GENERAL CHARACTERS OE BRITISH LICHENS. 37 



and Stereocaulon. These branch- or stem-like developments 

 of, or prolongations upward from, the thallus may be solid 

 or hollow. In the latter case they are frequently cylindrical, 

 and dilated at their apices into cup or funnel-shaped cavi- 

 ties, the margins of which bear the fructification. Such a 

 body may be regarded as a secondary or vertical thallus, 

 developed from the cortical substance of a primary, small, 

 horizontal thallus ; it is designated a podetium (77-01)9, Gh\, 

 pes, Lat., a foot), and is characteristic of the Cladonias. 

 On this podetium there is frequently to be found a ter- 

 tiary or second horizontal thallus, growing from its surface 

 more or less abundantly in the form of minute foliaceous 

 squamules. Pruticulose Lichens include some of the most 

 valuable dye-species, as the Roccellce ; and some of the 

 commonest species growing on our forest-trees, as the genus 

 Ramalina, — on our heaths, as the genus Cladonia, — and on 

 our hills, as the genera Sphterophoron and Stereocaulon. 



The filamentous thallus likewise forms a shrub-like mass 

 consisting of flaccid, or somewhat rigid, round, thread-like, 

 or filamentous segments, — sometimes, and especially in the 

 young state, growing erect, more frequently pendulous, and 

 even occasionally prostrate, such as the genus Usnea and 

 some species of Cornicularia. Filamentous Lichens are 



