H. Potonié first suggested the existence of a group of vascular plants that was a transition between pteridophytes and seed plants in 1899. Five years later, F.W. Oliver and D.H. Scott put together the stem, petiole, foliage and seed of the same plant and the existence of seed ferns was confirmed. Lyginopteris was first used as a generic name for the stem, but is now used to name the entire plant. In general, Lyginopterids have large fronds that are probably bigger than that of most modern ferns. They are vine-like, using surrounding vegetation as a means of support for the large leaves since the stems are long and slender. Adventitious roots come off all sides of the stem, sometimes falling in vertical rows. Within the Lyginopterids there is a lot of variation of morphology, so it was difficult to reconstruct the plant at first. However, the discovery of peculiar glands on all the parts except the roots led to the unification of the plant and confirmed its existence.
Leaves
Lyginopterid leaves are megaphyllous, meaning they have more than one vein and a trace associated with a region of parenchyma cells within the central cylindrical section. Species vary from having branch-like structures with leaves on them to having compound leaves with an arrangement of leaflets on either side of the central petiole, forming a feather-like shape. Like most modern ferns, the epidermis is cutinized on the upper surface, facing the stem. Stomata, pores in the leaves used for gas exchange, are on the underside.
Stems The stems range from 0.5 to 4 cm in diameter and are occasionally branched. There is a central pith with nodules of sclereids, thick wall cells that aid in support. The primary xylem, or water conducting tissue, differentiates both towards and away from the center. The primary phloem cells, or food conducting cells, are small, irregularly arranged and soon become crushed by secondary cells. The secondary phloem is generally one or two cells thick, with rays separating them. Outside the phloem is a zone of cells that resemble modern day pericycles - regions of cells that surround an inside layer of root cells. Surrounding this zone is a protective coat that forms as plants grow older. The cortex can be separated into two layers, the inner one consisting of thin-walled, metabolic cells and the outer one alternating between such cells and a network of fibrous strands. The fibers of this structure provide more strength than the cells of the inner layer and they also increase the flexibility. This could have aided in the vine-like nature of the plant, allowing it to curl around other plants. The epidermis of the stem is covered with multicellular glands called capitate glands that are also present all over Lyginopterids, except at the roots. The chemicals in these structures could have attracted potential pollinators or could also have repelled herbivores from eating the plant.
Some interesting questions that remain unanswered regard the lyginopterids' stem structure. The lyginopterid has a ratio of vascular tissue to stem width that r anges from 1:2 and 1:4. Reasons for such a close ratio are still unknown. Howeve r once revealed, the answer would help distinguish putative seed fern stems.
Roots The roots of Lyginopterids are abundant and well preserved. Most come out of the stems and are aerial, occasionally in vertical rows, very unlike modern ferns. They are very long in proportion to their diameter, again indicating that Lyginopterids were probably climbing plants. The roots have phloem alternating with xylem rays and the xylem develops centripetally. In larger roots, secondary xylem is developed. The cortex is divided into two regions - an outer layer of thin-walled cells and an inner layer about twice as thick with very dense cells.
Reproductive organs
The male reproductive structures are on the flattened blade segments of each frond. They are shaped like an arrowhead and contain about 30 sporangia. The shape probably originated from the fusion of three oval groups of sporangia on a small leaflet. Like many pteridophytes, the spores are trilete, meaning they have a Y-shaped mark on the surface. This intermediate characteristic does not occur in modern seed plants. The spores land so close to the female reproductive structure that there us no need for the pollen tubes that modern seed plants use to aid the path of the sperm to the egg.
The ovules are about 5.5 mm long and 4.4 mm in diameter. There is a cupule, an accessory set of structures, surrounding the ovule. There are plants alive today that also have a cupule however, their cupules surround an ovary, not a naked seed. The female reproductive structure is not attached to the plant at its tip, and at maturity, there is a central core surrounded by a pollen chamber. Botanists hypothesize that the central core could have served as a source of a pollination droplet that trapped windborne pollen, or it could have served as a pollen tube directing the pollen into the structure that stores the egg. A triangular pad of tissue, called the tent pole, below the central column is also a sign of a mature female plant. The tent pole is also present in several Paleozoic seeds. It could have functioned in sealing the pollen chamber so as to protect the developing embryo or it could have ruptured the membrane protecting the egg to allow sperm entrance.
