This surface layer in the typically subaerial shoot of the sporophyte in Pteridophytes and Phanerogams is known as the epidermis, though the name is restricted by some writers, on account of developmental differences, to the surface layer of the shoot of Angiosperms, and by others extended to the surface layer of the whole plant in both these groups.
One of the most striking characters common to the two highest groups of plants, the Pteridophytes and Phanerogams, is the Vascular possession of a double (hydrom-leptom) conducting .s system, such as we saw among the highest mosses, YS em.
The tracheids or vessels, indifferently called tracheal elements, together with the immediately associated cells (usually amylom in Pteridophytes) constitute the xylem of the plant.
This is a morphological term given to the particular~ type of hydrom found in both Pteridophytes and Phanerogams, together with the parenchyma or stereom, or both, included within the boundaries of the hydrom tissue strand.
The leptom found in Pteridophytes and Phanerogams with its associated cells, and is entirely parallel with the xylem.
When the protoxylems have an intermediate position the stele is inesarch (many Pteridophytes and some of the more primitive Phanerogams).
In many Pteridophytes the solid haplostele is maintained throughout the axis.
In the other groups of Pteridophytes internal phloem is not found and an internal endodermis but rarely.
To this type of steIn having a ground-tissue pith, whether with or without internal phloem, is given the name siphonostele to distinguish it from the solid haplostele characteristic of the root, the first-formed portion of the stem, and in the more primitive Pteridophytes, of the whole of the axis.
The vascular supply of the leaf (leaf-trace) consists of a single strand only in the haplostelic and some of the more primitive siphonostelic forms. In the microphyllous groups Leaf.trace of Pteridophytes (Lycopodiales and Equisetales) in and Petlolar which the leaves are small relatively to the stem, the Strands, single bundle destined for each leaf is a small strand whose departure causes no disturbance in the cauline stele.
It is found in most of P all I of those Pteridophytes which we have other reasons for e considering as primitive types, and essentially the same Ontogeny type is found, as we have seen, in the independently with developed primitive conducting system of the mossPh.~logeny.
I&Vertical section of a Palm-stem showing the root as it is found in most Pteridophytes vascular bundle,, Jr. curving and many Phanerogams has been already inwards and then outwards.
In the Pteridophytes, on the other hand, development from the egg is continuous.
In many Pteridophytes thi first leaf is formed very early, and the first vascular strand i!
In most Pteridophytes there is a single large apical cell at the end of each stem and root axis.
In most of the existing Pteridophytes, in the Monocotyledons and in annual plants among the Dicotyledons, there is n further growth of much structural importance in the ~ d ~
Stems. But in nearly all perennial Dicotyledons, in all dicotyledonous and gymnospermous trees and shrubs and in fossil Pteridophytes belonging to all the great groups, certain layers of cells remain meristematic among the permanent tissues, or after passing through a resting stage reacquire menstematic properties, and give rise to secondary tissues.
A considerable evolution in complexity can be traced in passing from the simplest forms of xylem and phloem found in the primary vascular tissues both among Pteridophytes and Phanerogams to these highly differentiated types.
This type is found in nearly all Pteridophytes and, so far as is known, in Cycadofihices, both in primary and secondary tissue.
In many annual plants no cambium is formed at all, and the same is true of most perennial Pteridophytes and Monocotyledons.
The plants showing it are not all forest trees, hut include also some Pteridophytes and some of the prothallia of the Ferns, Club-mosses, Liverworts and Horsetails.
This body has been called a blepharoplast, and in the Pteridophytes, Cycads and Ginkgo it gives rise to the spiral band on which the cilia are formed.
The Ferns form the great majority of existing Pteridophytes; the importance and interest of the other groups, of which the Club-mosses and Horsetails are the most familiar examples, depend largely on the fact that they are the surviving representatives of large families of plants which flourished in earlier geological periods.
For a discussion of this view, which regards the alternation of generations in Pteridophytes as antithetic and the two generations as not homologous with one another, reference may be made to the works of Celakovsky and Bower.
In addition to the three classes, Equisetales, Lycopodiales and Filicales, under which recent Pteridophytes naturally group themselves, a fourth class, Sphenophyllales, existed in Palaeozoic times, clearly related to the Horsetails and more remotely to the Ferns and perhaps the Club-mosses, but with peculiarities of its own demanding an independent position.
Consequently, the Pteridophytes, Gymnosperms and their allies had the field to themselves, so far as regards the higher plants, and filled places in nature which have now for the most part been seized on by families of more modern origin.
The evidence for terrestrial Silurian vegetation is still dubious; apart from some obscure North American specimens, the true nature of which is not established, Potonie has described well-characterized Pteridophytes (such as the fern-like Sphenopteridium and Bothrodendron among Lycopods) from supposed Silurian strata in North Germany; the horizon, however, appears to be open to much doubt, and the specimens agree so nearly with some from the Lower Carboniferous as to render their Silurian age difficult of credence.
Arborescent Pteridophytes are barely represented, and such dominant types as Lepidodendron, Sigillaria, Calamites and Sphenophyllum have practically ceased to exist; Cycads and Conifers have assumed the leading role, and the still luxuriant fern vegetation has put on a different aspect.