These trees have extensive root systems with rootlets that penetrate into the underlying sediment, which is either a coal seam (i.e. compressed plant material), or an intensely-rooted sandstone or mudstone (i.e. a soil horizon).
http://www.talkorigins.org/faqs/polystrate/trees.html
Other. 1  Fossil Stigmaria plant root, 1500g, 360 million years old. 2  Weight: 3 lb. 5 oz. = 1500 g. 3  Length: 10” inches = 25 cm.  Diameter: 2.5” x 2” = 6 cm x 5 1  cm. Condition: black fossilized wood, no cracks.  Age: 360 million of 1  years. Stigmaria are a type of branching tree root fossil found in Carboniferous  rocks.
http://www.ebay.com/itm/Fossil-Stigmaria-plant-root-1500g-360-million-years-old-/180972766404
In the [stratigraphic] section in the preceding chapter, the reader will observe the words 'Underclay, Stigmaria [a type of fossil tree trunk]' frequently recurring; and over nearly every underclay is a seam of coal.
http://www.talkorigins.org/faqs/polystrate/trees.html
Lepidodendron, extinct genus of tree-sized lycopsid plants that lived during the Carboniferous Period (about 359 million to 299 million years ago). Lepidodendron and its relatives—Lepidophloios, Bothrodendron, and Paralycopodites—were related to modern club mosses.
https://www.britannica.com/plant/Lepidodendron
Other. 1  Fossil Stigmaria plant root, 1500g, 360 million years old. 2  Weight: 3 lb. 5 oz. 3  Length: 10” inches = 25 cm.  Diameter: 2.5” x 2” = 6 cm x 5 1  cm. Condition: black fossilized wood, no cracks.  Age: 360 million of 1  years. Stigmaria are a type of branching tree root fossil found in Carboniferous  rocks.
http://www.ebay.com/itm/Fossil-Stigmaria-plant-root-1500g-360-million-years-old-/180972766404
Age: 360 million of years. Stigmaria are a type of branching tree root fossil found in Carboniferous rocks. They were the roots of coal forest lycopsid trees such as Sigillaria and Lepidodendron. Each trunk tended to have four of those roots.
http://www.ebay.com/itm/Fossil-Stigmaria-plant-root-1500g-360-million-years-old-/180972766404
the subterranean rootlike part of the trunk of arborescent Lycopodiophyta that existed in the end of the Paleozoic. Stigmariae consisted of thick, dichotomous axes (rhizo- phores) with spirally attached appendixes (roots), which left scars after falling off. Layers with large numbers of stigmariae often formed in the areas of maritime mangrovelike thickets.
http://encyclopedia2.thefreedictionary.com/stigmaria
Lepidodendron and its relatives lived in the extensive peat-forming swamps of the Early and Middle Pennsylvanian epochs (about 318 million to 307 million years ago) and became extinct when these swamps disappeared.
https://www.britannica.com/plant/Lepidodendron
Well, they were not a problem to explain in the 19th century, and are still not a problem now. John William Dawson (1868) described a classic Carboniferous-age locality at Joggins, Nova Scotia, where there are upright giant lycopod trees up to a few metres tall preserved mainly in river-deposited sandstones.
http://www.talkorigins.org/faqs/polystrate/trees.html
Description of an upright Lepidodendron with Stigmaria roots in the roof of the Sydney Main Coal in the island of Cape Breton. These grey sandstones, containing fossils of the clubmoss root Stigmaria, are low-angle cross-stratified or parallel laminated, and overlie >0.
http://encyclopedia2.thefreedictionary.com/stigmaria
how old are stigmaria roots
Stigmaria roots are 360 million years old.  