The squamulose type is well represented in Lecanora, and the species with that form of thallus have frequently been placed in a separate genus, Squamaria. These squamules are never very large; they possess an upper, somewhat amorphous, cortex; the medulla rests on the substratum, except in such a species as Lecanora lentigera, where they are free, a sort of fibrous cortex being formed of hyphae which grow in a direction parallel with the surface. In none of them are rhizinae developed.
cc. Parmeliaceae. The chief advance, apart from size, of the squamulose to the foliose type is the acquirement of a lower cortex along with definite organs of attachment which in Parmeliaceae are invariably rhizoidal and are composed of compact strands of hyphae extending from the cells of the lower cortex.
In the genus Parmelia rhizinae are almost a constant character, though in a few species, such as Parmelia physodes, they are scanty or practically absent. It is not possible, however, to consider that these species form a lower group, as in other respects they are highly evolved, and rhizinae may be found at points on the lower surface where there is irritation by friction. Soredia and isidia occur frequently and, in several species, almost entirely replace reproduction by spores. In one or two northern or Alpine species, P. stygia and P. pubescens, the lobes are linear or almost filamentous. They are retained in Parmelia because the apothecia are superficial on the fronds which are partly dorsiventral, and because rhizinae have occasionally been found. Some of the Parmeliae attain to a considerable size; growth is centrifugal and long continued.
Two monotypic genera classified under Parmeliaceae, Physcidia and Heterodea, are of considerable interest as they indicate the bases of parallel development in Parmelia and Cetraria. The former, a small lichen, is corticate only on the upper surface, and without rhizinae; and from the description, the cortex is of a fastigiate character. The solitary species grows on bark in Cuba; it is related to Parmelia, as the apothecia are superficial on the lobes. The second, Heterodea Mülleri, a soil-lichen from Australasia, is more akin to Cetraria in that the apothecia are terminal. The upper surface is corticate with marginal cilia, the lower surface naked or only protected by a weft of brownish hyphae amongst which cyphellae are formed; pseudocyphellae appear in Cetraria.
The genus Cetraria contains very highly developed thalline forms, either horizontal (subgenus Platysma), or upright (Eucetraria). Rhizinae are scanty or absent, but marginal cilia in some upright species act as haptera. Cetraria aculeata is truly fruticose with a radiate structure.
An extraordinary development of the under cortex characterizes the genera Anzia[1011] and Pannoparmelia: rhizinae-like strands formed from the cortical cells branch and anastomose with others till a wide mesh of a spongy nature is formed. They are mostly tropical or subtropical or Australasian, and possibly the spongy mass may be of service in retaining moisture. A species of Anzia has been recorded by Darbishire[1012] from Tierra del Fuego.
dd. Usneaceae. As we have seen, the change to fruticose structure has arisen as an ultimate development in a number of groups; it reaches however its highest and most varied form in this family. Not only are there strap-shaped thalli, but a new form, the filamentous and pendulous, appears; it attains to a great length, and is fitted to withstand severe strain. The various adaptations of structure in these two types of thallus have already been described[1013].
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. Kamtschadalis 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 frequently lodged here and there along the under side.
The extreme of strap-shaped fruticose development is reached in the genus Ramalina. In less advanced species 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 (siliquosa) which grows by preference on sea-cliffs and safely withstands the maximum of exposure to wind or weather.