84 



MORPHOLOGY 



in the molecular constituents of the cell-walls which permits the imbibition 

 and storage of water. The tissue, owing to the enormous increase of the 



. wall, is so closely pressed together that the individual 



hyphae become indistinct; the cell-lumen finally 

 disappears altogether, or, at most, is only to be 

 detected in section as a narrow disconnected dark 

 streak. The decomposed cortex is characteristic 

 of many lichens, crustaceous (Fig. 46) and squamu- 

 lose, as well as of such highly developed genera as 

 Usnea, Letharia, Ramalina, Cetraria, Evernia and 

 certain Pdrtneliae. 



Zukal took no note of the decomposed cortex, 

 but the omission is intentional and is due to his 

 regarding the structure of the youngest stages of the 

 thallus near the growing point as the most typical and as giving the best 

 indication as to the true arrangement of hyphae in the cortex. He thus 

 describes palisade tissue as the characteristic cortex of Evernia, since the . 

 formation near the growing point of the fronds is somewhat palisade-like ; 

 and he finds fibrous cortex at the tips of Usnea filaments. In both these 

 instances Hue has described the cortex as decomposed because he takes 

 account only of the fully formed thallus in which the tissues have reached 

 a permanent condition. 



4. Plectenchymatous : the last of Hue's types corresponds with the 

 first described by Zukal. It is the result of the lateral coherence and frequent 

 septation of the hyphae into short almost square or rounded cells (Fig. 47). 

 The simplest type of such a cortex can be studied in Leptogium, a genus of 



Fig. 46. Lecanora glazuo?na 

 yax.corrugata'NyX. Vertical 

 section of cortex x 500 (after 

 Hue). 



Fig. 47. Peltigera canina DC. Vertical section 

 of cortex and gonidial zone x 600. 



