Australia: The Land Where Time Began
Lycopods of the Giant Clubmoss Flora
These trees had unbranched trunks that forked repeatedly into a high crown. Leaves attached to leaf cushions were arranged in an ascending spiral on the smaller branches, the leafy twigs appearing like bottle brushes. Lepidodendron leaves are assigned to the form genus Lepidophyllum. The structure of the leaf-cushions continued to grow after the leaves had fallen, with the secondary growth of the branch, expanding the pattern of leaf bases. The surface of all branches was patterned with ascending patterns of leaf base spirals. The pattern on the bark of individual trees varies according to age of the tree and the branch size. The patterns also varies between fossils because the fossil can be of internal or external cast, or of a section of shed bark, or the outer bark has been removed prior to fossilisation, or even distorted by compression during the process of fossilisation or subsequent to the burial of the deposit. The position of the leaf traces, where the xylem strands continue from the stem into the leaf, are indicated by the leaf bases.
The cones of Lepidodendron are composed of sporophylls (scale-leaves) which carried the sporangia. Many small spores are contained in male sporangia, a few larger spores are contained in female sporangia. The cones are called Lepidostrobus. In European coal swamps some of the trees had grown to a height of about 45 m, and their cones were up to 50 cm long. It has been estimated that one of these male cones could have produced as many as 8 billion spores. The spores in the larger female cones would have numbered only in the hundreds. These primitive trees grew in dense forests in the tropical swamps in the Northern Hemisphere, apparently they were crowded together with their large trunks only metres apart. The apparent density of these forests may have been considered an artefact of preservation if actual instances of these forests in which the trunks and root systems were still intact, had not been found in situ, still standing in the positions they were in when they were buried, confirming the dense nature of the stands.
The root systems of the Lepidodendrons were composed of thick, forked branches on which were patterns of circular scars, in the centre of which there was a circular spot where the vascular trace enters the rootlet. The root buttresses are called stigmaria, the rootlets attached to the circular scars were stigmarian rootlets. Swamps were the only places giant club mosses are known to have grown. They no longer dominated the vegetation of such habitats when climatic change led to the reduction of rainfall in these tropical areas and the swampy areas were reduced. Their much smaller descendants still survive in places where the conditions have allowed their required tropical swamps to continue.
Unlike Lepidodendron, that produced cones, other giant club mosses had different reproductive structures. A common Australian genus Leptophloeum, from the Late Devonian, had regions of young stems that were used as reproductive structures, having sporophylls instead of leaves. The pattern of these zones differs from the rhombic pattern of leaf bases on mature stems.
Some European genera produced very large female spores that they retained inside the sporophyte plant, in effect these were seeds. The leaves of Leptophloeum australia were small and inconspicuous, while the Ledipophyllum leaves of Lepidodendron were long, narrow and substantial. The leaves of Leptophloeum australia were like forking spines and are very rarely found as fossils. The leaf-base pattern on young stems are narrow and rectangular, expanding later to the characteristic rhombic pattern of mature stems.
Mary E. White, The Greening of Gondwana, the 400 Million Year story of Australian Plants, Reed, 1994
|Floras of Ancient Australia|
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|