PDF - CES (IISc)

298
PHYLOGENY
The marginate apothecium has appeared once and again as we have
seen. It is probable however that its first development was in this group of
lichens, and even here there may have been more than one origin as there
is certainly more than one phylum.
aa. COURSE OF DEVELOPMENT. At the base of the series, the thallus is
of the crustaceous type somewhat similar to that of Lecidea, but there are
none of the very simple primitive forms. Lecanora must have originated
when the crustaceous lecideine thallus was already well established. Its
affinity is with Lecidea and not with any fungus: where the thallus is
evanescent or scanty, its lack is due to retrogressive rather than to primitive
characters.
bb. LECANORACEAE.. A number of genera have arisen in this large family,
but they are distinguished mainly if not entirely by spore characters, and
by some systematists have all been included in the one genus Lecanora,
since the changes have taken place within the developing apothecium.
There is one genus, Harpidium, which is based on thalline characters,,
represented by one species, H. rutilans, common enough on the Continent,
but not yet found in our country. It has a thin crustaceous homoiomerous
thallus, the component hyphae of which are divided into short cells closely
packed together and forming a kind of cellular tissue in which the algae are
interspersed. The dorsiventral stratose arrangement prevails however in the
other genera and a more or less amorphous "
decomposed " cortex is frequently
present. The medulla rests on the substratum.
With the stouter thallus, there is slightly more variety of crustaceous
form than in Lecideaceae: there occurs occasionally an outgrowth of the
thalline granules as in Haematomma ventosum which marks the beginning
of fruticulose structure. Of a more advanced structure is the thallus of
Lecanora esculenta, a desert lichen which becomes detached and erratic, and
which in some of its forms is almost coralline, owing to the apical growth of
the original granules or branches: a more or less radiate arrangement of
the tissues is thus acquired.
The squamulose type is well represented in Lecanora, and the species
with that form of thallus have frequently been placed in a separate genus,
Squamaria. These squamules are never very large; they possess an upper,
somewhat amorphous, cortex; the medulla rests on the substratum, except
in such a species as Lecanora lentigera, where they are free, a sort of fibrous
cortex being formed of hyphae which grow in a direction parallel with the
surface. In none of them are rhizinae developed.
cc. PARMELIACEAE. The chief advance, apart from size, of the squamulose
to the foliose type is the acquirement of a lower cortex along with definite
organs of attachment which in Parmeliaceae are invariably rhizoidal and