331 



state of hi<2;h composition. In short, it is here only that the physical properties 

 of vegetable matter can be usefully studied. 



LIST OF THE ORDERS. 



270. Lichenes. | 271. Fungi. [ 272. Algae. 



CCLXX. LICHENES. The Lichen Tribe. 



ALGiE, §3. Lichenes, Juss. Gen. 6. (1789) — Lichenes, Iloffm. Ennmerat. Liche- 

 num, (1784); Acharius Prodi: Lichen. (1798) ; Id. Methodus, (1803); Id. Liche- 

 nogr. Univers. (1810); Dec. Fl. Fr. 2. 321. (1815); Fries in Act. Holm. (1821); 

 Agardh Aph. 89. (1821); Eschweiler Syst. Lich. (1824); Wallroth Naturgeach. der 

 Flechten, (1824) ; Grev. Flora Edin. xix. (1824) ; Meyer uber die Entwickelung, ^c. 

 der Flecht. (1825); Fee Mtth. Lich. (1825); Fries Syst. Orb. Veg. 224. (1825); 



Martins in Bat. Zeitung, 193. (1826); Fee in Diet. Class. 9. 360. (1826) Hy- 



POXLA, in part, Dec. Fl. Fr. 2. 280. (1815); Grev. Fl. Edin. xx. (1824). — Gra- 

 PHiDE^E, Chevallier Hist, des Graphidees. (1824, &c.) 



Diagnosis. Aerial, leafless, flowerless, perennial plants, with a distinct 

 thallus, and external disk containing sporules. 

 Anomalies. 



Essential Character. — Perennial plants, often spreading over the surface of the 

 earth, or rocks or trees in dry places, in the form of a lobed and foliaceous, or hard and 

 crustaceous, or leprous substance, called a thallus. This thallus is formed of a cortical and 

 medullary layer, of which the former is simply cellular, the latter both cellular and fila- 

 mentous ; in the crustaceous species the cortical and medullary layer differ chiefly in tex- 

 ture, and in the former being coloured, the latter colourless ; but in the friiticulose or folia- 

 ceous species, the medulla is distinctly floccose, in the latter occupying the lower half of 

 the thallus, in the former enclosed all round by the cortical layer. Reproductive matter of 

 two kinds ; 1 , sporules lying in membranous tubes {thecce) immersed in nuclei of the medullary 

 substance, which burst through the cortical layer, and colour and harden by exposure to 

 the air in the form of little disks called shields ; 2, the separated cellules of the medullary 

 layer of the thallus. 



Affinities. According to Fries, Lichens are types of Algee born in 

 the air, interrupted in their developement by the deficiency of water, and 

 stimulated into forming a nucleus (or receptacle of sporules) by light. No 

 Lichen' is ever submersed ; there is none of which the vegetation is not inter- 

 rupted by the variable hygrometrical state of the atmosphere ; and, finally, 

 none that ever developes in mines, caverns, or places deprived of light. On 

 this account, their shields are more rare in the fissures of mountains, or in 

 shady groves, than in places fully exposed to light. In wet places, also, their 

 shields are not produced ; for so long as they are under the influence of 

 water they are hardly distina:uishable from Hydrophycse (forms of Algae) ; as, 

 for instance, Collema, &c. But these plants, when exposed to the sun, do 

 perfect their shields, as. is found by Nostoc Lichenoides, foliaceum, &c., 

 which, when dry, are ascertained to be Collema limosum, flacciduni, &c., 

 surcharged with water. By being acquainted with this rule, the same author 

 says, he has succeeded in discovering many Swedish Lichens with shields, 

 which have for many years been constantly found sterile ; as Parmelia cono- 

 plea, lanuginosa, gelida, &c. ; and he even asserts that he has succeeded 

 artificially in inducing sterile Lichens to become fruitful, as Usnea jubata, 

 and others. Plant. Horn. 224. Lichens consist, according to Eschweiler, 

 of a medullary and a cortical layer of tissue, of which the former is imper- 

 fectly cellular or filamentous, and bursts through the latter in the form of 

 shields (apothecia), which contain a nucleus, consisting of a flocculose-gela- 

 tinoas substance, among which lie the cases of sporules. These cases (thecae) 



