378 INTRODUCTION TO CRYPTOGAMIC BOTANY. 



by merely a slight stain in those parts of the bark which 

 produce perithecia. In Opegrapha, Arthonia, and the neigh- 

 bouring genera, a crustlike thallus of greater or less thickness 

 is produced, sometimes tartareous, but often hard and firm, 

 with the surface even, or cracked and granulated. Except in 

 the most obscure cases, gonidia are always present, however 

 thin the crust may be. 



415. The crustlike Lichens are, however, by no means con- 

 fined to the perithecial or opegraphoid species. Whole series 

 occur, belonging to Lecidea, Lecanora, &c., where the thallus 

 is crustaceous, and sometimes the medullary stratum approaches 

 in character to the vesicular structure of the cortical. By slow 

 gradation the crust exhibits foliaceous granules and expansions 

 on its surface, sometimes obscure, but at length distinct, and 

 then it is but a step to the expansion of these minute fronds 

 into regular leaflike processes, though still retaining the 

 same essential structure. These fronds may be smooth below, 

 or densely velvety or spongy, while occasionally in the midst 

 of the downy clothing there are regular pits exposing the 

 inner substance. If the fronds are very narrow, we have fucoid 

 forms, and much more so when the thallus is cylindrical and 

 either free from external organs, as in Usnea, or sprinkled 

 with little fronds, as in Genomyce. The medullary stratum in 

 the highly branched species forms a tough cord in the centre, 

 which is at length exposed by the cracking of the less extensile 

 cortical layer. 



416. The fructification is, in true Lichens, contained in dis- 

 tinct discs or perithecia, exactly as in Ascomycetous Fungi. 

 The hymenium, or thalamium as it is often called, gives 

 rise to asci and paraphyses, the tips of which are often charred 

 by exposure to the sun. In some cases, however, they produce 

 distinct bodies (Fig. 80, d), which are probably reproductive. 

 The sporidia themselves vary much in form and colour, and 

 are often more complicated than in Fungi. Few things can 

 exceed in beauty, as microscopical objects, the sporidia of 

 some species, though Fungi may vie with them even in this 

 respect. In Gassicourtia they are of a bright scarlet, in 

 S'plueroplioron of a more or less deep blue, while in other 



