Starting with the Permo-Carboniferous vegetation, and omitting for the moment the Glossopteris flora, we find a comparatively homogeneous flora of wide geographical range, consisting to a large extent of arborescent lycopods, calamites, and other vascular cryptogams, plants which occupied a place comparable with that of Gymnosperms and Angiosperms in our modern forests; with these were other types of the greatest phylogenetic importance, which serve as finger-posts pointing to lines of evolution of which we have but the faintest signs among existing plants.
In height according to the climate in which it grows, being arborescent in tropical latitudes.
We will first take the arborescent Lycopods, as in every respect the more important group. They may all be classed under the one family Lepidodendreae, which is here taken to include Sigillaria.
The arches bear on the convex outer side the delicate arborescent gills, and on the concave inner side develop a membranous septum with vermicular perforations, a special sifting or filtering contrivance through which the water absorbed by the mouth has to pass before reaching the respiratory organs of the branchial apparatus.
The upper limit of arborescent vegetation is considered to run at 7000-7500 ft., of shrubs such as rhododendrons at 850o ft., and of pasture-lands up to 9000 ft.
Ferns of the genera referred to Marattiaceae are common, but arborescent stems of the Psaronius type are still comparatively rare.
In Malaya and eastward the forests are rich in arborescent figs, laurels, myrtles, nutmegs, oaks and bamboos.
One of the chief troubles met with was the formation of arborescent growths around the edges of the cathode, due to the greater current-density in this region; this, however, was also obviated by the use of screens.
A small group of Australian genera closely approach the order Juncaceae in having small crowded flowers with a scarious or membranous perianth; they include Xanthorrhoea (grass-tree or blackboy) and Kingia, arborescent plants with an erect woody stem crowned with a tuft of long stiff narrow leaves, from the centre of which rises a tall dense flower spike or a number of stalked flower-heads; this group has been included in Juncaceae, from which it is doubtfully distinguished only by the absence of the long twisted stigmas which characterize the true rushes.
It was characterized by arborescent vascular Cryptogams and Gymnosperms of a type (Cordaiteae) which have left no descendants beyond it.