168 BRITISH LICHENS. 



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

 second, cells containing several granules, devoid of any cell membrane, 

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

 (Plate V., Fig. 10.) These gonidia often thrust themselves up 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 regard 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 pseudo-thallus, but do not produce apothecia. 

 Beneath the gonidial layer may be seen 



3rd. — The medullaryi layer composed of colourless interwoven 

 filaments, branched and tubular, being divided within at certain intervals 

 by walls or septa. (Plate V., Fig. 9c.) A section of the thallusof Peltigera 

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

 than Parmelia saxatilis. Beneath the medullary layer there are often 

 present infoliaceous species the rhizime which have already been referred 

 to, serving the purpose of attaching 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 geographica, (Plate V., Fig. 7,) we should have found the 

 cortical, gonidial, and medullary layers in precisely the same order as above, 

 but instead of rhizituevfe should have seen a very thin layer of dead cellular 

 black matter adhering closely to the surface of the stone, which is called 

 the' hypothalhis.t This is formed by the threads first thrown out by the 

 germinating spores, and is the earliest stage of a Lichen thallus. On 

 quartz rock this hypothallus of Lecidea geographica is often to be seen in 

 black radiating lines, while as yet no other part of the thallus has been 

 formed. 



In some of the gelatinous Lichens, as for example in Collema, the 

 cortical layer is absent, while in an allied genus, Leptogium, it is reduced 

 to the thinnest possible layer ; the gonidial layer and the medullary are 

 also mixed up together in one common stratum. (Plate V., Fig. 10.) 



While a well developed Lichen thallus will have the characters above 

 described, it must not be forgotten that in many species it is reduced to 

 such insignificance that it consists of little more than a few filaments, 

 accompanied by a few gonidia lying beneath the epidermis of the bark of 

 a tree, or running through the disintegrated surface of a rock. In species 

 with such an imperfectly developed thallus specimens will occur in which 

 it cannot be detected, it is then said to be evanescent ; but if present and 



* The term soredia is from the Greek word cwpos, a heap, a pile, indicatiog the 



little heaps of gonidia. 

 t Frommedulla, Latin for pith or marrow. 

 ; From vir& under, and OaXKbs a frond, i.e., that which underlies the thallus. 



