Plants of the Palaeozoic Period

A large number of fossil-stems are known which, judging from their general structure, in all probability belong to the Pteri dosperms. Nothing is known about their reproduction. In Megaloxylon, an Upper Carboniferous genus, the vascular cylin der consisted of a solid core of primary wood. The protoxylem elements are found in groups at the periphery and when followed upwards each group passes out to a leaf. On tracing them down wards each group spreads laterally round the stele in a tangential direction and finally becomes indistinguishable from the rest of the primary wood and so disappears. There was a considerable development of secondary wood round the primary cylinder.

The Calamopityeae, from the Lower Carboniferous though pos sibly also represented in the Upper Devonian, have in the centre of the stem a mixture of tracheids and parenchyma surrounded by a ring of xylem strands in each of which the first differentiated tracheids are in the centre. Surrounding the ring of strands is a sheath of secondary wood. The vascular supply to the leaf sepa rates from one of the large xylem strands as in Lyginopteris and then divides further out into two strands each of which is sur rounded by its own zone of secondary wood. This massive type of leaf supply is particularly characteristic of the Pteridosperms and is unlike the comparatively slender leaf-supply in the Gym nosperms. In the Protopityeae, unlike all plants known to be Pteridosperms, the leaves were arranged in two opposite series as in some species of Psaronius. The pith is elliptical with a sheath of primary xylem surrounding it. The leaf vascular supply is given off from the ends of the ellipse. Secondary xylem was present. Of more particular interest are the stems included in the Cladoxyleae. Cladoxylon scoparium (fig. 20) from the Middle Devonian of Germany is polystelic like Medullosa and the leaves which have been found in this species are deeply cleft and twig like and are only inch in length. The fertile leaves are not so deeply cleft and each division has a small terminal sporangium. Other Cladoxyla are known from the Lower Carboniferous; sev eral have secondary thickening. There still remains some doubt about the systematic position of Cladoxylon which may be either a Fern or a Pteridosperm.

Gymnosperms.

(Cordaitales, Coniferales, etc.)—Cordaitales. The Pteridosperms although they produced seeds had some very obvious features of agreement with the ferns : in the Cordaitales on the other hand there is little in common with the ferns and they may be regarded as fairly typical gymnosperms. There are three families provisionally grouped in the Cordaitales, the Poroxyleae, Pityeae and the Cor daiteae. Cordaites, the type genus of the Cordaiteae, of Carbonifer ous and Permian age, was a lofty tree (fig. unbranched except near the summit and bearing large leaves, linear or spatulate in shape. The leaves are traversed with numerous parallel veins and must have looked somewhat like the leaves of the Aspidistra; they are inserted in spiral sequence on the stem and branches. The name Noeggerathiopsis has been given to similar leaves found in the Carboniferous and Permian of the Southern Hemisphere where the Southern type of Palae ozoic flora is found: there is no important difference between them and leaves of Cordaites and a separate name seems unneces sary. The inflorescences consist of small lateral branches each bear ing numerous small catkins. Each catkin is about a centimetre long and has a short stout axis. In the staminate catkins the micro sporangia are borne in bunches of from two to five at the ends of slender filaments which contained vascular tissue. These fila ments bearing the sporangia arise from between the small, crowded scale leaves which formed the bulk of the catkin or were grouped together near the top. The pollen grains found in the sporangia are of large size. The ovulate or seed producing catkins like the staminate were formed of closely imbricating scales. Short stalks bearing ovules arose at intervals between the scales and according to one investigator corresponded in position to scales. In some specimens these stalks were very short but in others the seeds are found hanging out of the catkins on elongated stalks. It is possible that the stalk of the ovule lengthened as the ovule developed into a seed. The ovules and seeds are flattened in a plane tangential to the catkin. The integument of the ovule consisted of a hard inner layer and a fleshy outer layer while the nucellus had a well developed pollen chamber with a canal leading to it and was much like the nucellus of the living Cycas. In the pollen-chamber large pol len grains have been found simi lar to those found in the micro sporangia. The stem is con structed like that of a conifer ex cept that the pith is much larger and consisted of soft tissue which split transversely into a series of diaphragms in the older condition of the stem. When the stems started to decay these diaphragms broke down and internal casts in sandstone of these pith cavi ties exhibit transverse grooves which correspond to the remains of the diaphragms at the edge of the pith. These casts receive the

