The pycnides are minute dark-coloured conceptacles, like the 

 perithecia of Sphseriae, sessile on the thallus and containing simple 

 filaments (hasidia) bearing singly on their apices, bodies, often 

 oblong, but variable in form and size (stylospores). 



On the under-surface of the thallus of the Stictei are minute 

 urceolate pale excavations termed cyphella, whose function is 

 unknown. These are generally empty, but when filled with white 

 or citrine powder are called pseudocyphellce. 



The thallus of several genera bears on its upper surface orbicular 

 and flattened, or globular tubercles named cephalodia, consisting 

 internally of cells or granula gonima. Their use is unknown. 

 This upper-surface is also frequently found exuberant with minute, 

 erect, coral-like excrescences with dark-coloured apices, and then 

 constitutes the old pseudo-genus Isidium, which is now known to 

 be really an anomalous state of the thallus of some Pertusaria. 

 Black powdery patches or spots are scattered on the thallus not 

 unfrequently, which formerly constituted the old pseudo-genus 

 Spiloma, but are now considered to consist of minute parasitical 

 fungilli. 



In the genus Pertusaria the apothecia become abortive and 

 sterile and converted into soredia, on which state was anciently 

 founded a pseudo-genus Variolaria. 



The apothecia of certain lichens possess the power of dissolving 

 the surface of the calcareous rocks on which they grow, by means 

 of carbonic acid obtained from the atmosphere, thus forming 

 minute shallow cavities or foveoli, in which they partially bury 

 themselves. 



Of the uses to which Lichens may be applied it may suffice to 

 state generally that the " Iceland Moss," Getraria islandica, con- 

 tains starch and a bitter principle, and is used as a tonic and 

 demulcent in medicine, and in Iceland boiled in milk as food. 

 Cetraria nivalis is also nutritious. Roccella tinctoria, Lecanora 

 parella, Lecanora tartarea, Parmelia saxatilis, P. conspersa, &c, 

 afford valuable dyes. In Sweden, Norway, Finland, and Russia, 

 alcohol or brandy is distilled from Cladina rangiferina, which 

 lichen and other Cladoniw are also invaluable in the northern and 

 arctic regions as the food of Reindeer and cows. And in Africa 

 the curious Lecanora esculenta which rolls, in a detached state, over 

 the sandy plains, is collected as food for men and horses. 



The geographical distribution of Lichens over the world varies 

 according to the different zones, and may be stated generally 

 thus : — In the Arctic regions the Cladonice and Stereocaula occur 

 in immense abundance, covering tracts of country, many square 

 miles in extent, to the almost exclusion of other vegetation, but 

 diminish in profusion gradually as the warmer regions are 

 approached. In the Temperate zone the corticolar Lichens, such as 

 Lecanorei, Lecidei, Graphideos, and Verrucarei abound. In the Trop- 

 ical zone Lichens attain their maximum development in the Stictce, 

 andPawnefe/ theleaves of evergreen trees are covered with IStrigulce; 



