BRITISH LICHENS. 23 



If a thin, perpendicular section be taken of one of the foliaceouB 

 species, say for example Phi/Kcia parietina, and placed under the micro- 

 scope with a little water, there will be presented to the eye of the student 

 three distinct layers. (Plate V., Fig. 9.) 



1st. — The cortical* layer, which forms the upper surface or bark of 

 the thallus. It is composed of minute cells, closely compacted together, 

 constituting a firm membranaceous tissue. (Plate V., Fig. 9(-/.) The cells vary 

 in size and outline, the latter being modified by their mutual pressure. 

 The cell walls are comparatively thick, and by transmitted light show a 

 faint degree of colour, more conspicuous in verj- old specimens of the 

 Lichen. Beneath this cortical layer will be seen — 



2nd. — The gonidialj layer, which consists of cells filled with 

 chlorophj'll, sphei-ical, or nearly spherical, varying in size, and easily 

 separated from each other by the slightest pressure. Thei'e would 

 appear to be no actual union between these bodies, though produced one 

 from the other, but they lie loosely together in a tolerably well defined 

 stratum. (Plate V., Fig. 96.) They occur in the thallus of all Lichens, 

 though they differ considerably in form, being first, cells enclosing green 

 granular protoplasm, as in those we have now under the microscope ; 

 second, cells containing several gi-auules, devoid of any cell membrane, 

 called fjranula gonima ; and, third, cells arranged in chains or necklaces. 

 (Plate v., Fig. 10.) These gouidia often thrust themselves vip through the 

 surface of the cortical layer, in clusters or heaps, producing the conditions of 

 the thallus known as a sorediate\ thallus. Some very fanciful notions have 

 in late years been broached with regai'd to the relationship the gonidia 

 bear to the thallus, to which we cannot, in the limited space at our disposal, 

 give more than a passing notice ; at the same time it must be admitted that 

 they have the power of carrying on an existence independent of the other 

 constituents of the thallus, and will, on a favourable surface, multiply 

 themselves till they form a p seudo-thallus, but do not produce apothecia. 

 Beneath the gonidial layer ^ay be seen 



3rd. — The mediiUr: layer composed of colourless interwoven 



filaments, branched and tubular, being divided within at cex-tain intervals 

 by walls or septa. (Plate V., Fig. 9c.) A section of the thallus of Pe/fi't/era 

 canina will show both this and the preceding layers in a clearer manner 

 than Parmelia saxatilh. Beneath the medullary layer there are often 

 present in foliaceous species the rhizhuc which have already been referred 

 to, serving the purpose of attachmg the Lichen firmly to its place of 

 growth ; in Parmelia saxatilis they are black, short, and rigid, but in 

 Peltigera canina they are long, pale, and flaccid. 



If we had selected a crustaceous Lichen for examination, as for 

 example Lecidea geographiea, (Plate V., Fig. 7,) we should have found the 



* From cortex, Latin for rind or bark. 



t From "ybvT], generation, and «5os, resemblance, indicating their resemblance 

 in fimction to the spores. 



: The term soredia is from the Greek word (Tojpos, a heap, a pile, indicating the 



little heaps of gonidia. 

 11 From medulla, Latin for pith or marrow. 



