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 Lecanorales well defined by the polarilocular spores[1014]. It also arises from a Biatora species and forms a parallel development. Even in this phylum there are two series: one with colourless spores and mostly yellow or reddish either in thallus or apothecium, and the other with brown spores and with cinereous-grey or brown thalli. The dark spores are in many of the species typically polarilocular, though in some the median septum is not very wide and no canal is visible. Practically all of the lighter coloured forms contain parietin either in thallus or apothecia or in both; it is absent in the dark-spored series.
Among the lighter coloured forms it is difficult to decide which of these two striking characteristics developed first, the acid or the peculiar spore. Probably the acid has the priority: there is one common rock lichen in this country, Placodium rupestre (Lecanora irrubata), which gives a strong red acid reaction with potash, but in which the spores are still simple, and the fruit structure in the biatorine stage. Another species, Pl. luteoalbum, with a purplish reaction in the fruit only, shows septate spores but with only a rather narrow septum. The development continues through biatorine forms to lecanorine with a fully formed thalline margin. Among these latter we encounter Pl. nivale which is well provided with acid but in which the spores have become long and fusiform with little trace of the polar cells or central canal. We must allow here also for reversions, and wanderings from the straight road.
From crustaceous the advance is normal and simple to squamulose forms which in this phylum maintain a stiff regularity of thalline outline termed “effigurate”; the squamules, developing from the centre, extend outwards in a radiate-stellate manner. There are also foliose thalli in the genus Xanthoria and fruticose in Teloschistes. The cortex in the former horizontal genus is of plectenchyma, and no peculiar structures have emerged. In Teloschistes the cortex is of compact parallel hyphae (fibrous) which form the strengthening structure of the narrow compressed fronds (T. flavicans).
In the brown-spored series there is a considerable number of species that are crustaceous united in the genus Rinodina, all of which have marginate apothecia. One of them, Rinodina oreina, approaches in thalline structure the effigurate forms of Placodium; while in R. isidioides, a rare British species, there is an isidioid squamulose development.
Among foliose genera, the tropical genus Pyxine is peculiar in its almost lecideine fruit, a few gonidia occurring only in the early stages; its affinity with Physcia holds, however, through the one-septate brown spores with very thick walls and the reduced lumen of the cells. The more simple type of fruit may be merely retrogressive.
Physcia, the remaining genus, is mainly foliose and with dorsiventral thallus. A few species have straggling semi-upright fronds and these have sometimes been placed in a separate genus Anaptychia. Only one “Anaptychia,” Ph. intricata, has a radiate structure with fibrous cortex all round; in the others the upper cortex alone is fibrous—of long parallel hyphae—but that character appears in nearly every one of the horizontal species as well, sometimes in the upper, sometimes in the lower cortex.
In Physcia the horizontal thallus is of smaller dimensions than in Parmelia, and never becomes so free from the substratum: it is attached by rhizinae and soredia appear frequently. Very often the circular effigurate type of development prevails.
It is difficult to trace with any certainty the origin of this series of the phylum. Some workers have associated it with the purely lecideine genus, Buellia, but the brown septate spores of the latter are of simple structure, though occasionally approaching the Rinodina type. There are also differences in the thallus, that of Buellia, especially when it is saxicolous, inclining to Rhizocarpon in form. It is more consistent with the outer and inner structure to derive Rinodina from some crustaceous Placodium form with a marginate apothecium, therefore from a form of fairly advanced development. As the parietin content disappeared—perhaps from the preponderance of other acids—the colouration changed and the spores became dark-coloured.
Many genera and even families, such as Thelotremaceae, etc., have necessarily been omitted from this survey of phylogeny in lichens, but the tracing of the main lines of development has indicated the course of evolution, and has demonstrated not only the close affinity between the members of this polyphyletic class of plants, as shown in the constantly recurring thalline types, but it has proved the extraordinary vigour gained by both the component organisms through the symbiotic association.