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POPULAR HISTORY OP LICHENS. 



2. Umbilicaria pustulata {pustula, a pock or blister). 

 Thallus greyish-pruinose, pustulate, usually besprinkled with 

 dark greenish powdery masses, olive-coloured when moist- 

 ened, — below smoothish, brownish, reticulate-laeunose; apo- 

 thecia sessile, orbicular, somewhat simple, with a thick, often 

 roughened margin. 



A peculiar and distinctly-marked species, not uncommon 

 on granitoid rocks on the tops of various Highland moun- 

 tains : it is seldom found fertile, but Sir W. Hooker mentions 

 having gathered it in fructification in Skye. It possesses a 

 double cortical layer, the superior being thin and composed 

 of small polygonal cellules intimately united ; the inferior 

 comparatively thick, horny, and very hygrometric, formed 

 of globular, thick-walled cellules, so closely united that their 

 individual boundaries are not recognizable ; the free surface 

 of the latter is marked by the presence of an infinite number 

 of minute conical papillae composed of the same tissue. Its 

 thecse are somewhat short and broadened, containing one 

 perfect or sometimes two abortive spores; the spores are 

 large, oval, and muriform or cellular (containing a great 

 number of secondary cellules, arranged in parallel rows like 

 the bricks in a wall, or irregularly). Its spermogones are 

 rare, and occur in the form of isolated obtuse tubercles; 



