Classification of Cordaitales: 3 Families | Plants

The following points highlight the three families under which cordaitales has been classified. The families are:- 1. Pityaceae 2. Poroxylaceae 3. Cordaitaceae.

Family # 1. Pityaceae:

Pityaceae is known only from trunks of wood and a few vegetative shoots. Occur­ring in the Devonian and the Lower Carboniferous, they comprise the oldest of the Cordaitales. Arnold (1947) is of opinion that placing the Pityaceae within the Cordai­tales is only provisional pending future discoveries.

The recent discovery of the Callixylon-Archaeopteris association has brought into,doubt the- systematic positions of even the other members of Pityaceae..

The trunks fossils representing the family were of considerable girth signifying tall trees. In a trunk a large central pith is surrounded by strands of mesarch primary xylem some of which are in contact with secondary xylem while others are free and embedded in the pith. The pith is paienchymatous but often mixed with scattered tracheides or even with complete mesarch strands.

Leaf traces pass out as branches from the primary strands without causing leaf gaps. The secondary wood forms a compact cylinder without wood parenchyma and is composed of alternating narrow rays of bordered pitted tracheides and medullary rays as generally found in conifers. Some of the forms come very close to Calamopitys. The wood also shows certain definite Araucarian characters.

Of the three genera assigned to the Pityaceae, Callixylon and Archaeopitys are from the Upper Devonian while Pitys is from the Lower Carboniferous.

Callixylon from the-United States Upper Devonian (Fig. 698), which was placed within the Pityaceae from its anatomy, has aroused great interest of late. Trunks up to 5 ft. in diameter have been discovered signifying tall trees. The tracheides show multiseriate bordered pits placed in intermittent groups leaving clear pitless surface in between (Fig. 698B).

Beck (1960) has discovered a stem of Callixylon in association with Archatopteris fronds. It clearly assigns this to the pteridophytic Archaeopteridales.

Although it cannot be said that all the stems placed under the form genus Callixylon are Pteridophytes, the discovery points out that many members of the Pityaceae may be actually pteridophytic. The taxon Progymnosperm has, there­fore, been suggested as the heterosporous pteridophytic group from which the gymnosperms might have arisen.

Callixylon. A. T.s. of stem (after Arnold). B. Radial l.s. of secondary wood showing bord­ered pitted tracheides and medullary rays.

Pitys, from the Lower Carboniferous, is known not only by its wood (sometimes with feeble annual rings) but also by a leafy shoot, Pitys dayi. Trunks 50 ft. long and’ 10 ft. in diameter are known. The ‘leaves’ appear to be cylindrical fleshy cladodes 4 to 6 mm in diameter and up to 50 mm long. There is no lamina but there are hairs and sunken stomata in the epidermis.

These leaves were regularly shed, only the tip cluster remaining attached. At the tips the cladodes were clustered to form buds. This type of leaf is not in conformity with that of Cordaitaceae and brings Pitys nearer to Araucariaceae.

Family # 2. Poroxylaceae:

Poroxylaceae comprises of the single Permo-Carboniferous genus Poroxylort from France which, again, is known mainly as trunks and shoots. Although later in appear­ance, the stem structure of Poroxylon is more primitive. The primary xylem is exarch instead of mesarch and the secondary wood is made up of large cells and broad rays more like the Cycadofilicales than the Cordaitales.

The leaves and leaf traces are, however, cordaitean. Some cordaitean seeds (Rhabdocarpus) have also been found in association with these stems and leaves. A gradation is suggested from Poroxylon to Mesoxylon and finally to Cordaites.

Family # 3. Cordaitaceae:

Cordaites is the main genus of Cordaitaceae and is often considered as the only genus, the other generic names being considered as form genera of its different parts. The name Cordaites was at first used only for the leaves but gradually it has come to be applied to whole plants.

The wood of Cordaites has the named Cordaixylon, Dadoxylon, Noeggerathiopsis and Mesoxylon. Of these, Mesoxylon has a different anatomy (con­sidered later) so that it is often considered as a genus separate from Cordaites. Mesozoic. Araucarioxylon was sometimes included within Cordaitaceae but now its more probable affinity seems to be with Araucariaceae.

The root called Amyelon is usually associated with Mesoxylon. Inflorescences and strobili are named Cordaianthus. De­foliated leaf stalk compressions – Cordaicladus. Pith casts— Artisia (=Stemberhia). Seecls – Cordaicarpus, Cardioearpus, Mitrospermum and Samaropsis. Prominent cordaitean

Indian Lower Gondwana fossils are— Neoggerathiopsis hisolpi, Samaropsis spp. and Gordaicarpus sahnii from Upper Carboniferous:

Neoggerathiopsis hisolpi, N. stoliczkana, N. whittiana, Dadaxylon indicum, Samaropsis raniganjensis, S. milleri and Cordaicarpus indicus from Permian. Some Neoggerathiopsis hisolpi is present in the Panchet-Parsora Stage which is in between Upper and Lower Gondwana and probably extends to the Lower Triassic.

Cordaites formed the dominant forest vegetation during Carboniferous and Per­mian when it was widely distributed. It became extinct after this age though some (Noeggerathoiopsis) seem to have persisted in India up to the Lower Triassic. It formed tall trees with trunks often reaching 100 ft. in height and 3 ft. in diameter (Fig. 699A).

The trunk was usually branched only at the top with the final branches bearing dense clusters of leaves suggesting that the trees were crowded together forming a deep shade.

The leaves are the best preserved. In contrast to the compound leaves of the Cycadophytes, the leaves of Cordaites are simple. These are leathery with a highly xerophytes structure, entire margins and dichotomous venation (Fig. 699B). But, there is little forking of the vein above the leaf base so that the fine veins look parallel and simulate monocot leaves.

