THE SPORE-SAC LICHENS 507 
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Fic. 335.—Iceland moss. A small part of the apothecium and thallus shown 
in Fig. 334, magnified 400 diameters; showing the spore-sacs or asci (a), 
the paraphyses (p), the compact layer of mycelium (s) from which they 
arise, the alge (Pleurococcus) known as ‘‘gonidia’’ (g), the cottony 
mycelium (m), and the compact protective rind (r, r’). (Luerssen.) 
Fic. 336, I.—Beard-lichen (Usnea barbata, Beard-lichen Family, Usneacee). 
Natural size. Granules often appearing on the surface are soredia. 
(Baillon.)—Grayish or dull yellowish green throughout; often much 
longer than shown, with a tough central. Common on trees in various 
parts of the world. 
finally, keeps them supplied with all the carbon dioxid 
they need as a raw material for their food. Dissimilar or- 
ganisms living thus together with benefit to both are called 
