The second most important group of plants in the Riola forest were the tree ferns. These tree ferns are not closely related to those of the modern day. They are, instead, relatives of a small group of ferns known as the Marattiales. Members of this group of ferns are still present today, mainly in moist tropical environments; they can be found in many greenhouse botanical collections in temperate countries. Marattialean ferns are no longer trees, although they may have large leaves (fronds).
The tree ferns are spore producting plants. During the coal age, these plants were perhaps 30 feet (10 m) or so in maximum height with a crown of large, feathery fronds, each frond reaching many feet in length. Tree ferns were able to grow tall because their stems were surrounded by a thick layer, or "mantle" or roots, which supported the spongy, soft tissues of the trunk. Surprisingly, the roots of tree ferns were mostly air spaces, but with a tough outer layer. Individually not very strong, when all grown together, the root mantle, though light and cheaply constructed, gave the tree-fern trunk great strength.
All pictures are looking up at the roof of the mine. (Click on image for larger view.)