RADIATE THALLUS iii 



The sheath of R. phycopsis is a small structure about 3 to 4 mm. in width 

 and i'5 mm. thick. A few gonidia may be found below the dense cortical 

 layer, but they tend to disappear as the upright fronds become larger and 

 the shade, in consequence, more dense. Lower down the hyphae take an 

 intensely yellow hue; mixed with them are also some brown filaments. 

 A somewhat larger sheath 7 to 8 mm. wide forms the base of R. tinctoria. 

 In structure it corresponds — as do those of the other species — with the ones 

 already described. 



In purely filamentous species such as Usnea there is also primary sheath 

 formation: the medullary hyphae spread out in radiating strands which 

 force their way wherever possible into the underlying substance; on trees 

 they enter into any chink or crevice of the outer bark like wedges ; or they 

 ramify between the cork cells which are split up by the mere growth pressure. 

 By the vertical increase of the base, the fronds may be hoisted up and 

 an intercalary basal portion may arise lacking both gonidia and cortical 

 layer. Very frequently several bases are united and the lichen appears to 

 be of tufted habit. 



A basal sheath provides a similar firm attachment for Alectoria jubata 

 and allied species : these are slender mostly dark brown lichens which hang 

 in tangled filaments from the branches of trees, rocks, etc. 



These attaching sheaths differ in function as well as in structure from 

 the horizontal thallus of the Cladoniaceae. They may be more truly com- 

 pared with the primary thallus of the red algae Dumontia and Phyllophora 

 which are similarly affixed to the substratum, while upright fronds of 

 subsequent formation bear the fructifications. 



IV. STRATOSE-RADIATE THALLUS 

 I. STRATOSE OR PRIMARY THALLUS 



A. General Characteristics 



This series includes the lichens of one family only, the Cladoniaceae, the 

 genera of which are characterized by the twofold thallus, 

 one portion being primary, horizontal and stratose, the 

 other secondary and radiate, the latter an upright simple 

 or branching structure termed a "podetium' which nar- 

 rows above, or widens to form a trumpet-shaped cup or 

 "scyphus" (Fig. 66). The apothecia are terminal on the 

 podetium or on the margins of the scyphi ; in a few species 

 they are developed on the primary thallus. Some degree 

 of primary thallus-formation has been demonstrated in all Fig- 66. Ciadonia 

 the genera, if not in all the species of the family. The Basal "squamule" and 

 genus Cladina was established to include those species podetium. a, apo- 



thecia ; j, spermo- 



of Ciadonia in which, it was believed, only a secondary gonia (after Krabbe). 



