Fig. 75 B. Ramalina strepsilis Zahlbr. Transverse section of part of frond showing distribution of: a, air-pores, and f, strengthening fibres. × 37 (after Brandt).
Zukal[442] published drawings of channels of looser tissue between the exterior and the pith in Oropogon Loxensis and in Usnea barbata. He considered them to be of definite service in aeration. The fronds of Ramalina dilacerata by stretching develop a series of elongate holes. Reinke[443] found openings in Ramalina Eckloni which pierced to the centre of the thallus, and Darbishire[444] has figured a break in the frond of another species, R. fraxinea ([Fig. 75 A]), which he has designated as a breathing-pore. Finally Brandt[445], in his careful study of the anatomy of Ramalinae, has described as breathing-pores certain open areas usually of ellipsoid form in the compact cortex of several species: in R. strepsilis ([Fig. 75 B]) and R. Landroensis, and in the British species, R. siliquosa and R. fraxinea. These openings are however mostly rare and difficult to find or to distinguish from holes that may be due to any accident in the life of the lichen. It is noteworthy that Rosendahl found no further examples of breathing-pores in the brown Parmeliae that he examined in such detail. No other organs specially adapted for aeration of the thallus have been discovered.
b. Other openings in the Thallus. Lobaria is the only genus of Stictaceae in which neither cyphellae nor pseudocyphellae are formed; but in two species, L. scrobiculata and L. pulmonaria, the lower surface is marked with oblong or angular bare convex patches, much larger than cyphellae. They are exposed portions of the medulla, which at these spots has been denuded of the covering cortex. Corresponding with these bare spots there is a pitting of the upper surface.
A somewhat similar but reversed structure characterizes Umbilicaria pustulata, which as the name implies is distinguished by the presence of pustules, ellipsoid swellings above, with a reticulation of cavities below. Bitter[446] in this instance has proved that they are due to disconnected centres of intercalary growth which are more vigorous on the upper surface and give rise to cracks in the less active tissue beneath. These cracks gradually become enlarged; they are, as it were, accidental in origin but are doubtless of considerable service in aeration.
In some Parmeliae there are constantly formed minute round holes, either right through the apothecia (P. cetrata, etc.), or through the thallus (P. pertusa). Minute holes are also present in the under cortex of Parmelia vittata and of P. enteromorpha, species of the subgenus Hypogymnia. Nylander[447], who first drew attention to these holes of the lower cortex, described them as arising at the forking of two lobes; but though they do occur in that position, they as frequently bear no relation to the branching. Bitter’s[448] opinion is that they arise by the decay of the cortical tissues in very limited areas, from some unknown cause, and that the holes that pierce right through the thallus in other species may be similarly explained.
Still other minute openings into the thallus occur in Parmelia vittata, P. obscurata and P. farinacea var. obscurascens. In the two latter the openings like pin-holes are terminal on the lobes and are situated exactly on the apex, between the pith and the gonidial zone; sometimes several holes can be detected on the end of one lobe. Further growth in length is checked by these holes. They appear more frequently on the darker, better illuminated plants. In Parmelia vittata the terminal holes are at the end of excessively minute adventitious branches which arise below the gonidial zone on the margin of the primary lobes. All these terminal holes are directed upwards and are visible from above.
Bitter does not attribute any physiological significance to these very definite openings in the thallus. It has been generally assumed that they aid in the aeration of the thallus; it is also possible that they may be of service in absorption, and they might even be regarded as open water conductors.
C. General Aeration of the Thallus
Definite structures adapted to secure the aeration of the thallus in a limited number of lichens have been described above. These are the breathing-pores of Parmelia exasperata and the cyphellae and pseudocyphellae of the Stictaceae, with which also may be perhaps included the circumscribed breaks in the under cortex in some members of that family.