STRUCTURES PECULIAR TO LICHENS 131 



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 1 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 Par- 

 melia vittata and of P. enteromorpha, species of the subgenus Hypogymnia. 

 Nylander 2 , 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 3 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 open- 

 ings 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 illumi- 

 nated 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 con- 

 ductors. 



1 Bitter 1899. 2 Nylander i874 2 . 3 Bitter igoi 2 . 



92 



