Callistophytales
| Callistophytales | ||||||||||||
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| Temporal occurrence | ||||||||||||
| Upper Carbon ( Pennsylvania ) | ||||||||||||
| 318 to 299 million years | ||||||||||||
| Locations | ||||||||||||
|
USA, France |
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Callistophytales | ||||||||||||
The Callistophytales are an order of the extinct plant group of seed ferns . The genus Callistophyton probably grew in the understory of the carbon fiber forests.
features
Vegetative characteristics
Callistophyton was a small, splayed plant. It had a main axis from which about 50 cm long side branches branched off with large, pinnate leaves.
The largest surviving trunk has a diameter of around 3 cm. The marrow is parenchymal and slightly angular. It is surrounded by up to 13 axial bundles of the primary xylem . The number of bundles varies depending on the position on the trunk and whether straight leaf traces branch off. There are species with a mesarchic and exarchic xylem (it ripens from the middle to both sides, or from the outside to the inside). The tracheids of the metaxylema are ladder tracheids or have court pits . Around the primary xylem is a wide zone of secondary xylem up to 70 cells wide. Here the tracheids are arranged in rows one to five cells wide, separated by rays. The pits are limited to the radial cell walls and are slit-like. The cambium initially only produced xylem, later in equal parts xylem and phloem . The location and appearance of the primary phloem are unknown. The secondary phloem consists of sieve cells, phloem parenchyma and phloem rays. Amber inclusions in the sieve fields suggest callose plugs. The cortex consists of an inner, parenchymal layer, and an outer layer of longitudinal fibers that are embedded in the parenchyma. There are scattered cavities in the bark, filled with an amber-colored substance. Similar secretion cavities are found in all plant organs of the genus. A periderm can be seen in the inner bark , which forms the outer closure tissue in older tribes.
Callistophyton's vascular system is interpreted as independent axial bundles that run through the trunk and from which leaf traces branch off. The leaf traces are double at the base of the petiole, then individually towards the tip. Small axillary buds or side branches sit on the nodes . The buds are enclosed by two opposite leaves or cataphylls that extend beyond the tip of the bud. Side branches have the same structure as the stem axes described above.
The roots of Callistophyton are diarch. In older specimens, they contain abundant secondary tissue, including periderm. The roots are adventitious and arise on the stem axis in the axils of the buds or side branches.
A similar genus as Callistophyton is Johnhallia from the Middle Pennsylvania. They are trunks with an eustele about one centimeter in diameter. The primary trunk has five vascular cords. Each bundle has a core of parenchyma. The leaf fronds are flat and stand in a 2/5 phyllotaxis on the stem axis.
In Callistophyton , the leaves are arranged in a spiral. The largest leaves are around 30 cm long. The fronds are double to fourfold pinnate, the leaflets are flat, deeply lobed and narrowed at the base. Multicellular hair is sometimes found on the underside. The leaflets morphologically resemble the species Sphenopteris most closely .
Reproductive organs
The seeds are flattened and are known as callospermarion . It is not known where the seeds were formed on the plant. In analogy to the pollen organs, it is assumed that they were located on the underside of the leaflets.
The seeds (plants) are between 0.8 and 5 mm long, in one level up to 3.8, in the other up to 2.0 mm wide. The integument contains two vascular bundles in wing-like extensions. The integument is in three parts, the majority of the seed coat consists of cells of the outer sarcotesta and contains large secretion cavities. In Callistophyon pusillum , the nucellus is only fused with the integument in the base area (at the chalaza). The distal end forms a bottle-shaped pollen chamber. In a number of seeds considered immature, pollen grains of the Vesicaspora type were found in the pollen chamber . Probably the ovules formed a resinous drop of pollination that helped capture the pollen, similar to the recent naked samers .
The pollen organs sat on the abaxial side (underside) of the small leaflets. Idanothekion glandulosum from the middle Pennsylvania consists of a ring of six to nine sporangia , which are arranged around a central, vascular bundle-leading column and fused in the proximal part. Each of these synangia is around a millimeter long and just as wide. The wall of the sporangia is thick, except for the inwardly facing wall which is thin and where the opening took place. The conductive tissue is present as a broad band in the outer layers of each sporangium. Callandrium callistophytoides has practically the same structure, only the conductive tissue in the sporangia wall is missing. Both pollen organs contain small (around 40 µm), monosaccate pollen grains that resemble those of the conifers . As individual finds, the pollen grains would be placed in the genus Vesicaspora . The pollen is bilateral and has an elliptical shape at the equator. The body is surrounded by an equatorial air sac (saccus) that is finely ornamented. Between the two lobes of the saccus there is an elliptical germ fold (sulcus).
Stages of development of the microgametophyte consisting of two, three and four axially stacked cells were observed in pollen sacs. The cells standing proximally are interpreted as prothallium cells, the largest cell as embryonic cells or antheridium cells. The pollen was released at the 4-cell stage. The pollen was probably spread by the wind and caught by the fertilization drops of the ovules. The pollen germinated via the distal sulcus. In one case a branched pollen tube was observed in an ovule , an indication of a haustorium function of the pollen tube similar to that of Cycadales . This would be a preliminary stage to real siphonogamy .
supporting documents
- Thomas N. Taylor, Edith L. Taylor: The Biology and Evolution of Fossil Plants , pp. 550-558. Prentice Hall, Englewood Cliffs 1993. ISBN 0-13-651589-4