Lepidodendron is the best known genus of the Palaeozoic and is with more than 100 species. It appeared in the Late Devonian, flourished most luxuriantly in the Carboniferous and probably declined during the Late Permian.
The Sporophyte of Lepidodendron:
The plant is a large tree with stigmarian root-system and a straight trunk, which remains un-branched for a considerable height and then gives rise to freely dichotomously dividing branches. The leaves are simple and deciduous, acicular to linear in shape, and ligulate. These are borne either spirally or in alternating whorls. The pyramidal leaf bases are persistent in nature and form the leaf cushions, which have got a great taxonomic importance.
The leaf base possesses a single, small, median vascular strand, flanked on both sides by a strand of parenchymatous cells forming the perichnos. The parichni run parallel to the vascular strands for some time but ultimately become lost in the mesophyll tissue; these are supposed to be associated with the intake of air and its circulation through the body of the plant.
Internally, the stem can be differentiated into extrastelar and intrastelar regions.
The cortex is very thick and is characteristically differentiated into:
(1) A parenchymatous inner cortex,
(2) A secreting zone,
(3) A homogeneous middle cortex, similar to the inner one, and
(4) An outer cortex composed of thin- and thick-walled cells.
The stele is either a protostele or a siphonostele with exarch and polyarch xylem. In some cases, secondary growth has been recorded in the intrastelar region. The periderm is formed very early and may appear even before the intrastelar secondary thickening.
Numerous fossil lepidodendroid cones have been discovered and these are usually referred to the form genus Lepidostrobus. The cones are cylindrical or elliptical in outline, about 3.30 cm. in length and 2.6 cm. in breadth; these are usually slightly tapering both at the apex as well as at the base.
The sporophylls are either spirally arranged or whorled, and standing out approximately at right angles from the central axis. Each sporophyll bears a ventral sporangium, whose wall is made up of a single layer of prism-shaped cells. The sporangia contain either microspores or megaspores.
The Gametophyte of Lepidodendron:
No male gametophyte has yet been recorded, but Gordon (1910) discovered cellular, female gametophytes bearing archegonia; this suggests a possible relationship with the living Selaginella.
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