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PHYLOGENY
The thallus varies from crustaceous or minutely squamulose, to lobes of
fair dimension in Parmeliella and in Hydrothyria venosa, an aquatic lichen.
Plectenchyma appears in the upper cortex of both of these, and in the
proper margin of the apothecia, while the under surface is frequently provided
'
with rhizoidal filaments.
These two families form a transition between the gelatinous, and mostly
homoiomerous thallus, and the more developed entirely heteromerous thallus
of much more advanced structure. The fructification in all of them, gelatinous
and non-gelatinous, is a more or less open apothecium, sometimes immar-
ginate, and biatorine or lecideine, but often, even in species nearly related
to these, it is lecanorine with a thalline amphithecium. Rarely are the spori-
ferous bodies sunk in the tissue, with a pseudo-perithecium, as in Phylliscum.
It would be difficult to trace advance in all this group on the lines of fruit
development. The two genera with bright-green gonidia, Psoroma and
Psoromaria, have been included in Pannariaceae owing to the very close
affinity of Psoroma hypnorum with Pannaria rubiginosa; they are alike in
every respect except in their gonidia. Psoromaria is exactly like Psoroma,
but with immarginate biatorine apothecia, representing therefore a lower
development in that respect.
These lichens not only mark the. transition from gelatinous to non-
gelatinous forms, but in some of them there is an interchange of gonidia.
The progression in the phylum or phyla has evidently been from blue-green
up to some highly evolved forms with bright-green algae, though there may
have been, at the beginning, a substitution of blue-green in place of earlier
bright-green algae, Phycolichens usurping as itwerethe Archilichen condition.
e. PELTIGERACEAE AND STICTACEAE. The two families just examined
marked a great advance which culminated in the lobate aquatic lichen
Hydrothyria. This lichen, as Sturgis pointed out, shows affinity with other
Pannariaceae in the structure of the single large-celled cortical layer as well
as with species of Nepkroma (Peltigeraceae). A still closer affinity may be
traced with Peltigera in the presence in both plants of veins on the under
surface. The capacity of Peltigera species to grow in damp situations may
also be inherited from a form like the submerged Hydrothyria. In both
families there are transitions from blue-green to bright-green gonidia, or
vice versa, in related species. Thus in Peltigeraceae we find Peltigera con-
taining Nostoc in the gonidial zone, with Peltidea which may be regarded
as a separate genus, or more naturally as a section of Peltigera; it contains
bright-green gonidia, but has cephalodia containing Nostoc associated
its thallus.
with
The genus Nephroma is similarly divided into species with a bright-green
gonidial zone, chiefly Arctic or Antarctic in distribution, and species with
Nostoc (subgenus Nephromium) more numerous and more widely distributed.