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.