460 A TEXTBOOK OF BOTANY [On. X 



familiar grayish, or bright colored (other than bright green) 

 incrusting growths on old trees, fences, and rocks. They 

 differ from all other Fungi in the fact that they incorporate 

 Algae in their bodies ; and the resultant symbiotic structure 

 has a much more compact, definitely shaped, and even thal- 

 lus-like construction than has the mycelium of other Fungi. 

 While several are Basidiomycetes nearly all are Ascomycetes, 



FIG. 322. The Tree Lichen, Physcia stellaris, a foliose form. 



Left, plant on a tree trunk ; natural size. Center, above, vertical sec- 

 tion through apothecium and thallus ; X 15. Below, vertical section into 

 thallus, showing hyphse of Fungus in relation to the Alga (Pleurococcus) ; 

 X 200. Right, section through hymenium of the apothecium, showing 

 asci, each with 2-celled ascospores, developing among paraphyses ; X 200. 

 (Based on figures by Bergen and Davis, in part from nature.; 



though belonging to different orders. While thus a some- 

 what diversified group phylogenetically, they have promi- 

 nent characteristics in common, on which account it is con- 

 venient to consider them together. Some 4000 species are 

 known, of which a few have minor economic uses. 



A typical form is the common Tree Lichen, which grows 

 as a grayish radiate incrustation on the bark of old trees 

 (Fig. 322). On the thallus occur many little cups like the 

 apothecia of the Discomycetes, with which, indeed, this 

 form really belongs. The thallus-mycelium is composed of 

 felted hyphse, which are compact towards the surf ace, but 



