GENERAL CHARACTERS OF BRITISH LICHENS. 57 



thallus in a hypertrophic or abnormal condition is productive, 

 as we have to some extent already seen, of many thalline mal- 

 formations; it constitutes various epithalline growths, in the 

 form of warts, tubercles, squamules, or folioles ; it sometimes 

 covers the surface of the thallus with a thick, powdery coat- 

 ing, — nay, it is the basis frequently of a pulverulent dege- 

 neration, characterized by the destruction of all other tissues 

 of the thallus. In a word, the pulverulent, sorediiferous, 

 variolarioid, and isidioid forms of the thallus, to which a 

 hypertrophic condition of the gonidia gives birth, so com- 

 pletely change its aspect, as to have led the older authors 

 to constitute species possessing such a vegetative system 

 into distinct genera. 



The Reproductive system of Lichens is divisible into the 

 Primary — normal or typical, wherein it resembles,— and the 

 Secondary, oy supplementary, whereby it is distinguished 

 from, that of plants higher in the scale of vegetation. The 

 primary reproductive system consists of the following parts 

 or organs : — 1. Apothecia, which generate and protect the 

 cellular embryos or spores, by whose germination and sub- 

 sequent development the species is reproduced ; 2. Spermo- 

 gones, which produce and discharge at certain periods of the 

 growth of the plant minute bodies, Spermatia, whose function 



