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PHARMA COGNOSY. 
A number of distinct types of lichens are recognized, 
depending upon the form of the thallus and how it is 
attached to the substratum, as crustaceous, foliaceous, 
fruticose, etc. 
They occur on trees and rocks, but are never found 
on decaying organic matter. A large number form 
products useful to mankind. Several furnish food 
materials, as the reindeer moss and manna lichen, 
and several furnish valuable coloring principles, as 
litmus and orseille. Iceland moss is also used as a 
medicine. 
CETKAK1A (Iceland Moss). 
The entire plant of Cetraria islandica, one of the 
Ascoiichenes and widely distributed over the northern 
part of both continents. The chief commercial sup- 
plies are obtained from Scandinavia, Germany, Swit- 
zerland and parts of Austria. 
Description. — Consisting of a number of somewhat 
dichotomously branching, more or less curled, papery, 
fringed segments, 5 to 10 cm. long and about 5 mm. 
wide; upper surface greenish brown, with occasional 
dark reddish-brown cupular apothecia ; under surface 
grayish, with numerous small whitish depressed spots; 
tough when damp, but brittle when dry ; odor slight ; 
taste mucilaginous and bitter. 
Constituents. — The principal constituents are liche- 
nin and isolichenin (about 70 per cent.); the former 
appears to be intermediate between starch and cellu- 
lose, and is soluble in hot water, the solution becom- 
ing gelatinous on cooling but is not colored blue with 
iodine ; isolichenin (dextrolichenin) somewhat resem- 
bles soluble starch, being soluble in cold water and 
giving a blue reaction with iodine ; the drug also con- 
tains a bitter crystalline principle cetraric acid, about 
2 per cent. ; a tasteless crystalline principle, lichenos- 
