an incrustation adhering closely 

 to a substratum of rocks or to the soil, which the hyphae to a certain 

 extent penetrate. FOLIACEOUS LICHENS (Fig. 367), whose flattened, 

 leaf-like, lobed or deeply-cleft thallus is attached more loosely to the 



