230 
PHARMACEUTICAL BOTANY 
According to the manner of growth of the thallus and nature of 
attachment to the substratum, three different sub-groups of lichens 
may be distinguished, viz.: (i) Foliaceous where the thallus is flat, 
leathery and leaf-like and attached to the substratum at different 
points. To this group belong Physcia and Parmelia. (2) Crus- 
taceous, where the thallus closely adheres to rocks and bark of trees. 
To this group belong Graphis and Pertusaria. (3) Fruticose, where 
the thallus is upright and branching. To the last group belong 
Cetraria islandica, species of Cladonia, and Usnea. 
Fig. 106. — Cetraria islandica. (Sayre.) 
To the pharmacist and chemist lichens are chiefly of interest 
because of the coloring principles which they contain. Species of 
Lecanora and Rocella tinctoria yield, when subjected to fermentation, 
the dyes orcein and litmus. Litmus is one of the best indicators in 
volumetric analysis. Cudbear, a purplish-red powder, used exten- 
sively for coloring pharmaceutical preparations in the form of tinc- 
ture, is prepared by treating species of Rocella, Lecanora or other 
lichens with ammonia water. Other lichens, such as Cetraria 
islandica, various species of Parmelia, Usnea and Alectoria, have 
been used in medicine because of demulcent principles which they 
contain. 
