GENERAL CHARACTERS OF BRITISH LICHENS. 67 



from the difficulty of its recognition — that it is sometimes 

 absent. The contents of the young theca are, first, a hyaline, 

 mucous, or hazy granular matter. In progress of develop- 

 ment this becomes more coarsely granular, and in some spe- 

 cies, such as Lecanora pallescens var. Parella and Pertusaria 

 communis, large oil globules, in greater or less abundance, 

 make their appearance ; a number of the larger granules be- 

 come aggregated in nuclear masses : these assume a definite 

 form and acquire a membranous investment. The outline 

 of the young spores is now distinctly visible ; the granules 

 fuse themselves into a more homogeneous mass ; the spore 

 enlarges, its wall thickens, and it gradually acquires colour. 

 Each theca contains usually, when fully developed, eight 

 spores, — sometimes only four, two, or even one ; very seldom 

 does it contain an innumerable quantity, as in some jEndo- 

 carpons. When the spores arrive at maturity, the theca be- 

 comes ruptured at the apex, so as to allow of their escape ; 

 sometimes it is ruptured or partially decayed considerably 

 prior to this period; and in exceptional cases it is even 

 said to be altogether absent, its place being supplied by 

 the spore-sac. After it has fulfilled its office — the genesis 

 and bringing to maturity of the spores — the theca becomes 

 shrivelled and disappears ; but a constant succession of 



