96 MORPHOLOGY 



a. BY DEVELOPMENT OF THE CORTEX. Such a transformation of tissue 

 is well illustrated in Heppia Guepini. The thallus consists of rigid squamules 

 which are attached at one point only ; the cortex of both surfaces is plecten- 

 chymatous and very thick and even the medulla is largely cellular. 



The much larger but equally rigid coriaceous thallus of Dermatocarpon 

 miniatum (Fig. 56) has also a single central attachment or umbilicus, and 



Fig- 56. Dermatocarpon miniatum Th. Fr. (S. H., Photo.). 



both cortices consist of a compact many-layered plectenchyma. The same 

 structure occurs in Umbilicaria pustulate and in some species of Gyrophora, 

 which, having only a single central hold-fast, gain the necessary stiffening 

 through the increase of the cortical layers. 



In the Stictaceae there are a large number of widely-expanded forms, 

 and as the attachment depends mostly on a somewhat short tomentum, 

 strength is obtained here also by the thick plectenchymatous cortex of both 

 surfaces. When areas denuded of tomentum and cortex occur, as in Lobaria 

 pulmonaria, the under surface is not sensibly weakened, since the cortical 

 tissue remains connected in a stout and firm reticulation. 



b. BY DEVELOPMENT OF VEINS OR NERVES. Certain ground lichens 

 belonging to the Peltigeraceae have a wide spreading thallus often with 

 very large lobes. The upper cortex is a many-layered plectenchyma, but 

 the under surface is covered only by a loose felt of hyphae which branch 

 out info a more or less dense tomentum. As the firm upper cortex continues 

 to increase by intercalary growth from the branching upwards of hyphae 

 from the meristematic gonidial zone, there occurs an extension of the upper 



