A plectenchymatous cortex marks some progress here also in Leptodendriscum, Leptogidium and Polychidium, all of which are associated with Scytonema. These genera may well be derived from an elementary form such as Thermutis. They differ from each other in spore characters, etc., Polychidium being the most highly developed with its cortex of two cell-rows and with two-celled spores.
Nostoc forms the gonidium of Collemaceae. In its free state it is extremely gelatinous and transmits that character more or less to the lichen. In the crustaceous genus Physma, which forms the base of the Collema group or phylum, there is but little difference in form between the thalline warts of the lichen crust and the original small Nostoc colonies such as are to be found on damp mosses, etc.
In Collema itself, the less advanced species are scarcely more than crusts, though the more developed show considerable diversity of lobes, either short and pulpy, or spreading out in a thin membrane. The Nostoc chains pervade the homoiomerous thallus, but in some species they lie more towards the upper surface. There is no cortex, though once and again plectenchyma appears in the apothecial margin, both in this genus and in Leprocollema which is purely crustaceous.
Leptogium is a higher type than Collema, the thallus being distinguished by its cellular cortex. The tips of the hyphae, lying close together at the surface, are cut off by one or more septa, giving a one- or several-celled cortical layer. The species though generally homoiomerous are of thinner texture and are less gelatinous than those of Collema.
c. Pyrenidiaceae. This small family of pyrenocarpous Phycolichens may be considered here though its affinity, through the form of the fruiting body, is with Archilichens. The gonidia are species of Nostoc, Scytonema and Stigonema. There are only five genera; one of these, Eolichen, contains three species, the others are monotypic.
The crustaceous genera have a non-corticate thallus, but an advance to lobate form takes place in Placothelium, an African genus. The two genera that show most development are both British: Coriscium (Normandina), which is lobate, heteromerous and corticate—though always sterile—and Pyrenidium which is fruticose in habit; the latter is associated with Nostoc and forms a minute sward of upright fronds, corticate all round; the perithecium is provided with an entire wall and is immersed in the thallus.
If the thallus alone were under consideration these lichens would rank with Pannariaceae.
d. Heppiaceae and Pannariaceae. The next stage in the development of Phycolichens takes place through the algae, Scytonema and Nostoc, losing not only their gelatinous sheaths, but also, to a large extent, their characteristic forms. Chains of cells can frequently be observed, but accurate and certain identification of the algal genus is only possible by making separate cultures of the gonidia.
Scytonema forms the gonidium of the squamulose Heppiaceae consisting of the single genus Heppia. The ground tissue of the species is either wholly of plectenchyma with algae in the interstices, or the centre is occupied by a narrow medulla of loose filaments.
In the allied family Pannariaceae, a number of genera contain Scytonema or Nostoc, while two, Psoroma and Psoromaria, have bright-green gonidia. 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.