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 immarginate, and biatorine or lecideine, but often, even in species nearly related to these, it is lecanorine with a thalline amphithecium. Rarely are the sporiferous 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 it were the 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 Nephroma (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 containing 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 with its thallus.
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.
Peltigera and Nephroma are also closely related in the character of the fructification. It is a flat non-marginate disc borne on the edge of the thallus: in Peltigera on the upper surface, in Nephroma on the under surface. The remaining genus Solorina contains normally a layer of bright-green algae, but, along with these, there are always present more or fewer Nostoc cells, either in a thin layer as in S. crocea or as cephalodia in others, while, in three species the algae are altogether blue-green.
The members of the Peltigeraceae have a thick upper cortex of plectenchyma and in some cases strengthening veins, and long rhizinae on the lower side. Some of the species attain a large size, and, in some, soredia are formed, an evidence of advance, this being a peculiarly lichenoid form of reproduction.
The Stictaceae form a parallel but more highly organized family, which also includes closely related bright-green and blue-green series. They are all dorsiventral, but they are mostly attached by a single hold-fast and the lobes in some species suggest the fruticose type in their long narrow form. A wide cortex of plectenchyma protects both the upper and the lower surface and a felt of hairs replaces the rhizinae of other foliose lichens. In the genus Sticta (including the section Stictina) special aeration organs, cyphellae or pseudocyphellae, are provided; in Lobaria these are replaced by naked areas which serve the same purpose.
Nylander[1002] regarded the Stictaceae as the most highly developed of all lichens, and they easily take a high place among dorsiventral forms, but it is generally conceded that the fruticose type is the more highly organized. In any case they are the highest reach of the phylum or phyla that started with Pyrenopsidaceae and Collemaceae; the lowly gelatinous thalli changing to more elaborate structures with the abandonment of the gelatinous algal sheath, as in the Pannariaceae, and with the replacement of blue-green by bright-green gonidia. Reinke[1003], considers the Stictaceae as evolved from the Pannariaceae more directly from the genus Massalongia. Their relationship is certainly with Pannariaceae and Peltigeraceae rather than with Parmeliaceae; these latter, as we shall see, belong to a wholly different series.