In this article we will discuss about the habit, habitat and structure of lepidodendron.
Habit and Habitat of Lepidodendron:
A new groups of arborescent lycopods, popularly known as Lepidodendrales, had evolved from the Middle Devonian lycopods.
These arborescent plants would grow to 164 ft (50 m) and form extensive coal measures swamp forest of the Northern Hemisphere Euramerican province during the Carboniferous period. Among these, Lepiclodendron was the most successful of all the arborescent members and is the best-investigated genus (Fig. 7.36).
In early stage of development, an endosporic prothallus grew straight up and subsequently it expanded its diameter by secondary growth until its apical meristem divided. Thus, in the successive dichotomy, the size of the branches decreased until the growth finally ceased with terminal twig. So the plant had a determinate growth pattern.
The Lepidodendron was a large tree (50-60 m tall) with a prominent trunk (up to 35 m height). The ultimate dichotomies formed the leaves. The branches and the foliage formed a spreading crown bearing cones at their tips.
The plant had bipolar growth, thus the main axis developed branches at both ends. The aerial branches formed three-dimensional dichotomies bearing branches and foliages, similarly the basal branches formed three-dimensional dichotomies bearing stigmarian root system.
Structure of Lepidodendron:
Anchoring and Water-Absorbing Structure:
In all the members of Lepidodendrales, the root-bearing underground axes are called rhizomorph and the detached rhizomorph and their roots are called Sigmaria which are mostly found as siliceous casts or molds. Stigmaria ficoides, the commonest species of Stigmaria, was a large trunk base that divided dichotomously into four large massive descending axes (Fig. 7.37).
These four axes penetrated the substrate of the swamp shallowly and again formed repeated dichotomous branches in the horizontal plane. The Stigmaria spread over an area of about 20 ft (6 m) across. The younger portions of the Stigmaria had spirally arranged roots, known as Stigmarian rootlets, while the older portions are marked by spirally arranged root scars that might have abscissed.
Anatomically, the main Stigmarian axes showed a distinct primary vascular system with endarch xylem. Secondary growth has been observed by the unifacial activity of cambium which only formed secondary xylem, while abundant extrastelar secondary cortical tissues were produced from the diffuse phellogen. In T.S., the free roots and root trace strand showed a monarch collateral vascular bundle comprised of protoxylem, metaxylem and phloem in centripetal sequence.
The root trace, surrounded by inner cortex, is slightly eccentrically placed within the large cavity formed by the dissolution of the middle cortex, which is again delimited by an outer cortex (Fig. 7.38). The Stigmarian rootlets are comparable with the roots of Isoetes.
The stem form-genus is called Lepidodendron which has been reported mostly as casts or compressions. In most species, the trunks attained a height up to 98-115 ft (30-35 m), because the first branching at a distal end appeared up to 30-35 m in height. At the base, the trunks are known to be 3.3 ft (1 m) in diameter. Numerous leaf cushions arrange spirally on the stem surface.
The leaf cushions-are rhomboidal in shape and broader in their vertical dimension than their transverse length (Fig. 7.39). A leaf scar is situated just above the middle line of the cushion. The leaf scar comprised of a vascular bundle scar at the centre and is flanked by two parichnos scars on either side of the bundle scar just above the middle of the cushion.
A ligule pit is situated just above the cushion. Two more parichnos scars (infrafoliar parichnos) are situated on either side of the leaf scar at lower level. The parichnos were the longitudinal channels traversing the length of the leaf parallel to the vein which are believed to be aerating organs.
T.S. of the permineralised stem shows a pro- tostele or a siphonostele (Fig. 7.40). The primary xylem is situated just outside the pith, comprised of metaxylem tracheids. The small protoxylem tracheids form vertical ridges at the periphery and leaf traces develop spirally at the steep angle from these protoxylem ridges.
In most species, secondary growth is characteristic of the genus, which was initiated by the unifacial activity of the cambium. Thus, only secondary xylem was produced externally and the cambium did not produce secondary phloem. There was massive extrastellar secondary growth by the meristematic activity of cortical parenchyma.
The periderm thus formed composed of secondary cortex which forms the massive volume of the stem, and exceeds 50% of the volume of the stem. The tracheids are scalariform and have delicate strands of secondary wall material extending between adjacent bars and are termed as fimbrils. The periderm provided the main mechanical support to the stem and branches.
The primary cortex is divided into three regions, viz.:
(i) The outer cortex, just outside the secondary cortex, bearing leaf cushion,
(ii) Middle cortex consisting of homogeneous mass of parenchyma cells, interspersed with leaf traces, and
(iii) The narrowest inner cortex having parenchyma cells. Some of the cells aggregated to form secretory cells.
