TAXONOMY 
281 
Lichens are found on the bark of trees, on rocks, logs, old fences, 
etc. The body of a lichen shows a differentiation into two regions: 
a more or less compact row of cells on both surfaces, called the epider¬ 
mis; and an inner portion composed of the mycelium of the fungus. 
The alga is imbedded in this portion. In most cases the spores are 
borne in asci, which are themselves found in closed or open Apothecia. 
Scales or soredia are found on many lichens. These consist of a 
network of hyphae enclosing algal cells. By becoming detached 
from the parent plant, they develop new lichens and so constitute 
a means of vegetative propagation. 
Pig. 150. —Cetraria islandica. (Sayre.) 
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.: (1) Foliaceous where the thallus is flat, 
leathery and leaf-like and attached to the substratum at different 
points. To this group belong Physica 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. 
To the pharmacist and chemist lichens are chiefly of interest 
because of the coloring principles which they contain. Species of 
Lecanora&nd Rocella tinctoria yield, when subjected to fermentation, 
