4. =Plectenchymatous=: the last of Hue’s types corresponds with the first described by Zukal. It is the result of the lateral coherence and frequent septation of the hyphae into short almost square or rounded cells (Fig. 47). The simplest type of such a cortex can be studied in _Leptogium_, a genus of gelatinous lichens in which the tips of the hyphae are cut off at the surface by one or more septa. The resulting cells are wider than the hyphae and they cohere together to form, in some species, disconnected patches of cells; in others, a continuous cortical covering one or more cells thick, while in the margin of the apothecium they form a deep cellular layer. The cellular type of cortex is found also, as already stated, in some crustaceous _Pertusariae_, and in a few squamulose genera or species. It forms the uppermost layer of the _Peltigera_ thallus and both cortices of many of the larger foliose lichens such as _Sticta_, _Parmelia_, etc. 5. The “fibrous” cortex must be added to this series, as was pointed out by Heber Howe[348] who gave the less appropriate designation of “simple” to the type. It consists of long rather sparingly branched slender hyphae that grow in a direction parallel with the surface of the thallus (Fig. 48). It is characteristic of several fruticose and foliose lichens with more or less upright growth, such as we find in several of the _Physciae_, and in the allied genus _Teloschistes_, in _Alectoria_, several genera of Roccellaceae, in _Usnea_ _longissima_ and in _Parmelia pubescens_, etc. Zukal would have included all the _Usneae_ as the tips are fibrous. [Illustration: Fig. 48. _Physcia ciliaris_ DC. Vertical section of thallus. _a_, cortex; _b_, gonidial zone; _c_, medulla. × 100.] More than one type of cortex, as already stated, may appear in a genus: a striking instance of variability occurs in _Solorina_ where, as Hue[349] has pointed out, the cortex of _S. octospora_ is fastigiate, that of all the other species being plectenchymatous. Cortical development is a specific rather than a generic characteristic. _b._ ORIGIN OF VARIATION IN CORTICAL STRUCTURE. The immediate causes making for differentiation in cortical development are: the prevailing direction of growth of the hyphae as they rise from the gonidial zone; the amount of branching and the crowding of the filaments; the frequency of septation; and the thickening or degeneration of the cell-walls which may become almost or entirely mucilaginous. In the plectenchymatous cortex, the walls may remain quite thin and the cells small as in _Xanthoria parietina_, or the walls may be much thickened as in both cortices of _Sticta_. As a result of stretching the cell may increase enormously in size: in some instances where the internal hyphae are about 3 µ to 4 µ in width, the cortical cells formed from these hyphae may have a cell cavity 15 µ to 16 µ in diameter.