distinguishing name Artisia and it is not always possible to say whether they belong to Cordaites itself or to some nearly allied genus. The wood is entirely cen trifugal in Cordaites but in the closely allied genus Mesoxylon there was a small amount of centripetal primary wood. The primary strands project into the pith. In cellular organisation the wood is very like that of the living Araucaria but like most other Palaeozoic woods from the Northern area there is no evidence of seasonal periodicity in the growth as indicated by the presence of "annual rings." The vascular supply to the leaf is represented by two of the primary strands surrounding the pith, which at higher level pass out wards and on reaching the base of the leaf fork repeatedly and link up with the numerous veins of the leaf lamina. This double vascular connection with the axis is also found in Ginkgo (maiden hair tree) and is also found in the Pteridosperms. In the struc ture of the leaf some species of Cordaites show close resemblance to the cycads, for in each vein the xylem is in two parts ; the larger strand lying next the upper surface of the leaf consisting of centripetally developed secondary wood while the underlying strand is centrifugal with the bast lying immediately below it. In other species there is no strand of centrifugal wood. Both these types of vein are found in the living cycads sometimes in the same leaf for the base of a cycad leaf centrifugal xylem may be present while higher up it may completely disappear. The veins in some species are surrounded with a sheath of cells with bordered pits just like the corresponding tissue in the leaf of the cycads. Thus in Cordaites we find a remarkable combination of pteridosperm, cycad and conifer characters. The fossil roots called Amyelon radicans though as yet they have not been found actually attached to the rest of the plant from their almost constant asso ciation and agreement in certain microscopical characters are almost certainly parts of cordaitean plants. They are typical gymnosperm roots and some of the smaller ones have been found to be infested with fungal mycelium. From the position of the fungus in the roots it is almost certain that they afford an example of the association of a fungus with the higher plant which is com mon in living gymnosperms as well as in some of the angiosperms and is called Mycorrhiza (q.v.). The leaves of the Poroxyleae, a Permian family, attained the length of a metre and a fifth of a metre in width and show resemblance in structure to leaves of the cycads. The branches arise in the arils of the leaf as in some species of Lyginopteris while the vascular tissue of the branch connects up with that of the stem as in the pine. Structurally the stem is more like Lyginopteris than Cordaites. The xylem of the primary bundles surrounding the pith is entirely centripetal. Where the vascular tissue to the leaf connects with the vascular tissue of the axis it consists of two bundles similar in construction to those of cycad leaves. When traced further down the stem these two bundles appear to fuse and still lower they are only rep resented by a strand of secondary xylem. In the Pityeae, a Lower Carboniferous and Upper Devonian group, while there are char acters indicating a relationship to the rest of the Cordaitales there are also features in their construction which mark them out as a rather distinct group. Pitys Dayi, a plant of arborescent habit from Lower Carboniferous rocks on the shore of the Firth of Forth, had short conical projections on its branches representing the leaves which had no flat, expanded surface. These short squat leaves must have made the tree appear like a coarse Araucaria ex celsa. The stem had a wide pith with a ring of vascular bundles near the periphery with other smaller bundles scattered through it. The first differentiated elements are in the centre of these bundles. The relation between these strands and the rest of the vascular system is unknown. The lower part of the bundle supplying the leaf was situated in the peripheral ring and the upper part forked into three strands in the base of the leaf. A zone of secondary wood surrounded the ring of primary strands but was separated from them by a narrow zone of parenchymatous tissue. Nothing is known about the fructifications of these plants. In Callixylon an Upper Devonian genus there are no scattered bundles in the pith but otherwise the structure is not unlike that of Pitys. There is little doubt that Callixylon and Pitys are closely allied. Al though an older group than the Cordaiteae and Poroxyleae the Pityeae show closer resemblance to the Araucarian conifers and less evidence of affinity to the Pteridosperms and cycads. A con sideration of the comparisons that have been made between these groups leads to the belief in a common ancestral group from which they have all descended.