1. The upper cortex, as in crustaceous lichens, is generally of the “decomposed”[341] or amorphous type: interlaced hyphae with thick gelatinous walls. A more highly developed form is apparent in _Parmeliella_ and _Pannaria_ where the upper cortex is formed of plectenchyma, while in the squamules of _Heppia_ the whole structure is built up of plectenchyma, with the exception of a narrow band of loose hyphae in the central pith. 2. The gonidia are Myxophyceae or Chlorophyceae; the squamules in some instances may be homoiomerous as in _Lepidocollema_, but generally they belong to the heteromerous series, with the gonidia in a circumscribed zone, and either continuous or in groups. Friedrich[342] held that, as in crustaceous lichens the development of the gonidial as compared with the other tissues depended on the substratum. The squamules of _Pannaria microphylla_ on sandstone were 100 µ thick, and the gonidial layer occupied 80 or 90 µ of the whole[343]. With that may be compared _Placodium Garovagli_ on lime-containing rock: the gonidial layer measured only 50 µ across, the pith hyphae 280 µ and the rhizoidal hyphae that penetrated the rock 500 µ. 3. The medullary layer, as a rule, is of closely compacted hyphae which give solidity to the squamules; in those of _Heppia_ it is almost entirely formed of plectenchyma. 4. The lower cortex is frequently little developed or absent, especially when the squamules are closely applied to the support as in some species of _Dermatocarpon_. In some of the squamulose _Lecanorae_ (_L. crassa_ and _L. saxicola_) the lowest hyphae are somewhat more closely interwoven; they become brown in colour, and the lichen is attached to the substratum by rhizoid-like branches. In _Lecanora lentigera_ there is a layer of parallel hyphae along the under surface. Further development is reached when a plectenchyma of thick-walled cells is formed both above and below, as in _Psoroma hypnorum_, though on the under surface the continuity is often broken. The squamules of _Cladoniae_ are described under the radiate-stratose series. 3. FOLIOSE LICHENS A. DEVELOPMENT OF FOLIOSE THALLUS The larger leafy lichens are occasionally monophyllous and attached at a central point as in _Umbilicaria_, but mostly they are broken up into lobes which are either imbricate and crowded, or represent the dividing and branching of the expanding thallus at the circumference. They are horizontal spreading structures, with marginal and apical growth. The several tissues of the squamule are repeated in the foliose thallus, but further provision is made to meet the requirements of the larger organism. There is the greater development of cortical tissue, especially on the lower surface, and the more abundant formation of rhizoidal organs to attach the large flat fronds to the support. There are also various adaptations to secure the aeration of the internal tissues[344]. B. CORTICAL TISSUES