Squamulose and foliose lichens grow mostly in close relation with the support, and the flat expanding thallus, as in the _Parmeliae_, is attached at many points to the substance—tree, rock, etc.—over which the plants spread. Special provision for support is therefore not required, and the lobes remain thin and flaccid. Yet, in a number of widely different genera the attachment to the substratum is very slight, and in these we find an adaptation of existing tissues fitted to resist tearing strains, resistance being almost invariably secured by the strengthening of the cortical layers. _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 plectenchymatous 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 both cortices consist of a compact many-layered plectenchyma. The same structure occurs in _Umbilicaria pustulata_ 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. [Illustration: Fig. 56. _Dermatocarpon miniatum_ Th. Fr. (S. H., _Photo._).] 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.