PDF - CES (IISc)

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PHYLOGENY
various adaptations of structure in these two types of thallus have already
been described 1 .
In Parmelia itself there are indications of this line of development in
P. stygia, with short stiff upright branching fronds, and in P. pubescens,
with its tufts of filaments, but these two species are more or less dorsiventral
in structure and do not rise from the substratum. In Cetraria also there
is a tendency towards upright growth and in C. aculeata even to radiate
structure. But advance in these directions has stopped short, the true line
of evolution passing through species like Parmelia physodes with raised, and
in some varieties, tubular fronds, and the somewhat similar species P. Kamt-
schadalis with straggling strap-like lobes, to Evernia. That genus is a true
link between foliose and fruticose forms and has been classified now with
one series, now with the other.
In Evernia furfuracea, the lobes are free from the substratum except
when friction causes the development of a hold-fast and the branching out
of new lobes from that point. It is however dorsiventral in structure, the
under surface is black and the gonidial zone lies under the upper cortex.
Evernia prunastri is white below and is more fruticose in habit, the long
fronds all rising from one base. They are thin and limp, no strengthening
tissue has been evolved, and they tend to lie over on one side; both surfaces
are corticate and gonidia sometimes travel round the edge, becoming fre-
quently lodged here and there along the under side.
The extreme of strap-shaped fruticose development
genus Ramalina. In less advanced species
is reached in the
such as R. evernioides there is a
thin flat expansion anchored to the substratum at one point and alike on
both surfaces. In R.fraxinea the fronds may reach considerable width (var.
ampliata), but in that and in most species there is a provision of sclerotic
strands to support and strengthen the fronds. One of those best fitted to
resist bending strains is R. scopulorum (siliquosd) which grows by preference
on sea-cliffs and safely withstands the maximum of exposure to wind or
weather.
The filamentous structure appears abruptly, unless we consider it as
foreshadowed by Parmelia pubescens. The base is secured by strong sheaths
of enduring character; tensile strains are provided for either by a chondroid
axis, as in Usnea, or by cortical development, as in Alectoria; the former
method of securing strength seems to be the most advantageous to the plant
as a whole, since it leaves the outer structures more free to develop, and there
is therefore in Usnea a greater variety of branching and greater growth in
length, which are less possible with the thickened cortex of Alectoria.
ee. PHYSCIACEAE. There remains still an important phylum of Lecano-
rales well defined by the polarilocular spores 2 . It also arises from a Biatora
1 See p. 101. 2 See p. 188.