STRATOSE-RADIATE THALLUS 125 



the primary thallus is able to retain moisture for a long time 1 . When the 

 primary thallus is small and feeble the podetia are generally much branched 

 and live in close colonies which retain moisture. Sterile podetia are long- 

 lived and grow indefinitely at the apex though the base as continually 

 perishes and changes into humus. Wainio 2 cites an instance in which the 

 bases of a tuft of Cl. alpestris had formed a gelatinous mass more than a 

 decimetre in thickness. 



I. PlLOPHORUS AND STEREOCAULON 



These two genera are usually included in Cladoniaceae on account 

 of their twofold thallus and their somewhat similar fruit formation. 

 They differ from Cladonia in the development of the podetia which are 

 not endogenous in origin as in that genus, but are formed by the growth 

 upwards of a primary granule or squamule and correspond more nearly to 

 Tulasne's conception of the podetium as a process from the horizontal 

 thallus. In Pilophorus the primary granular thallus persists during the life 

 of the plants; the short podetium is unbranched, and consists of a some- 

 what compact medulla of parallel hyphae surrounded by a looser cortical 

 tissue, such as that of the basal granule, in which are embedded the algal 

 cells. The black colour of the apothecium is due to the thick dark hypo- 

 thecium. 



Stereocaulon is also a direct growth from a short-lived primary squamule 3 . 

 The podetia, called " pseudopodetia " by Wainio, are usually very much 

 branched. They possess a central strand of hyphae not entirely solid, and 

 an outer layer of loose felted hyphae in which the gonidia find place. A 

 coating of mucilage on the outside gives a glabrous shiny surface, or, if 

 that is absent, the surface is tomentose as in St. tomentosum. In all the 

 species the podetia are more or less thickly beset with small variously 

 divided squamules similar in form to the primary evanescent thallus. Gall- 

 like cephalodia are associated with most of the species and aid in the work 

 of assimilation. 



Stereocaulon cannot depend on the evanescent primary thallus for attach- 

 ment to the soil. The podetia of the different species have developed various 

 rooting bases: in St. ramulosum there is a basal sheath formed, in St. coral- 

 loides a well-developed system of rhizoids 4 . 



1 Aigret 1901. ' 2 Wainio 1897. 3 Wainio 1890, p. 67. 4 Reinke 1895. 



