_b._ DEVELOPMENT OF CYPHELLAE. The cortex of both surfaces in the thallus of _Sticta_ is a several-layered plectenchyma of thick-walled closely packed cells, the outer layer growing out into hairs on the under surface of most of the species. Where either cyphellae or pseudocyphellae occur, a more or less open channel is formed between the exterior and the internal tissues of the lichen. In the case of the cyphellae, the medullary hyphae which line the cup are divided into short roundish cells with comparatively thin walls (Fig. 73). They form a tissue sharply differentiated from the loose hyphae that occupy the medulla. The rounded cells tend to lie in vertical rows, though the arrangement in fully formed cyphellae is generally somewhat irregular. The terminal empty cells are loosely attached and as they are eventually abstricted and strewn over the inside of the cup they give to it the characteristic white powdery appearance. According to Schwendener[434] development begins by an exuberant growth of the medulla which raises and finally bursts the cortex; prominent cyphellae have been thus formed in _Sticta damaecornis_ (Fig. 73). In other species the swelling is less noticeable or entirely absent. The opening of the cup measures usually about 1/2 mm. across, but it may stretch to a greater width. _c._ PSEUDOCYPHELLAE. In these no margin is formed, the cortex is simply burst by the protruding filaments which are of the same colour—yellow or white—as the medullary hyphae. They vary in size, from a minute point up to 4 mm. in diameter. _d._ OCCURRENCE AND DISTRIBUTION. The genus _Sticta_ is divided into two sections: (1) _Eusticta_ in which the gonidia are bright-green algae, and (2) _Stictina_ in which they are blue-green. Cyphellae and pseudocyphellae are fairly evenly distributed between the sections; they never occur together. Stizenberger[435] found that 36 species of the section _Eusticta_ were cyphellate, while in 43 species pseudocyphellae were formed. In the section _Stictina_ there were 38 of the former and only 31 of the latter type. Both sections of the genus are widely distributed in all countries, but they are most abundant south of the equator, reaching their highest development in Australia and New Zealand. In the British Isles _Sticta_ is rather poorly represented as follows: § _Eusticta_ (with bright-green gonidia). Cyphellate: _S. damaecornis._ Pseudocyphellate: _S. aurata._ § _Stictina_ (with blue-green gonidia). Cyphellate: _S. fuliginosa_, _S. limbata_, _S. sylvatica_, _S. Dufourei_. Pseudocyphellate: _S. intricata_ var. _Thouarsii_, _S. crocata_.