In _Peltidea_ (_Peltigera_) _aphthosa_ they are especially evident near the growing edges of the thallus; and they take part in the development of the superficial cephalodia[354] which are a constant feature of the lichen. They tend to disappear with age and leave the central older parts of the thallus smooth and shining. In several other species of _Peltigera_ (_P. canina_, etc.) they are present and persist during the life of the cortex. In these lichens the cells of the cortical tissue are thin-walled, all except the outer layer, the membranes of which are much thicker. The hairs rising from them are also thick-walled and septate. Generally they branch in all directions and anastomose with neighbouring hairs so that a confused felted tangle is formed; they vary in size but are, as a rule, about double the width of the medullary hyphae as are the cortical cells from which they rise. They disappear from the thallus, frequently in patches, probably by weathering, but over large surfaces, and especially where any inequality affords a shelter, they persist as a soft down. Hairs are also present on the upper surface of some _Parmeliae_. Rosendahl[355] has described and figured them in _P. glabra_ and _P. verruculifera_—short pointed unbranched hyphae, two or more septate and with thickened walls. They are most easily seen near the edge of the thallus, though they persist more or less over the surface; they also grow on the margins of the apothecia. In _P. verruculifera_ they arise from the soredia; in _P. glabra_ a few isolated hairs are present on the under surface. In _Nephromium tomentosum_ there is a scanty formation of hairs on the upper surface. They are abundant on the lower surface, and function as attaching organs. A thick tomentum of hairs is similarly present on the lower surface of many of the Stictaceae either as an almost unbroken covering or in scattered patches. In several species of _Leptogium_ they grow out from the lower cortical cells and attach the thin horizontal fronds; and very occasionally they are present in _Collema_. C. GONIDIAL TISSUES With the exception of some species of _Collema_ and _Leptogium_ lichens included under the term foliose, are heteromerous in structure, and the algae that form the gonidial zone are situated below the upper cortex and, therefore, 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 penetrate in groups well into the medulla.