46 



LICHENALES. 



[Thallogens. 



universallv of a green colour, and either lie singly or in clusters beneath the cortical 



layer of the thallus, or break out in clus- 

 3 r 4 ters called soredia, or in cups called 



cypheha. 



" Nothing can be more varied than the 

 appearance of Lichens. If the grey, and 

 yellow, and brown stams upon old walls, 

 ancient chiu'ches, and other buildings are 

 carefully exammed, those appearances will 

 always be found to arise from minute 

 Lichens ha^ing taken possession of the 

 sui'face of the stones, to which they adhere, 

 dra'\\'ing their food from the atmosphere ; 

 small shields are scattered over their sur- 

 face, sometimes round,but not imfrequently 

 like dark clefts or lines, gi\'ing the Lichen 

 the appearance of being covered with 

 broken letters. Others are found on trees 

 and pales, forming broad patches of various 

 colours, some being of the richest golden 

 yellow ; others spread upon the ground in plantations and heaths — these have usually a 

 much larger growth ; some again hang from the branches of venerable trees, which they 

 clothe with a shaggy beard of grey ; and, finally, a few start up upon the heath, grey and 

 deformed, but eventually fashioning themselves into tiny goblets, the border of which is 

 studded with crimson shields. According to Fries, Lichens " are types of Algals born in 

 the air, interrupted in their development by the deficiency of water, and stimulated into 

 formmg a nucleus by hght. No Lichen is ever submersed (Verrucaria submersa is an 

 exception) ; there is none of which the vegetation is not mterrupted by the variable 

 hygrometrical state of the atmosphere ; and, finally, none that ever develop in mines, 

 caverns, or places deprived of light. On this account, their shields are more rare m the 

 fissures of momitams, 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 distmguishable from Hydrophycse (forms of Algals) ; as, for 

 instance, CoUema, &c. But these plants, when exposed to the sun, do perfect their 

 sliields, as is found by Nostoc lichenoides, foliaceum, &c., which, when dry, are ascer- 

 tained to be Collema limosum, flaccidum, &c., surcharged with water." By being 

 acquauated with this rule, the same author says, he has succeeded in discovering many 

 Swedish Lichens with shields, which have for many years been constantly fomid sterile ; 

 as Parmelia conoplea, lanuginosa, gelida, &c. ; and he even asserts that he has suc- 

 ceeded artificially m inducing sterile Lichens to become fniitful, as Usnea jubata, and 

 others, — Plant. Horn. 224. Lichens consist, according to EscliAveiler, of a medullary 

 and a cortical layer of tissue, of Avhich the former is imperfectly cellular or filamentous, 

 and bursts through the latter in the form of shields (apothecia), which contain a nucleus, 

 consisting of a flocculent gelatinous substance, among which lie the cases of sporules. 

 These cases (thecse) are transparent membranous tubes, either simple or composed 

 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 Lichens are stated 

 to be in all cases developed in humidity, and to be, in fact, at that time, mere 

 Phycese or Confervse ; but as soon as the humidity diminishes, the under part dies, 

 and an inert leprous crvist is formed, which ultimately becomes the basis of the 

 plant. Hence Lichens consist of two distinct sorts of tissue, — livmg cellules fonnmg 

 the vegetating part, and dead cellules the cohesion of which is lost ; when separate, 

 the former is Palmella botryoides, and the latter Lepraria. Of these two sorts of mat- 

 ter, the leprous is incapable of perpetuatmg the Lichen, while every part of the firing 

 stratum has been ascertamed to become reproductive matter. See Fries, as above 

 quoted, and Meyer Veljer die Entivickehing, dr., cler Flechten. The investigations of the 

 latter are exceedingly interesting. By sowmg Lichens, he arrived at some curious con- 

 clusions, the chief of which are, that, like other imperfect plants, they may owe their 

 origin either to an elementary, or a reproductive, generating power — the latter 

 capable of development hke the plant by which they are borne : that decomposed 

 vegetable, and some morganic, matter, are equally capable of assumuig organisation 

 under the influence of water and light ; and that the pvdverulent matter of Lichens is 



Fig. XXXI. — Section of the shield of Parmelia tiliacea ; the green gonidia are the black dots beneath 

 the skin ; 2. a portion of the same more magnified, showing the spore-cases and paraphyses ; 3. a 

 morsel of the shield of Cladonia coccifera ; 4. spores of Parmelia parietina. 



