Quick Notes on Cordaitales. After reading this article you will learn about: 1. Introduction to Cordaitales 2. Systematic position of Cordaitales 3. Features 4. Reproductive Structures 5. Strobilus 6. The Ovule 7. Affinities.
Introduction to Cordaitales:
The Cordaitales is another fossil group which lived side by side with the Cycadofilicales in Palaeozoic times. Both the groups were very common, the Cordaitales being then the prevailing gymnospermous forest plants. The group extends from Devonian through Carboniferous upto the Permian times, and in India they had been found even in the lower Mesozoic, i.e., in Lower Gondwana strata of India.
The plants of this group were trees some even more than ninety feet in height and three feet in diameter. The plant branched at only tips and bore generally narrow and simple leaves in clusters at top.
According to Scott, the order may be divided into three groups:
1. Poroxyleae.
2. Pityeae and
3. Cordaiteae.
The details, especially of first two groups are not fully known. Their separation from Cordaiteae is based on structure of vegetative parts.
1. Poroxyleae:
This had a single genus known as Poroxylon. Here the stem was slender with long internodes and within the stem were collateral exarch primary bundles, the pith was large, and double leaf traces for each leaf which had thick parallel venation in it.
2. Pityeae:
It included a single genus Pitys where the bundles were mesarch, the wood was with multi-ciliated pits, so it differed from Cordaiteae having mesarch bundles.
3. Cordaiteae:
It included a number of genera. The best known genus was Cordaites.
Systematic position of Cordaitales:
Division. Gymnospermae
Class. Coniferopsida
Order. Cordaitales
Family. Cordaitaceae
Genus. Cordaites.
Features of Cordaitales of Cordaitales:
(A) Morphological Features:
The plant was tall and slender. It had a crown of branches near the top, leaves were large simple and pendulate and not found in any living gymnosperms.
(B) Anatomical Features:
The internal structure of stem in Cordaites showed a combination of Cycas and conifers characters. Pith like cycads layer but discoid in form, i.e., it was in form of plates at intervals. The vascular cylinder was like those of conifers. It was made up of thick collateral endarch primary bundles which sometimes however, seem to have been mesarch as in Mesoxylon.
There was good deal of normal secondary growth. Secondary xylem was made up of living pitted tracheids with pits on radial walls. In fact it is clear that majority of plants found in Carboniferous period belonging to Cycadofllicales, Cordaitales and a number of fossil pteridophytes. The stems had good deal of secondary growth in them, but in these no annular rings.
The stem anatomy of Cordaitales shows variations from exarch to end-arch collateral primary vascular bundles with good deal of secondary growth in stem. So far anatomical structure is concerned the group is related to Cycadofilicales in having exarch and mesarch bundles and to gymnosperms especially to conifers in having developed endarch bundles.
Reproductive Structures of Cordaitales:
The plants were monoecious or dioecious but the strobili were always monosporous and never bisporangiate like Cycadeoideales (Bennettitales) or living Gnetales. Cordaitanthus was the reproductive shoot of Cordaites itself It was not definitely organized to form a cone but it was more or less a strobilus.
The male and female strobili were borne separately in lateral sides of short simple axis. The strobili were enclosed when young by bracts so outwardly they could not be distinguished unless sections were cut and examined. Both were usually of small size.
Strobilus of Cordaitales:
Male Strobilus:
The male strobilus had a thick axis bearing spirally arranged bracts with stamens on them, the stamens themselves were either solitary or grouped near the apex and in between the stamens there were sterile bracts present. These bracts might have been sterile microsporophyll’s.
Each individual stamen was peculiar in form, in that it had a cluster of three to six terminal sporangia similar types of stamens have also sometimes described belonging to Cycadofilicales, but the important difference was that at the tips were borne synangia and separate or individual sporangia.
If in this male catkin of Cordaites there were no sterile bracts the structure would be very much like the staminate strobilus of Ginkgo but there is a very important difference and that in the Cordaitanthus the sporangia were terminal while in Ginkgo they were pendant and only two in number.
In fact the morphology of this male strobilus of Cordaites is not quite clear. Different interpretations are put on male catkin of Cordaites. Some think that whole of strobilus acting as single male flower in which there are large number of stamens, the stalk being the filamentous bearing a number of pollen sacs were simply sterile filaments.
