The Smithsonian.com has an article about how the ancient “trees” grew.  Researchers from Cardiff University, Nanjing Institute of Geology and Paleontology, and State University of New York have published a paper in PNAS which studied 374 million year old fossils of a group of “trees” known as cladoxylopsids.

The team’s findings, published in Proceedings of the National Academy of Sciences, reveal that cladoxylopsids had an intricate anatomy unlike anything that has been seen before. Within cross-sections of the fossilized trees, researchers were surprised to discover an interconnected web of woody strands.

The strands, known as xylem, are tubes that carry water from the roots of a tree to its branches and leaves. Trees today typically form a single cylinder of xylem, “to which new growth is added in rings year by year just under the bark,” according to a Cardiff University statement. The cladoxylopsids, by contrast, had their xylem dispersed throughout the outer two inches of the tree. And the middle of the trunk was hollow.

Also remarkable was the fact that each strand of xylem had its own concentric rings—“like a large collection of mini trees,” according to the statement.

As the trees grew, soft tissue in their trunks would push the strands apart. The strands would sometimes split in a controlled manner to accommodate the expansion, reports Mary Beth Griggs of Popular Science, but examination of the fossils showed that the webs would subsequently repair themselves.