This, of course, cannot signify any affinity with the very distant monocots.

The leaves are arranged spirally on the ultimate branches. A trans­verse section of the leaf (Fig. 699C) shows a cutinised thick-walled epidermis and a supporting hypodermis of thick-walled cells which latter expands near vascular bundles to reach them. In some leaves there is a distinction between palisade and spongy tissues while in others the mesophyll is uniform.

Haplocheilie stomata occur in longitudinal rows or irregularly on the lower epidermis. When preserved, they show 4 to 6 subsidiary cells. The parallel leaf veins are mesarch or sometimes exarch. The veins are connected by transversely elong­ated cells somewhat like the modern Cycad leaves. Thus, the leaf anatomy, on the whole, resembles that of the modern Cycads.

Grand’ Eury classified Corda­ites according to leaf forms into Eu-Cordaites (broad leaf), Dory- Cordaites (lanceolate leaf) and Poa-Cordaites (linear leaf). The leaves were 2 cm to 1 metre long and up to 20 cm broad.

The stem of Cordaites should be considered under two heads— Mesoxylon and Cordaites proper (which latter includes Dadaxylon). Mesoxylon stems are most abun­dant among Carboniferous Cordaitales and. are so different that they are often considered to form a separate genus.

Its stem structure is more primitive than that of Cor­daites and is intermediate (hence meso between Poroxylon and Cor­daites. Its cortex is differentiated into two layers as in the Cycadofilicales (there is a periderm and the wood, with visually large dis­coid pith) as in other Cordaites. The leaf traces are mesarch as in Poroxylon and are double at first but divide further into eight on entering the leaf.

The Cordaites stem (Fig. 700) shows a large pith in most species (it may be smaller in some), a thin zone of primary wood and a large cortex resembling the Cycads.

But, the secondary wood is well developed, thick and coniferous, rendering it distinct. The pith, 1 to 10 cm or more in diameter, is rather peculiar. It is entirely par­enchymatous but in course of growth it cracked transversely (Fig. 701 A) giving the appearance of a pile of concave discs.

Pith casts named Artisia (Fig. 701B- former name Stembergia, also c.f. Fig. 507) are obtained separately. The primary wood is a ring of endarch bundles. The mesarch primary bundles giving rise to leaf traces as found in Mesoxylon are absent here. The primary tracheides are annular, spiral, scalariform and finally pitted. Round this discontinuous ring is a thick layer of secondary wood.

The tracheides in secondary wood are long and slender with multiseriate (in 1 to 3 or more rows) bordeied pits on radial walls. The hprdered pits are hexagonal or flattened oval resembling Araucaria. The rays are usually uniseriate (Fig. 701C). Dark resinous material may be present in the rays but resin canals and xylem parenchyma are conspicuously absent.

Annual rings are not found in Carboniferous Cordaites but are frequent in Permian specimens signifying the change in climate. Dadoxylon shows both types. The cortex is parenchymatous with gum canals. A periderm develops during secondary growth.

The wood, specially the pits are Araucarian and for this it merges into the Mesozoic fossil trunk Araucarioxylon which is now assigned to Araucariaceae.

The cordaitean root fossil (Fig. 702) is named Amyelon. It is suspected to belong to Mesoxylon although no actual association has been found. It shows a central triangular protostele of diarch to tetrarch xylem surrounded by a thick secondary wood. The cortex shows a periderm.

Cordaites is monoecious or dioecious but the fructifications are always unisexual. The fructifications are lax inflorescences 10 cm or more long, borne on the stem (not necessarily the axils of leaves) among the leaves (Fig. 699B). Each inflorescence axis is a slender stalk bearing stiff bracts each of which subtends a short axillary bud-like strobilus.

The inflorescence and strobili fossils are placed in the form genus Cordaianthus.

Sometimes the strobili may be arranged in four rows but usually they are arranged spirally in two rows (Fig. 703A).

Eachlstrobilus is a dwarf shoot consisting of a short woody axis bearing spirally arranged woody appendages some of which are sterile bracts and others sporophylls. A male strobiliu (Fig. 703B) contains one to six or more microsporophylls or stamens each supporting one to six terminal microsporangia or unilocular anthers.

The female strobilus (Fig. 703C) similarly contains one to four megasporophylls or carpels each bearing a terminal megasporangium or ovule. In some cases the megasporophylls may be forked bearing two terminal ovules. The ovule or megasporangium (Fig. 703D) shows an elongated oval nucellar mass completely free from the single integument which is extended into a short micropylar canal at top.

A gap in the place of an elongated female gametophyte is noticed in some specimens.

In some mature ovules the nucellar beak projects into the micropyle and there is a pollen cavity in this beak where pollens are found lodged (Fig. 704A). These pollens show thin intines and thick exines. The exine contains a big air sac which rendered the pollen buoyant. The pollen contains a multicellular gametophyte.

There seems to be a lining of prothallial cells on the circumference and some spermatogenous cells in the centre. There is no evidence of any pollen tube formation. Even motile sperms are not known and may only be conjectured. If there was no pollen tube, fertilisation in Cordaites was almost as in the Pteridophytes. If a pollen tube was formed, it was nearer to the Cycadophyta.

By the time the seeds were mature the stalk-like sporophylls greatly increased in length and the seeds were suspended in air far beyond the bracts (Fig. 704B & C). Such seed fossils go by the names Cordaicarpus, Cardiocarpus, Samaropsis of Mitrospermum.

The seed (Fig. 704D) is flattened and heart-shaped, the nucellar portion of which is surrounded by an integument whose outer layer is expanded into a wing-like outer testa and the inner layer forms a hard, sclerotic inner testa. A vein from the base divides into two to supply the integument. The structures clearly signify pollination and seed dispersal by wind.