414 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.: (1) Foliaceous, where the 
thallus is flat, leathery and leaf-like and attached to the sub- 
stratum at different points. To this group belong Physica, 
Sticta and Parmelia. (2) Crustaceous, where the thallus closely _ 
adheres to rocks and bark of trees. To this group belong 
Graphis and Pertusaria. (3) Fructicose, where the thallus is 
upright and branching. To the last group belong Cetraria 
islandica, species of Cladonia, and Usnea. 
Fic. 308.—Cetraria islandica, or Iceland Moss, a medicinal lichen. (Sayre.) 
STRUCTURE.—The body of a typical lichen shows a differ- 
entiation into the following regions: 
(1) A tegumentary layer of several rows of hyphal cells. 
(2) An upper cortex of densely arranged hyphal cells. 
(3) An algal layer containing green algal cells, often called 
gondia. 
(4) A medullary layer of loosely interwoven fungal hyphe, 
with large air-spaces. This is usually the broadest layer and 
sometimes also contains algal cells. 
(5) A lower cortex of hyphal cells, less dense than the upper 
cortex. From the surface cells of this layer rhizoids are 
developed which absorb water and nutrient salts from the 
substratum. 
