LICHENS. 



[ 464 ] 



LICHENS. 



niedullary layer very solid ; and in some of 

 them its lowest filaments are seen growing 

 put all round the borders, in the direction 

 in which the plant is extending, the upper 

 filaments with the gonidia and the cortical 

 layer by degrees overgrowing these lowest 

 filaments, which in the meantime have 

 extended further out. Some of the crus- 

 taceous 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 (Physcid) parietina, 

 the yellow Lichen so abundant on walls, 

 and Borrera ciliaris, common on branches 

 of trees (fig. 396), may serve as examples. 

 The thin paper-like thallus of the former 

 exhibits four distinct regions (PL 37. 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. These are in- 

 terlaced and imprison air between them. 

 From the lower face arise laminae or fibrous 

 processes, like roots (rhizinee), 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 cor- 

 tical layer, the filamentous medullary struc- 

 ture forming the irregular veined surface, 

 prolonged here and there into pseudo-radical 

 processes. In Endocarpon and other fronds 

 of solid texture, the medullary layer is 

 formed of slender linear cells, closely packed, 

 with few air-passages. The species of Cla- 

 donia exhibit a structure of the thallus 

 intermediate between that of the foliaceous 

 kind just referred 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 goni- 

 dial layer, which again rests on the closely- 

 felted filamentous medullary substance. In 

 the branches of C. rangi/erina, 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 

 filaments, as is the case also with those of 

 Ramalina 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 cel- 

 lular, gonidia are here and there to be ob- 

 served. 



In many Lichens, when exposed to excess 

 of moisture, the proper fructification is not 

 developed, and the gonidial structure is 

 produced so abundantly as to burst through 

 the superficial cortical layer and become 

 naked, giving a mealy appearance to the 

 thallus. Lichens reproduced by gonidia 

 commonly grow at first into a pulverulent 

 stratum by the multiplication of the cells, 

 giving rise to the forms which were at one 

 time thought distinct genera, such as Le- 

 praria. 



The gonidia have been stated to produce 

 motile zoospores; but these are now con- 

 sidered molecular granules. 



The fronds of Collema are remarkable foi 

 their gelatinous texture, and approach the 

 Nostochacese (Algae) in the simplicity of 

 their structure ; thus, the thallus of C. 

 clieileum consists of branched and colourless 

 filaments Or tubes imbedded in an abundance 

 of mucilage ; in C.jacobecefolium, there exist 

 in addition very numerous granula gonima, 

 almost all arranged in long beaded lines 

 (PI. 37. fig. 13), some being larger than 

 others, the whole mixed with the con- 

 tinuous filaments and imbedded in mucus. 

 Both species have long, whitish, branched, 

 filamentous pseudo-radical processes. 



Putting aside the gonidia or gemmule- 

 cells of the thallus, the reproductive organs 

 of the lichens are of three kinds: 1, the 

 apothecia, which, according to their forms, 

 receive different names, and are all charac- 

 terized by producing the sacs (thecce) con- 

 taining spores; 2, the spermogonia, which 

 some regard as antheridia, and which pro- 

 duce extremely minute cylindrical bodies 

 (spermatia) growing at the ends of short 

 pedicels, from which they are ultimately 

 detached, like the spores of many Fungi ; 



