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In this essay we will discuss about:- 1. Root of Cordaitales 2. Stem of Cordaitales 3. Leaf 4. Fructification 5. Seed.
Essay # 1. Root of Cordaitales:
The roots of cordaitales are highly branched and shallow forming stilt roots like those of mangroves. This indicates the halophytic nature of the plants which grew along the sea shores.
Anatomically, the root of cordaitales has an actinostelic structure with diarch to tetrarch primary wood which is surrounded by enormous secondary wood (Fig. 1.51). The periderm is well-developed in the cortical zone associated with many lenticels.
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The cortex may be distinguished into two zones, sometimes infected with endotrophic mycorrhizal fungi. In older roots, a pith is formed at the centre by the medunation of metaxylem, thus the protostelic root transformed into a siphonostelic configuration.
Essay # 2. Stem of Cordaitales:
Though Cordaites trees are very large, they bear very slender branches.
Anatomically, the stem of cordaitales shows siphonostelic configuration with a narrow band of primary vasculature in between the large pith and pycnoxylic secondary wood (Fig. 1.52A, B). The primary xylem in Pennsylvanioxylon is endarch, while it is mesarch in Mesoxylon.
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The hexagonal bordered pits, restricted to radial walls, are in two or more rows and are very crowded. There are frequent uniseriate rays in the wood (Fig. 1.52C). The secondary phloem is made up of radially arranged sieve cells, parenchyma and fibres.
The cortex in younger stem exhibits secretary sacs, in older stem the cortex is replaced by periderm (Fig. 1.52B). The pith shows the evidence of large air chambers separated by diaphragms (Fig. 1.52D). During the elongation of stem, the pith cracks transversely with intact periphery, thus appearing like a pile of concave discs. Hence, such a pith is called discoid pith.
The stem of cordaitales exhibits many leaf traces which appear to be double, arising from sympodia, and each leaf trace is flanked by two branch traces. In Pennsylvanioxylon, the primary xylem in leaf trace is endarch and centripetal, while it is mesarch and both centripetal as well as centrifugal in Mesoxylon.
Essay # 3. Leaf of Cordaitales:
The leaves of cordaitales are simple, sessile and long slender, up to 1 m in length (Fig. 1.53A). They are mostly linear, but may be lanceolate, spathulate or obovate. The venation is parallel. The Cordiates leaves may be sub-divided into three sub-genera based on the leaf forms, viz., subgenus Dory-Cordaites (leaves—strap-like with pointed tip), Eu-Cordaites (leaves strap-like with blunt tip) and Poa-Cordaites (leaves grass-like).
Anatomically, the leaves of cordaitales exhibit xerophytic characteristics. Both the upper and lower epidermis are heavily cutinised (Fig. 1.53B). The stomata are haplocheilic and mostly confined to the lower epidermis. The stomatal apparatus is comprised of two guard cells (parallel to veins) surrounded by 4-6 subsidiary cells.
The palisade and spongy tissue are poorly differentiated with a large number of lacunae in between the veins. The thick-walled sclerenchymatous hypodermal cells are present on two surfaces of the leaf which often enclose the veins or may form l-shaped girdle between veins. Each mesarch vascular bundle is surrounded by two-layered bundle sheath.
Essay # 4. Fructification in Cordaitales:
In cordaitales, there is a homology between the male and the female fructifications; both the fructification are assigned to the genus Cordaianthus.
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Cordaianthus is a compound structure comprising of a primary axis bearing many secondary (dwarf) shoots subtended by bracts (Fig. 1.54A). Each secondary shoot is radially symmetrical and consists of a central axis with spirally arranged scales (Fig. 1.54B, C).
The primary axis is somewhat flattened and bilaterally symmetrical containing an endarch medullated stele. The main vascular cylinder of primary axis forms traces that depart alternately from two areas situated at the opposite end of the stele to the four ranked bracts and secondary shoots. The single trace that enters the secondary shoot again branches, so that each scale receives vascular supply.
i. Male Fructification:
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The secondary shoot in male fructification is about 6 mm long. The shoot consists of 25-40 scales, of which 5-10 distally arranged scales (towards apex) are fertile, while the remaining proximal scales (away from the apex) are sterile (Fig. 1.54C).
The fertile scales bear 4 (C. saportanus) to 6 (C. concinnus, C. penjoni) microsporangia (pollen sacs) at their tip (Fig. 1.54C). The microsporangia are fused at the base. The vascular trace supplying the fertile scale dichotomously branches at the tip so that each microsporangium receives vascular supply.
The microsporangium is an elongated finger like, single-layered structure containing numerous microspores or pollen grams. Florinites is the best known pollen type having a single wing (monosaccate).
ii. Female Fructification:
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The secondary shoot in female fructification consists of 16-20 scales, of which 4-6 distal scales are fertile and the remaining proximal scales are sterile (Fig. 1.54B). Each fertile scale bears a single reflexed (pendulous) ovule at its tip. Sometimes, the fertile scale is forked and contains two to three terminal, pendulous ovules.
Essay # 5. Seed in Cordaitales:
The seeds of cordaitales are cordate, flattened and bilaterally symmetrical (Fig. 1.55A, B).
The integument is three-layered:
(i) the outer fleshy sarcotesta which is laterally expanded into a wing,
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(ii) the middle sclerotesta with spine-like projection into sarcotesta, and
(iii) the endotesta made up of thin-walled cells.
The integument of cordaitales is free from the nucellus except at the base and the nucellar beak of the pollen chamber projects into the microphyle.
There is a single vascular strand that enters the ovule. In Cardiocarpus, the single trace enters up to the base of the nucellus, then it divides to form two branches that extend through the inner side of sarcotesta.
A second pair of strands passes through the peripheral zone of the nucellus. In Mitrospermum, the two lateral strands enter the sarcotesta moving through the sclerotesta. In the sarcotesta, each strand divides further to form several bundles (Fig. 1.55A).
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Origin of Seed-Cone Complex:
It has been established that the cones of Pinus are modified compound shoots which show homology with the Cordaianthus fructification. The ovuliferous scales are the modified secondary shoots of Cordaianthus and the bract scales of Pinus are the modified bracts of Cordaianthus.
In the evolutionary stages, the radially symmetical secondary shoots of Cordaianthus (Fig. 1.56A) have gradually been planated to become bilateral shoots of Voltziales (e.g., Ernestiodendron, Wallchiostrobus).
In the next stage of evolution, both the fertile and sterile scales of secondary shoots are laterally fused to form a common platform (ovuliferous scale) bearing ovules (e.g., Pseudovoltzia, Glyptolepis) (Fig. 1.56B, C). Thus, an ovuliferous scale is formed which, in turn, is fused with a bract to form a bract-scale complex.
In the present day conifers, the cone axis (= primary axis) bears spirally arranged seed-cone complexes (= secondary shoots). Each seed-cone complex is comprised of ovuliferous scale bearing ovulus and bract scale (Fig. 1.56D). During evolution, the female cones of living conifers have undergone further morphological changes.
The evolutionary pathways that followed in the origin and evolution of conifer cones are:
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1. Change in the secondary shoots from the radially symmetrical condition to bilateral symmetry.
2. Lateral fusion of fertile or both fertile and sterile scales to form ovuliferous scale.
3. Shifting from erect ovules to inverted ovules.
4. Reduction in the number of ovules.
