208 



PLANT ORGANS OR PARTS OF PLANTS. 



The drug is odorless ; the taste bitter and mucilaginous. 



Histology. — There are three varieties of tissue. The 

 outer cuticle, four to six cell-rows deep, of very small, 

 thick- walled, difficultly distinguishable cells, with, how- 

 ever, visible lumens, merges into the compact mass of 

 colorless, filiform hyphae, which in turn give way to the 

 dense branched interlocking cells of the central portion. 

 In the latter tissue are numerous intercellular spaces con- 

 taining round gonidia about i micron in diameter, 



Fig. 56. — Cetraria Islandica. 



filled with the green coloring-matter, thallochlor. In 

 many places the colorless middle and outer layers of 

 tissue entirely replace the central body, producing the 

 white dots seen on the surface of the membrane. Here 

 are found crystals of cetrarin. The cilia of the margins 

 are the spermagonia. These are short, cylindrical, often 

 forked, filled with rod-shaped antherozoids, 6 microns long. 

 Chemistry. — Lichenin or lichen starch, 70 per cent. ; 

 cetraric acid, 2 per cent. ; lichen-stearic acid, about i per 



