GENERAL CHARACTERS OE BRITISH LICHENS. 63 



nucleus in the medullary thalline tissue ; it gradually becomes 

 enlarged and approaches the surface. In some cases — the 

 majority of the Gymnocarpi — it bursts through the cortical 

 layer, which may or may not form a margin, in the former 

 case constituting a rudimentary exciple; in a few instances 

 the cortical layer, an extension thereof, or a peculiar mem- 

 branous veil, covers the apothecium up to a certain stage of 

 its development, as in Peltigera ; while in the Angtocarpi 

 the cortical layer would appear to be extended over the 

 nucleiform thalamium in the shape of the perithecium. 

 This capsule or outer covering — the perithecium — which is 

 frequently black, and sometimes red or pale-coloured, is 

 generally lined by a somewhat tough membrane, dark or 

 pale-coloured, which directly encloses the thalamium. The 

 colour of the thalamium is not fully developed until the 

 apothecium arrives at maturity; it is as varied as that of 

 the thallus. In the very young state it is pale or colourless ; 

 when mature it has various shades or combinations of 

 brown, black, red, and yellow. In some species its colour 

 is very vivid and beautiful ; for instance, the bright scarlet 

 apothecia of a section of the Cladonias. The colour of the 

 thalamium resides in, and is due to, the terminal cells of 

 the paraphyses, its chief constituent elements, of which we 



