PDF - CES (IISc)

292
PHYLOGENY
stereocauloides, the thallus is described by Wainio 1 as consisting of minute
clavate stalks of interwoven thick-walled hyphae, with gelatinous algae, like
Gloeocapsa, interspersed in groups, though with a tendency to congregate
towards the outer surface.
The highest development along this line of advance is to be found in the
Gyrophoraceae, a family of lichens with a varied foliose character and dark
lecideine apothecia. The thallus may be monophyllous and of fairly large
a central stout hold-
dimensions or polyphyllous; it is mostly anchored by
fast and both surfaces are thickly corticate with a layer of plectenchyma;
the under surface is mostly bare, but may be densely covered with rhizinalike
strands of dark hyphae. They are all northern species and rock-dwellers
exposed to severe extremes of illumination and temperature, but well
protected by the thick cortex and the dark colouration common to them all.
cc. CLADONIACEAE. This last phylum of Lecideales is the most interesting
as it is the most complicated. It possesses a primary, generally sterile,
thallus which is dorsiventral and crustaceous, squamulose or in some instances
almost foliaceous, along with a secondary thallus of upright radiate
structure and of very varied form, known as the podetium which bears at
the summit the fertile organs.
A double thallus has been suggested in the spreading base, containing
gonidia, of some radiate lichens such as Roccella, but the upright portion
of such lichens, though analogous, is not homologous with that of
Cladoniaceae.
The algal cells of the family belong to the Protococcaceae. Blue-green
algae are associated in the cephalodia of Pilophorus and Stereocaulon.
The primary thallus is a feature of all the members, though sometimes very
slight and very short-lived, as in Stereocaulon or in the section Cladina of
the genus Cladonia. Where the primary thallus is most largely developed,
the secondary (the podetium) is less prominent.
This secondary thallus originates in two different ways: (i) the primary
in the new
granule may grow upward, the whole of the tissues taking part
development; or (2) the origin may be endogenous and proceed from the
hyphae only of the gonidial zone: these push upwards in a compact fascicle,
as in the apothecial development of Lecidea, but instead of spreading outward
on reaching the surface, they continue to grow in a vertical direction and
form the podetium. In origin this is an apothecial stalk, but generally it is
clothed with gonidial tissue. The gonidia may travel upwards from the
base or they may possibly be wind borne from the open. The podetium
thus takes on an assimilative function and is a secondary thallus.
The same type of apothecium is common to all the genera ; the spores
1 Wainio 1890.