86 . MORPHOLOGY 



become almost or entirely mucilaginous. In the plectenchymatous cortex, 

 the walls may remain quite thin and the cells small as in Xanthoria parie- 

 tma, or the walls may be much thickened as in both cortices of Sticta, 

 As a result of stretching the cell may increase enormously in size: in some 

 instances where the internal hyphae are about 3 p to 4 //, in width, the 

 cortical cells formed from these hyphae may have a cell cavity i$/j, to i6/i 

 in diameter. 



c. Loss AND RENEWAL OF CORTEX. Very frequently the cortex is 

 covered over by a layer of homogeneous mucilage which forms an outer 

 cuticle. It arises from the continual degeneration of the outer cell-walls 

 and it is liable to friction and removal by atmospheric agency as was 

 first described by Schwendener 1 in the weather-beaten cortex of Umbi- 

 licaria pusttdata. He had noted the irregular jagged outline of the cross 

 section of the thallus, and he then suggested, as the probable reason, the 

 decay of the outer rind with the constant renewal of it by the hyphae from 

 the underlying gonidial zone, though he was unable definitely to prove his 

 theory. The peeling of the dead outer layer (with its replacement by new 

 tissue) has however been observed many times since his day. It has been 

 described by Darbishire 2 in Pertusaria: in that genus there is at first a 

 primary cortex formed of hyphae that grow in a radial direction, parallel 

 to the surface of the thallus. The walls of these hyphae become gradually 

 more and more mucilaginous till the cells are obliterated. Meanwhile 

 short-celled filaments grow up in serried ranks from the gonidial layer and 

 finally push off the dead "fibrous" cortex. The new tissue takes on a 

 plectenchymatous character, and the outer cells in time become decomposed 

 and provide a mucilaginous cuticle which in turn is also subject to wasting. 



The same process of peeling was noted by Rosendahl 3 in some species of 

 brown Parmeliae, where the dead tissues were thrown off in shreds, though 

 only in isolated patches. But whether in patches or as a continuous sheath, 

 there is constant degeneration, with continual renewal of the dead material 

 from the internal tissues. 



The cortex is the most highly developed of all the lichen structures and 

 is of immense importance to the plant as may be judged from the various 

 adaptations to different needs 4 . The cortical cell-walls are frequently 

 impregnated with some dark-coloured substance which, in exposed situa- 

 tions, must counteract the influence of too direct sunlight and be of 

 service in sheltering the gonidia. Lichen acids sometimes very brightly 

 coloured and oxalic acid are deposited in the cortical tissues in great 

 abundance and aid in retaining moisture; but the two chief functions to 



1 Schwendener 1863, p. 180. 2 Darbishire 1897. 3 Rosendahl 1907. 



4 See p. 96. 



