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PHYLOGENY
scantiest and very soon disappears, while the podetial thallus continues to
grow indefinitely and to considerable size.
4. COURSE OF PODETIAL DEVELOPMENT. In Baeomyces the podetial
primordium is wholly endogenous in some species, but in others the
outer cortical layer of the primary thallus as well as the gonidial hyphae
take part in the formation of the new structure which, in that case, is simply
a vertical extension of the primary granule. This type of podetium called
by Wainio 1 a pseudopodetium also recurs in Pilophorus and in Stereocanlon.
To emphasize the distinction of origin it has been proposed to classify these
two latter genera in a separate family, but in that case it would be necessary
to break up the genus Baeomyces. We may assume that the endogenous
origin of the "apothecial stalk" is the more primitive, as it occurs in the
most primitive lecideine lichens, whereas a vertical thallus is always an
advanced stage of vegetative development.
Podetia are essentially secondary structures, and they are associated
both with crustaceous and squamulose primary thalli. If monophyletic in
origin their development must have taken place while the primary thallus
was still in the crustaceous stage, and the inherited tendency to form podetia
must then have persisted through the change to the squamulose type. In
species such as Cl. caespiticia the presence of rudimentary podetia along
with large squamules suggests a polyphyletic origin, but Wainio's 1
opinion is
that such instances may show retrogression from an advanced podetial form,
and that the evidence inclines to the monophyletic view of their origin.
The hollow centre of the podetium arises in the course of development
and is common to nearly all advanced stages of growth. There are however
some exceptions : in Glossodium aversum, a soil lichen from New
Granada, and the only representative of the genus, a simple or rarely forked
stalk about 2 cm. in height rises from a granular or minutely squamulose
thallus. The apothecium occupies one side of the flattened and somewhat
wider apex. There is no external cortex and the central tissue is of loose
hyphae. In Thysanothecium Hookeri, also a monotypic genus from Australia,
the podetia are about the same height, but, though round at the base, they
broaden upwards into a leaf-like expansion. The central tissue below is of
loose hyphae, but compact strands occur above, where the apothecium
spreads over the upper side. The under surface is sterile and is traversed
by nerve-like strands of hyphae.
5. VARIATION IN CLADONIA. It is in this genus that most variation is to
be found. Characters of importance and persistence have arisen by which
secondary phyla may be traced within the genus: these are mainly (i) the
relative development of the horizontal and vertical structures, (2) formation
1 Wainio 1897.