LICHENS. 



[ 389 ] 



LICHENS. 



to the naked eye, the thallus being com- 

 posed of very small collections of micro- 

 scopic elements, more or less concealed in 

 the matrix on which the plants grow. 



In the simplest kind of Lichens, the frond 

 or thallus consists of microscopic branched 

 filaments penetrating among the superficial 

 layers of the cells of the bark upon which 

 the plants grow. These filaments present 

 globular cells here and there growing out 

 from them, filled with green matter, which 

 globular cells are capable of reproducing the 

 plant when detached ; they are called goni- 

 dia, and are regarded as analogous to the 

 buds of the Flowering plants and the cellular 

 gemmce of the higher Cryptogams. In the 

 simple forms here alluded to the gonidia are 

 not sufficiently numerous to give a coloured 

 tinge to the structure as seen by the naked 

 eye; in some even the filaments make no 

 show, while in others they form whitish 

 patches (Opegrapha, Verrucaria). In the 

 forms rather more developed we find a layer 

 of globular epidermal cells, with whitish 

 contents, closely coherent together, consti- 

 tuting a ' cortical layer' covering the upper 

 surface, to which the filamentous structure 

 then forms the * medullary layer/ The 

 crustaceous kinds overgrowing stones have 

 this filamentous medullary layer very solid, 

 and in some of them its lowest filaments are 

 seen growing out all round the borders, in 

 the direction in which the plant is extend- 

 ing, the upper filaments with the gonidia 

 and the cortical layer by degrees overgrow- 

 ing these lowest filaments, which in the 

 meantime have extended farther out. Some 

 of the crustaceous Lichens grow out in more 

 or less regular lobes at their borders, and 

 thence lead to the pseudo-foliaceous forms, 

 of which the common Parmelia parietina, 

 the yellow Lichen so abundant on walls, and 

 Borrera ciliaris, common on branches of 

 trees (fig. 401), may serve as examples. The 

 thin paper-like thallus of the former exhi- 

 bits four distinct regions (PI. 29. fig. 2) : 

 1. on the upper face a layer of thick cells, 

 firmly connected together, coloured yellow 

 at the surface (upper cortical or epidermal 

 layer); 2 a layer like the preceding, but 

 white, forming the inferior surface of the 

 thallus (lower cortical or epidermal layer) ; 

 3. beneath the upper cortical layer lie the 

 gonidia ; and 4, under these lie the medul- 

 lary filaments forming the central substance, 

 at the upper part of which lie the gonidia 

 arising from these filaments, which are in- 

 terlaced and imprison air between them. 



From the lower face arise laminae or fibrous 

 processes, like roots, serving as cramps by 

 which the plant attaches itself to the surface 

 on which it grows. In Peltigera canina there 

 is no inferior epidermal or cortical layer, the 

 filamentous medullary structure forming the 

 irregular veined surface, prolonged here and 

 there into pseudo-radical processes. In 

 Endocarpon and other fronds of solid tex- 

 ture, the medullary layer is formed of slen- 

 der linear cells, closely packed, with few 

 air-passages. The species of Cladonia ex- 

 hibit a structure of the thallus intermediate 

 between that of the foliaceous kind just re- 

 ferred to and the shrubby sort. In the 

 foliaceous expansion resting on the ground, 

 of C. pyxidata, for example, we detect the 

 upper epidermis, next the gonidial layer, 

 which again rests on the closely-felted 

 filamentous medullary substance. In the 

 branches of C. ranaiferina, as in a great 

 number of its congeners, there is no well- 

 defined epidermis. The branches are tubes, 

 vacant in the centre, formed of a cartilagi- 

 nous structure, in which only two zones can 

 be distinguished, the inner and more solid 

 of which is composed of almost simple, pa- 

 rallel, solid filaments intimately glued toge- 

 ther by mucous substance ; the outer zone 

 is formed of a felted mass of filaments, like- 

 wise solid, but branched and divaricated. 

 The solidity of these filaments arises from 

 the obliteration of the cell-cavity by second- 

 ary layers on its walls, giving the filaments 

 a horny texture. In the outer loose layer 

 are found scattered groups of gonidia. In 

 Stereocaulon denudatum the branches are 

 solid and formed exclusively of parallel fila- 

 ments, as is the case also with those of Ra- 

 malina scopulorum. In Evernia vulpina 

 there is a solid axis formed of parallel fila- 

 ments enclosed in a layer of interlaced fibres, 

 between which and the horny coat, which is 

 either solid or very obscurely cellular, goni- 

 dia are here and there to be observed. 



The fronds of Collema are remarkable foi 

 their gelatinous texture, and differ greatly IE 

 organization from the foregoing, approach- 

 ing the simplicity of the Nostochacese( Algae). 

 The thallus of C. cheilewn consists of branched 

 and colourless filaments or tubes, imbedded 

 in an abundance of mucilage; in C.jacobece- 

 folium, there exist in addition very numerous 

 green granules, almost all arranged in long 

 beaded lines (PL 29. fig. 13), some being 

 larger than others, the whole mixed with 

 the continuous filaments and imbedded in 

 mucus. Both species have long, whitish, 



