GENERAL CHARACTERS OE BRITISH LICHENS. 69 



there is great variation in size. We would recommend the 

 student to study the spore and the tissues which enclose it 

 in Pertusaria communis, Physcia ciliaris, Parmelia stellaris, 

 and Parmelia parietina, all of which are common species. 

 Their colour is most frequently a pale straw-yellow ; some- 

 times they are brownish or reddish ; a few have an intense 

 olive colour. In the young state most spores are colour- 

 less ; and even when mature, many spores appear colourless 

 individually, while they have a distinctly yellow tint en 

 masse. The spore-wall varies in thickness ; typically, it is 

 composed of two membranes, the outer constituting the 

 JEpispore (iirl, upon or on the outside), the inner being de- 

 nominated the Endospore (evSov, within). Frequently only 

 one of these can be observed, usually the outer. The epi- 

 spore is generally pellucid and smooth; sometimes it is 

 coated with the remnants of the protoplasmic or mucous 

 matter from which it was originally developed ; in one 

 species (Solorina saccata) its surface has been described as 

 granular, and in another [Thelotrema exanthematicum) it is 

 said to be bristled over with very fine, transparent points. 

 The contents of the young spore consist usually of mucous 

 and granular matter, intermixed in some species with a 

 quantity of large oil globules ; as it advances to maturity, 



