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THE THALLUS 291
aa. COENOGONIACEAE. There are two types of gonidial algae in this
family, and both are filamentous forms, Trentepohlia in Coenogonium and
Cladophora in Racodium. The resulting lichens retain the slender thread-like
form of the algae, their cells being thinly invested by the hyphae and both
symbionts growing apically. The thalline filaments are generally very
sparingly branched and grow radially side by side in a loose flat expansion
attached at one side by a sheath, or the strands spread irregularly over the
substratum. Plectenchyma appears in the apothecial margin in Coenogonium.
Fruiting bodies are unknown in Racodium.
Coenogoniaceae are a group apart and of slight development, only the
one kind of thallus appearing; the form is moulded on that of the gonidium,
and is, as Reinke 1
to receive the maximum of
illumination and aeration.
remarks, perfectly adapted
bb. LECIDEACEAE AND GYROPHORACEAE. The origin of this thalline phylum
is distinct from that of the previous family, being associated with a different
type of gonidium, the single-celled alga of the Protococcaceae.
The. more elementary species are of extremely simple structure as
exemplified in such species as Lecidea (Biatora) uliginosa or Lecidea granu-
losa. These lichens grow on humus-soil and the thallus consists of a spreading
mycelium or hypothallus with more or less scattered thalline granules containing
gonidia, but without any defined structure. The first advance takes
place in the aggregation and consolidation of such thalline granules and
the massing of the gonidia towards the light, thus substituting the heteromerous
for the homoiomerous arrangement of the tissues. The various
characters of thickness, areolation, colour, etc. of the thallus are constant and
are expressed in specific diagnoses. Frequently an amorphous cortex of
swollen hyphae provides a smooth upper surface and forms a protective
covering for such long-lived species as Rhizocarpon geographicum, etc.
The squamulose thallus is well represented in this phylum. The squamules
vary in size and texture but are mostly rather thick and stiff. In
Lecidea ostreata they rise from the substratum in serried rows forming a
dense sward; in L. decipiens, also a British species, the squamules are still
are thick and firm and the
larger, and more horizontal in direction ; they
upper cortex is a plectenchyma of cells with swollen walls. Solitary hyphae
from the medulla pass downwards into the support.
Changes in spore characters also arise in these different thalline series,
as for instance in genera such as Biatorina and Buellia, the one with colour-
less, the other with brown, two-celled spores. These variations, along with
changes in the thallus, are of specific or generic importance following the
significance accorded to the various characters.
In one lichen of the series, the monotypic Brazilian genus Spltaerophoropsis
1 Reinke 1895, p. no.
192