Other workers described this male catkin as inflorescence, each catkin bearing a flower itself, the filament is stalk of flower, the pollen sacs the stamens.
Female Strobilus:
The female strobilus when young was enclosed by bracts. It had a thick conical axis bearing a number of spirally arranged bracts. In the axils of some of these bracts there were dwarf shoots present. These dwarf shoots in turn had minute bracts. On the tip of dwarf shoot there was a terminal ovule.
The Ovule of Cordaitales:
In the ovule the nucellus was completely separate from the integument. The integument formed a micropyle; at the base of micropyle the nucellus formed a prominent nucellar beak and a large pollen chamber. Round the ovules there were two coats outer thick and fleshy and the inner one delicate in young ovule but hard in older ovules.
Both these coats were separate below but more or less united above. So these coats may be interpreted as two separate integuments which are formed at upper part or a case of single integument which differentiated into three coats one of which not preserved.
The female strobilus is distinctly compound in form as compared to male strobilus. The ovule bearing stalks were distinctly axillary in position just like those of present day conifers.
In Cycadofilicales, Cycadeoideales (Bennettitales) and the living cycads the ovule bearing structures are not axillary in position and the ovules in these groups were borne on leaves while in Cordaitales and the Coniferales the ovules are borne on stem structure. Dr. Sahni according to the position of ovules recognized two groups.
1. Phyllosperms:
The ovules borne on leaves as seen in pteridosperms and Cycas.
2. Stachysperms:
The ovules as seen in Cordaitales, Ginkgo and the Coniferales.
A large number of detached seeds have been described from Palaeozoic times and when they separated from branches they cannot definitely be assigned to Cycadofilicales and to Cordaitales. Generally according to Dr. Sahni those belonging to Cordaitales were flat and described as platy-sperms and more or less heart shaped in appearance and no embryo in them like those of Cycadofilicales.
Affinities of Cordaitales:
Cordaitales and Cycadofilicales lived side by side in Palaeozoic time, both were very abundant in Carboniferous and disappeared just about Permian. Backwards both groups extend to Devonian, which of two is more ancieant cannot be definitely stated.
Both seem to be related is clear from their habitat, the general form and structure of plants, the form of strobili especially male one, structure and form of ovule and finally of seed was very primitive in the two in the ovule of the two the presence of free nucellus.
The form of integument with two sets of vascular bundles in it, and the form of pollen chamber and lastly the absence of embryo are really primitive features which naturally present in the primitive groups. All these resemblances mean that either the Cycadofilicales give rise to Cordaitales or the Cordaitales to Cycadofilicales.
All combined features show that Cycadofilicales are more primitive while the Cordaitales show advanced features.
The other possibility is that both of them might have arisen from some common ancestral form as they lived side by side. So this second possibility is more accepted but what the ancestral form is not known.
Evidently it must be some heterosporous pteridophyte which lived near about the Devonian might have given rise to Cordaitales and Cycadofilicales which is clear from the stem anatomy of Cordaitales in which endarch siphonostele was more common.
The strobili of Cordaitales are better organized. The sporophylls here completely lost their leaf like appearance. The female strobilus was more complicated in that ovule bearing branch was always axillary in position. In Cycadofilicales the strobilus was simple and ovules were borne on leaves.
So these Cycadofilicales and Cordaitales can be said to represent two different lines of evolution; the Cycadofilicales was left behind with more primitive features in it and before it disappeared it gave rise to two branches of Cycadophyta line the fossil Cycadeoideales (Bennettitales) and living Cycadales.
The Cordaitales made some advance both in anatomy of stem organization and complexity of its reproductive parts. Being a primitive group naturally it had resemblances with Cycas which is seen in the presence of large pith, general leaf anatomy, general structure of seed and ovule and presence of sperms.
Chamberlain is of opinion that Cordaitales represents one line of Gymnosperms, the Coniferophyte line characterized by the presence of large sized plant with pro-fused branched stem and simple leaf, small pith, abundant wood and narrow cortex. The reproductive structure organized to form more or less cones.
The Cordaitales seems to have given rise to Ginkgoales and conifers before it disappeared in or about in Permian times. In Ginkgoales there are good deal of resemblances. Both the plants are lofty and branched. The stem anatomy, the structure of ovule, and in presence of sperms two groups seem to be more or less closely related.
There is great similarity in the outward appearance of male strobili of Ginkgo and that of Cordaitales.