88 MORPHOLOGY 



fore, in the most favourable position for photosynthesis. Whether belonging 

 to the Myxophyceae or the Chlorophyceae, they form a green band, straight 

 and continuous in some forms, in others somewhat broken up into groups. 

 In certain species they push up at intervals among the cortical cells, as in 

 Gyrophora and in Parmelia tristis. In Solorina crocea a regular series of 

 gonidial pyramids rises towards the upper surface. The green cells are 

 frequently more dense at some points than at others, and they may pene- 

 trate in groups well into the medulla. 



The fungal tissue of the gonidial zone is composed of hyphae which 

 have thinner walls, and are generally somewhat loosely interlacing* In 

 Peltigera* the gonidial hyphae are so connected by frequent branching and 

 by anastomosis that a net-like structure is formed, in the meshes of which 

 the algae a species of Nostoc are massed more or less in groups. In 

 lichens with a plectenchymatous cortex, the cellular tissue may extend 

 downwards into the gonidial zone and the gonidia thus become enmeshed 

 among the cells, a type of formation well seen in the squamulose species, 

 Dermatocarpon lachneum and Heppia Gttepini, where the massive plecten- 

 chyma of both the upper and lower cortices encroaches on the pith. In 

 Endocarpon and in Psoroma the gonidia are also surrounded by short cells. 



A similar type of structure occurs in Cora Pavonia, one of the Hymeno- 

 lichenes: the gonidial hyphae in that species form a cellular tissue in which 

 are embedded the blue-green Chroococcus cells 2 . 



D. MEDULLA AND LOWER CORTEX 



a. MEDULLA. The hyphal tissue of the dorsiventral thallus that lies 

 between the gonidial zone and the lower cortex or base of the plant is 

 always referred to as the medulla or pith. It is, as a rule, by far the most 

 considerable portion of the thallus. In Parmelia caperata (Fig. 49), for 

 instance, the lobes of which are about 300 n thick, over 200 //, of the space 

 is occupied by this layer. It varies however very largely in extent in 

 different lichens according to species, and also according to the substratum. 

 In another Parmelia with a very thin thallus, P. alpicola growing on quart- 

 zite, the medulla measures scarcely twice the width of the gonidial zone. 

 It forms a fairly massive tissue in some of the crustaceous lichens in some 

 Pertusariae and Lecanorae attaining a width of about 600 /*. 



Nylander 3 distinguished three types of medullary tissue in lichens: 



(1) felted, which includes all those of a purely filamentous structure; 



(2) cretaceous or tartareous, more compact than the felted, and containing 

 granular or crystalline substances as in some Pertusariae; and lastly 



(3) the cellular medulla in which the closely packed hyphae are divided 



1 Meyer 1902. * See p. 52. 3 Nylander 1858. 



