334 



PART IV. VEGETABLE TAXONOMY. 



Graphis scripta, from its fancied resemblance to hand-writing, is 

 common on the trunk of the Beech ; and another, Rhizocarpon 

 geographicus, forms a bright yellow incrustation of indefinite 

 extent on granitic and other silicious rocks. 



(C). The Foliaceous Lichenes form flattened leaf-like 

 expansions, which may be variously lobed or crispate on the 

 margins and adhere, often in the form of rosette-like patches, 

 more or less closely to the substratum. Many of these, as the 

 Parmelias and Stictas, form greenish-grey or yellowish patches 

 on tree-trunks, fences, etc. ; others, as the species of Peltigera, 

 grow on damp hillsides among the moss, and bear their apothecia 

 on the lobed borders of the thallus. Still other species, as those 

 of Gyrophora and Umbilicaria, form dark-colored patches on 

 rocks. See Figs. 536 and 537. 



(D). The Fruticose Lichenes have a shrub-like growth, 

 often branching profusely, and are attached to the substratum 

 only at the base. The branches of the thallus are in some species 

 cylindrical, in others flattened, but even in the latter there is 

 usually little, if any, structural difference between the upper and 

 under surface. For the most part the gonidia are arranged in the 

 form of a hollow cylinder running lengthwise of the branches ; the 

 interior of the cylinder is filled with hyphse, and on the exterior 

 they enclose it as a sheath. Many of the species cling to tree- 

 trunks, logs or rocks, but some grow upon damp earth. 



To the forms having cylindrical branches belong the Usneas 

 (see Fig. 538), Roccella tinctoria, a Lichen that constitutes one 

 of the principal sources of Litmus (see Fig. 539), and the familiar 

 Cladonia rangifarina, or Reindeer Lichen ; and to those with 

 flattened branches belong the so-called Iceland Moss, Cetaria 

 islandica, prized as a food, and used in medicine for its demul- 

 cent and tonic properties. See Fig. 540. 



