PDF - CES (IISc)

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 pitstnlata 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 into 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