121 



are well marked and present some interesting features in lichen- 

 ological development. 



The thallus, as far as the representatives in our territory are con- 

 cerned, is typically crustaceous, there being no trace of a cortical 

 tissue. Even in the highly developed southern forms (J3. imbricatus) 

 in which the thallus has a distinctly foliose appearance, branched 

 with ascending lobes, there is no cortical tissue proper, but simply a 

 network of agglutinate hyphae with thickened gelatinous walls. The 

 thallus usually covers a considerable area ; it is of uniform thick- 

 ness and adapts itself to the conformations of the substratum to which 

 it is closely adherent ; its color is usually bright green, becoming 

 light colored on drying. 



On examining vertical sections it is found that the algae extend 

 nearly to the upper surface, there being only a thin upper layer con- 

 sisting of the ends of hvphal branches mingled with the remnants of 

 dead algae. The algal layer is well developed, followed below by a 

 medullary tissue ; the rhizoidal hyphae are rather deficient, and the 

 hyphae throughout the thallus are slender ; in the algal zone they 

 are much branched, entirely enclosing the algal cells with numer- 

 ous ectotrophic haustoria ; the endotrophic haustoria are also very 

 numerous and exceptionally distinct. The algae are probably Cys- 

 tococcus humicola but differ from the usual forms by the presence of 

 numerous pyrenoid bodies, usually two or three larger ones or many 

 smaller ones in each cell ; algal cells in all stages of division may 

 be found. Baeomyces affords the best opportunity of studying the 

 life-history of lichen-algae and their relation to the fungal symbiont. 

 It need scarcely be mentioned that it is necessary to study fresh ma- 

 terial. In some species (J3. rosens, B. aeruginosus} a form of Gloe- 

 ocapsa occurs in association with the normal algal form, below the 

 Cystococcus, sometimes 'forming a layer of considerable extent. It 

 is, however, not constantly present and should, therefore, be looked 

 upon as a case of contingent symbiosis. 



The apothecia are of medium size with either a flattened or con- 

 vex disk and are borne upon thick simple stipes, which are some- 

 times so much reduced that the apothecia appear almost sessile 

 (^. aeruginosus}. These stipes may terminate in a single globose 

 apothecium {B. roscus) or in several more or less agglutinate apothe- 

 cia (B. byssoides). In the approximately sessile apothecia the disk 

 is flattened ; the epithecium and upper ends of the simple, rather 



