Structures resembling cyphellae, with an overarching rim, are sprinkled over the brown under surface of the Australian lichen, _Heterodea Mülleri_; the thallus is without a lower cortex, the medulla being protected by thickly woven hyphae. _Heterodea_ was at one time included among Stictaceae, though now it is classified under Parmeliaceae. Pseudocyphellae are also present on the non-corticate under surface of _Nephromium tomentosum_, where they occur as little white pustules among the brown hairs; and the white impressed spots on the under surface of _Cetraria islandica_ and allied species, first determined as air pores by Zukal[436], have also been described by Wainio[437] as pseudocyphellae. There seems no doubt that the chief function of these various structures is, as Schwendener[438] suggested, to allow a free passage of air to the assimilating gonidial zone. Jatta[439] considers them to be analogous to the lenticels of higher plants and of service in the interchange of gases—expelling carbonic acid and receiving oxygen from the outer atmosphere. It is remarkable that such serviceable organs should have been evolved in so few lichens. B. BREATHING-PORES [Illustration: Fig. 74. _Parmelia exasperata_ Carroll. Vertical section of thallus. _a_, breathing-pores; _b_, rhizoid. × 60 (after Rosendahl).] _a._ DEFINITE BREATHING-PORES. The cyphellae and pseudocyphellae described above are confined to the under surface of the thallus in those lichens where they occur. Distinct breathing-pores of a totally different structure are present on the upper surface of the tree-lichen, _Parmelia aspidota_ (_P. exasperata_), one of the brown-coloured species. They are somewhat thickly scattered as isidia- or cone-like warts over the lichen thallus (Fig. 74) and give it the characteristically rough or “exasperate” character. They are direct outgrowths from the thallus, and Zukal[440], who discovered their peculiar nature and function, describes them as being filled with a hyphal tissue, with abundant air-spaces, and in direct communication with the medulla; gonidia, if present, are confined to the basal part. The cortex covering these minute cones, he further states, is very thin on the top, or often wanting, so that a true pore is formed which, however, is only opened after the cortex elsewhere has become thick and horny. Rosendahl[441], who has re-examined these “breathing-pores,” finds that in the early stage of their growth, near the margin or younger portion of the thallus, they are entirely covered by the cortex. Later, the hyphae at the top become looser and more frequently septate, and a fine network of anastomosing and intricate filaments takes the place of the closely cohering cortical cells. These hyphae are divided into shorter cells, but do not otherwise differ from those of the medulla. Rosendahl was unable to detect an open pore at any stage, though he entirely agrees with Zukal as to the breathing function of these structures.