In this article we will discuss about the life cycle of cordaites. Also learn about its affinity.
The Sporophyte of Cordaites:
Tree-like in habit, tall and slender trunks, branching only at the top, and bearing long, slender, spirally arranged leaves. The leaves are tough, leathery, linear, lanceolate to spathulate in shape, and there is no midrib. The veins are arranged in a parallel fashion.
The structure of the stem shows a large parenchymatous pith, which is partly septate due to the formation of diaphragms, as is found in the modern Fuglans. The primary wood is small and endarch. The secondary wood forms a thick layer; tracheids are pitted. Pits are uni-, di-, or tri-seriate; the rays are simple. Growth rings are rare.
The central cylinder of the root is a di-, tri-, or tetrach protostele, and is surrounded by secondary wood. The periderm is much well-developed.
Internally, the leaf shows xerophytic characters, and possesses a thick cuticle. The upper epidermis is highly cuticularized, below which there is a thin hypodermis, composed of one or two layers of thick-walled cells. The mesophyll is mainly composed of palisade parenchyma cells; the cells towards the lower epidermis are angular, closely set and without any intercellular spaces.
The vascular bundles are arranged in a parallel series; each bundle is surrounded by a sheath, and is connected with the hypodermis by means of a band of thick-walled cells. Though the lower epidermis is not found in a very well-state of preservation, the guard cells are noted to be provided with 4-6 subsidiary cells.
Cordaites may be either monoecious or dioecious, and bear small monosporangiate strobili on stalks, the male ones being more numerous than the female ones.
Staminate (or Male) and Pistillate (or Female) Strobilus:
It consists of a short axis bearing spirally arranged, sterile bracts, and fertile microsporophylls, each of the latter supporting a number of microsporangia or pollen sacs. The microspores or pollen grains are globose, and the exine is inflated forming a big air-containing sac, which, with the exception of a small area at one corner of the pollen grain, can be regarded essentially as an antheridium, comparable to that as found in the present-day gymnosperms, and probably gave rise to the pollen tube.
Pistillate (or Female) Strobilus:
Like the male one, it also bears a stout axis, bracts and a few (usually 1-4) megasporophylls. The ovules are borne on slender stalks, and in fully ripened specimens, the seed completely outgrows the bracts. The general plan of construction of the ovule is of the cycadean type. The nucellus is practically free from the three-layered, solitary integument, excepting at the base; the nucellar beak projects into the micropyle. The pollen chamber is conspicuous.
The Gametophytes of Cordaites:
Male Gametophyte:
Soon after the shedding of the winged pollen grain, a large number of cells develop within its body. The nature of the cells is not properly known.
Female Gametophyte:
There are two archegonia present and the structure is an elongated one.
In all probability the fertilization was effected with the help of motile sperms.
Seed:
The seed is flattened and winged.
Embryo:
Regarding the embryo nothing is practically known, excepting the recent findings of Darrah from the coal balls of Iowa.
Affinity of Cordaites:
This order exhibits a combination of characters of the Pteridosperms, Cycadeoideales, Cycadales, Ginkgoales, and Coniferales, together with peculiarities of its own. As to the structure of the seed, large pollen chamber and two sets of vascular strands, it resembles the Pteridosperms. It shows an affinity with the Cycadeoidales in the general organization of strobili and reduction of bracts, but if differs from the latter in its monosporangiate nature.
Its resemblances with the Cycadales are also very prominent, because of the large pith, the abundance of sclerenchyma in the leaf, the general structure of seed, and the probable presence of swimming sperms. The anatomical evidence, general habit, leaf structure, and nucellar beak suggest its affinity with the Ginkgoales. The resemblance between Cordaitales and Conifers in the lofty habit, and the inflorescence (Cordaianthus) can be homologized to the abietinean cone.
Cordaitales represent a state of comparatively high development among the Paleozoic plants and phylogenetically occupy an intermediate position between the recent Cycads and Conifers. They were more or less on the same level of modern Gymnosperms.
(a) Affinities with Cycads:
1. Large pith and cortex.
2. Abundant sclerenchyma in the leaf.
3. Relatively simple and fairly large seeds.
4. Probability of motile sperms.
5. Relatively simple structure of the wood.
(b) Affinities with Ginkgoales:
1. Lofty and branching habit as in Pityeae and Poroxylaceae.
2. Fan-shaped leaves were probably, borne by some cordaitalean trunk.
3. Probable motility of sperms.
4. General anatomical plan and the occurrence of double leaf trace—indicate the similarity of both the groups.
(c) Affinities with Pteridosperms:
1. Structure of the seed—from nucellus and integument in certain seeds of both the groups; large pollen chamber, two sets of vascular strands and the absence of embryo in both the groups.
2. Anatomy—large pith, primary wood mesarch in Mesoxylon, lyginopterid-like secondary wood in Poroxylon, double leaf trace, etc.
(d) Affinities with Coniferales:
1. Lofty and arborescent habit—a feature shared by Pityeae and possibly by Poroxylaceae.
2. Leaves of Pityeae have been compared to those of Araucaria by Prof. Gardon.
3. Similarities in general plan of anatomy:
(a) The presence of multiseriate bordered pits and
(b) Absence of resin canal in the wood of Cordaitae suggest they are more related to the Araucariaceae than the Pinaceae.
4. Cordaitean inflorescence (Cordaianthus) and abietinean cone are probably homologous with intermediate graduations noticed in some Paleozoic Conifers, e.g., Ernestiodendron, Lebachia, Pseudovoltzia and Ullmannia.
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