327 



LIST OF THE ORDERS. 

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



CCLXX. LICHENES. The Lichen Tribe. 



Alge, §3. Lichenes, Juss. Gen. 6. (1789.)— Lichenes, Hojm. Enumerat. Lichenum, (1784) ; 

 Acharius Prodr. Lichen. (1798); Id. Me t hodus, (1803) ; Id. Lichcnogr.Univers. (1810); 

 Dec. Fl. Fr. 2. 321. (1815) ; Fries in Act. Holm. (1821 ) ; Agardk Aph. 39, (1821) ; Eschweiler 

 Syst.Lich. (1824); WaUroth JS'aturgesch. der FLcchten. (1824); Grev. Flora Edin. xix. (1824); 

 Meyer iibcr d>c Entuickelung, <$*c. der Flecht. (1825); Fee Meth. Lich. (1825); Fries 

 Syst. Orb. Vcg. 224 (1825) ; Murtius in Bot. Zeitung, 193. (1826) ; Fee in Diet. Class 9, 360. 

 (1826);— Hypoxyla. in part, Dec. Fl. Fr. 2, 280 (1815); Grev.Fl. Edin. xx. (1824).— Gra- 

 fhidejE, ChevaJ.lier' 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 1 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 filamentous ; in the 

 crustaceous species the cortical and medullary layer differ chiefly in texture, and in the for- 

 mer being coloured, the latter colourless ; but in the fruticulose or foliaceous species, the me- 

 dulla 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 1 

 in membranous tubes, (thecal) 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 1 to Fries, Lichens are types of Alg& 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 interrupted 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 

 distinguishable from Hj'drophycaj (forms of Alga?) ; 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, flaccidum, &c, surcharged with water. By being ac- 

 quainted 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 conoplea, lanuginosa, gelida, <fcc. ; 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 imperfectly cellular or filamentous, and bursts through 

 the latter in the form of shields (apothecia), which contain a nucleus, consist- 

 ing of a flocculose-gelatinous substance, among which lie the cases of sporules. 

 These cases (thecal) are transparent membranous tubes, either simple or com- 

 posed of several placed end to end, which either lie free in the nucleus, or are 

 themselves contained in other membranous cases (asci). In the beginning 



