74 



A TEXT-BOOK OF BOTANY. 



of the gelatinous carbohydrate lichenin which they contain. Be- 

 sides those given, the following may be mentioned : Cladonia 

 rangiferina (reindeer moss), Lecanora esculenta (supposed to be 

 the manna of the Israelites). The Lichens are, however, chiefly 

 of interest because of the coloring principles which they contain. 



FIG. 43! Iceland Moss (Cetraria islandica). A-F, various forms of thalli showing 

 apothecia (a); I, cross-section of an apothecium showing the hymenium (h), the hypothe- 

 cium (p), the algal layer (e), the medullary layer (m), and lower or ventral surface (1); K, 

 an ascus with eight ascospores and two paraphyses from the hymenium (h). 



Roccella tinctoria, Lecanora tartarea, and other species of Leca- 

 nora, yield upon fermentation the dyes orcein and LITMUS, the 

 latter of which finds such general use as an indicator in volu- 

 metric analysis. Cudbear, a purplish-red powder, is prepared by 

 treating the same lichens with ammonia water ; while in the prep- 



