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In this article we will discuss about:- 1. Habit of Lepidodendron 2. Internal Structure of Lepidodendron 3. Reproduction.
Habit of Lepidodendron:
Like other ancient lycopods, Lepidodendron was also tree like in habit (Fig.55). In general appearance it was not unlike that of present day Lycopodium. But in size the genus enormously exceeded the herbaceous Lycopodium. The petrified trunks were sometimes as long as 100 feet. Judging from this it may be safely assumed that the plant reached a height of over 120 feet.
The stem was erect and did not branch up to some distance from the ground. The branching of the stem was typically dichotomous. The ultimate dichotomies produced the leaves. The branches and the foliage formed a sort of crown at the apex of the stem.
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The leaves which clad the young stems and branches were acicular or linear in shape having a length of 5-9 inches. The arrangement of the leaves was spiral or very rarely they showed a whorled arrangement. The leaves were ligulate.
Each leaf had a single vein with the stomata situated in two bands on the ventral surface. The leaves were deciduous. Upon abscission a flat rhomboidal scar persisted on the stem resembling a small cushion. The base of the stem had a stigmarian type of root system.
Internal Structure of Lepidodendron:
1. Stem:
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In majority of the species, secondary growth is characteristic. But some species seem to lack a cambium. A transverse section of the trunk, of L. vasculare shows three regions, stele, cortex and a periderm.
In the primary structure there was an epidermis but soon i.e. even before the initiation of secondary growth in the vasculature, it was replaced by the periderm.
The periderm was produced by a phellogen which produced phelloderm towards the interior and phellem towards the exterior. The outline of the bark was wavy due to the presence of leaf bases (Fig. 56).
Internal to this was the cortex.
It consisted of four regions viz.:
(1) Outer cortex consisting of alternating bands of sclerotic and parenchymatous cells,
(2) A middle cortex having a homogenous mass of parenchyma cells. Interspersed with the parenchyma cells were the leaf traces,
(3) Secretory zone consisting of glandular cells which were filled with a dark coloured substance. They probably secreted the waxy material which covered the surface of the stem,
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(4) An inner cortex having parenchyma cells.
The central region of the stem was occupied by the stele which was either protostelic or siphonostelic. The protoxylem was exarch and polyarch. In many species (L. vasculare) there was a secondary growth initiated by the cambium.
This produced secondary xylem to the interior and secondary phloem to the exterior. The cambial activity was not uniform, as a result there was a tendency for the formation of an eccentric vascular ring. The secondary xylem had radial rows of tracheitis separated by xylem rays which were uniseriate.
2. Leaf:
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Anatomically the leaves showed a single vascular bundle flanked on either side by parichnos cavities. These are believed to be aerating organs.
Reproduction in Lepidodendron:
The strobili of Lepidodendron are given the name Lepidostrobus. In general structure they had a central axis bearing spirally arranged or whorled sporophylls. The sporophylls were ligulate and somewhat peltate bearing a single, sessile, elongate sporangium on their adaxial face.
It is quite possible that some sporangia were trabeculate, the trabeculae being concerned with nutrition. The strobili were heterosporous with the megasporophylls aggregated towards the base.