Development of Stem:
In Lepidodendron it has been observed that in the successive dichotomy the size of the aerial branches decreased and ultimately showed a determinate growth with terminal twig. This determinate growth was accompanied by anatomical changes.
The primary vascular system at the base of a mature tree was protostelic, while it contained a large siphonostele at the upper portion of the stem, characterised by a wide pith at the centre. Hence there must have been a change in stelar configuration from the protostelic sporeling state to the siphonostele at the higher level in the trunk (Fig. 7.41).
This pattern of development, where the primary xylem increased its complexity by progressive expansion, has been termed epidogenesis. At more distal levels, the branches had smaller steles accompanied by successive decrease in size.
Thus, the penultimate branch showed slightly medullated stele, while the ultimate branch contained a tiny protostele with only a few delicate tracheids, no secondary xylem or periderm. This type of growth pattern — in which the plant produced progressively smaller and simpler anatomy by decreasing the amount of secondary xylem and periderm — has been termed apoxo- genesis (Fig. 7.41).
The leaves were microphyllous ligulate, generally linear, acicular or awl-shaped and were borne on the small penultimate or ultimate branches. The leaves were deciduous and had swollen photosynthetic bases (leaf cushion) that remain attached even after the shedding of laminae. The size of the leaf cushion were related to the diameter of the shoots, the smallest twigs bore smallest leaf cushions.
The C.S. of the leaves shows variability in their shape depending upon the place of cross- section. Hence the cross-section at the basal region covering leaf cushion appears to be rhomboidal, while the shapes change from angular rhomboidal to triangular in successive distal positions. T.S. of the acicular part of the leaves shows two prominent furrows on the abaxial surface (Fig. 7.42).
Several rows of stomata were arranged parallel to the long axis on the furrow region. There is a thick-walled, well-developed hypodermis all round the leaf, except the furrows. There are thin-walled mesophyll cells in the centre that encircled the sheathed vein.
The sheath is composed of transfusion cells, perhaps made up of tracheidal parenchyma. The centre is occupied by a vascular bundle made up of scalariform xylem and phloem cells.
Lepidodendron formed bisporangiate cones called Flemingites that were borne terminally. The sporophylls were helically attached to the central cone axis. The microsporophylls bearing microsporangia were usually borne in the apical portion, while megasporophylls bearing megasporangia occupied the basal portion of the cones (Fig. 7.43).
Morphologically, both the sporophylls were identical, except for their spore content. The microspores were small, about 25 pm in diameter, with smooth or granular exine. The cones containing only microspores are assigned to the form genus Lepidostrobus, possibly a monsporangiate cone of Lepidodendron.
Megaspores were spherical, slightly elongated, showing trilete aperture with echinate(spinous) exine.
Megaspore tetrads were attached tetrahedrally and their three contact faces raised into an apical prominence. The megaspore germinated endosporically to form multicellular prothallus projecting between the parted contact faces. Prothallus formed rhizoids and interdispersed archegonia comprised of one to three tiers of neck cells.
Lepidocarpon: a False Seed:
The female gametophyte of Lepidodendron is called Lepidocarpon (Fig. 7.44). Like a spermatophyte (especially gymnosperm), the niegagametophyte (Lepidocarpon) is retained within the megasporangiun and the sides of the pedicel were extended to lorm lateral laminae, called integuments —which completely enveloped the sporangium.
This unique feature of Lepidocarpon has not been observed in any other lycopod, thus showing a significant step towards the seed habit. The affinity of such structure is, however, open to question.
Seed characteristics of Lepidocarpon (similarity with an ovule of gymnosperm):
1. Formation of a single functional megaspore in the megasporangium.
2. Retention of functional megaspore in the megasporangium.
3. Formation of endosporic megagametophyte within megasporangium.
4. Formation of an integument that delimited a microphyle.
All of the above characteristics show that Lepidocarpon apparently possessed the attributes of a seed habit. However, it did not reach the level of a seed (ovule), because the basic characteristics of a seed (ovule) are totally absent in Lepidocarpon.
1. The megasporangium wall of Lepidocarpon is dehiscent. However, it is indehiscent in an ovule (seed).
2. In Lepidocarpon, fertilisation took place with the help of water, where archegonia were exposed and flagellated sperms had direct access to the ovum of archegonia. However, siphonogamous fertilisation through pollen tube is the characteristic feature of spermatophytes.
3. The integument of Lepidocarpon is not a true integument, it is foliar in nature (evidenced by the presence of parichnos).
The seed-like features of Lepidocarpon is not an unusual phenomenon. The seed-like feature e.g. permanent retention of the female gametophyte has also been reported in some members of Selaginellales and Isoetales.
The so-called integument of Lepidocarpon has been described as water or moisture retaining structure or as protective adaptation. So, Lepidocarpon should at best be regarded as a false seed or pseudo- spermatophyte.