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
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. The gonidia of the immediately underlying zone are sparsely arranged and a few of them are found in the lower half of the cone; the hyphae of the medulla can be traced up to the apex. Zukal[442] claims to have found breathing-pores in Cornicularia (Parmelia) tristis and in several other Parmeliae, notably in Parmelia stygia. The thallus of the latter species has minute holes or openings in the upper cortex, but they are without any definite form and may be only fortuitous.
Fig. 75 A. Ramalina fraxinea Ach. A, surface view of frond. a, air-pores; b, young apothecia. × 12. B, transverse section of part of frond. a, breathing-pore; f, strengthening fibres. × 37 (after Brandt).
