42 



POPULAR HISTORY OF LICHENS. 



with, they usually appear first towards the centre, sometimes 

 extending centrifugal ly over its whole surface, so as to give 

 it a general pulverulent appearance; sometimes they are 

 developed at the extremities of the thalline lobules, or la- 

 cinicBy as in Parmelia ceratophylla, var. p/iysocles ; some- 

 times they occur in orbicular or globular masses, sprinkled 

 over a fruticulose thallus, as in Ramalina farinacea. Scat- 

 tered sparingly over the surface of a thallus or apothecium, 

 they give it a pruinose or frosted, farinose or mealy appear- 

 ance, according to their quantity and colour. The disc of an 

 apothecium is sometimes rendered abortive by sorediiferous 

 degeneration ; the soredia then become discoid or globular, 

 assuming the form of the abortive apothecium. This is the 

 basis of the old genus Variolaria, which is now found to 

 be merely an abnormal condition of the thallus or its fructi- 

 fication, depending on a hypertrophy, or excessive develop- 

 ment, of the gonidic element. This variolarioid condition 

 is not uncommon in many crustaceous species, as those of 

 the genera Pertusaria and Lecanora. On the under surface 

 of the thallus of the genus Sticta, soredia occur in the form 

 of minute urn or cup-shaped bodies, called .Cyphelltf, which 

 appear to be excavated in its substance,— sometimes white, 

 more frequently yellow or greenish ; on the upper surface 



