The fungal tissue of the gonidial zone is composed of hyphae which have thinner walls, and are generally somewhat loosely interlacing. In _Peltigera_[356] 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 Guepini_, where the massive plectenchyma 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 Hymenolichenes: the gonidial hyphae in that species form a cellular tissue in which are embedded the blue-green _Chroococcus_ cells[357]. 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 µ thick, over 200 µ 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 quartzite, 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 µ. Nylander[358] 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 into short cells and a kind of plectenchyma is formed, as in _Lecanora_ (_Psoroma_) _hypnorum_, in _Endocarpon_, etc. [Illustration: Fig. 49. _Parmelia caperata_ Ach. (S. H., _Photo._).] The felted medulla is characteristic of most lichens and is formed of loose slender branching septate hyphae with thickish walls. This interwoven hyphal texture provides abundant air-spaces. Hue[359] has noted that the walls of the medullary hyphae in _Parmeliae_ are smooth, unless they have been exposed to great extremes of heat or cold, when they become wrinkled or scaly. They are very thick-walled in _Peltigera_ (Fig. 50). [Illustration: Fig. 50. Hyphae from lower medulla of _Peltigera canina_ DC. × 600.]