etal, as in Cetraria. In Cladonia and Stereocaulon, we observe a 

 double thallus, that is, a vertical, which we call podeiium, arising 

 from the primary horizontal ; and the former is, again, by the evolu- 

 tion of gonidia, also besprinkled with another kind of horizontal 

 thallus in the shape of leaf-like scales. The horizontal thallus is 

 either, I. Crustaceous ; or, II. Foliaceous ; which are well distin- 

 guished by habit, though their limits are not strictly definable. 

 Thus a crustaceous lichen may become effigurate at the margin, or, 

 with age, all over ; and a foliaceous lichen, either by its evolution 

 being hindered, or by anamorphosis, becomes often crustaceous. 

 (This suggests another reason for caution in judging of lichens from 

 a hasty view of the mere habit.) A crustaceous thallus of the most 

 simple kind is one equally explicated and concrete of the hypothal- 

 lus (th. contiguus), which, with age, becomes often cracked and 

 chinky. Next to this is the thallus coming forth from the hypothal- 

 luSy here and there conglobated, which is either innate in the hypo- 

 thallus (th. areolatus\ or more discrete from the hypothallus (th. 

 granulosus), the granula being rounded in the latter, and flattened 

 (areolas) in the former. By the coalition of many areolas or granula 

 these become lobate, and pass thus into a new form (th. crustaceo- 

 squamulosus.) In the younger state of a crustaceous thallus, it is, 

 sometimes, from the presence of cylindrical cells, plicato-radiose at 

 the margin, while yet in its older state, the lichen, in consequence 

 of the vegetation of these cells becoming suppressed, returns to a 

 uniform crustaceous habit A foliaceous tballus is either originally 

 composite (th. squamulosus, to be cautiously distinguished from the 

 th. crustaceo-squamulosus), or it is more simple and entire (th.fron- 

 dosus,th. monophyllus.) Of the vertical thallus there are two kinds, 

 the compressed (th. subfoliaceus), and the terete (th.fructiculosus). 

 Of both of these the filamentous and the pendulous thalli are degen- 

 erations ; of which, indeed, there occur instances in the merely fo- 

 liaceous thallus. 



Among the lowest tribes of lichens, we find the thallus innate in 

 the matrix of the plant (th. hypophlaodes) wherein the hypothallus is 

 deficient, and the thallus is inconspicuous. Much as these recede 

 from other lichens, it is only as a degeneration, nor are the limits 

 determinable between the thallus which is innate in the matrix, and 

 that which is superficial (th. ectopMceodes). 



Auctde Morphosi: Fries, Lichenogr. p. 58; Meyer, Entwick. p. 

 316, &c. ; Wallroth, Naturgesch. der Flechten 1. p. t4, &c. 



III. METAMORPHOSIS. 



The only normal metamorphosis in Lichenes is the formation of 

 Jlpothecia. These repeat, in narrower limits, the elementary organs 

 ot the thallus, which appear in them more defined and ennobled. 

 In the structure of the parts of fructification, lichens agree with 

 fungi, and hence these pirts in both classes have the same names. 

 We distinguish in the apothecium first the Thalamium, or fructifica- 

 tion itself, and second, the Excipulum, or that which contains it. In 

 the thalamium we find sporidia and asci ; the sporidia being either 

 naked, or included in the asci, which are themselves sometimes in- 



