WASHINGTON, Oct. 23 (Xinhua) -- Fossils from a 374-million-year-old tree found in northwest China showed that the first trees to have ever grown on Earth were also the most complex, scientists said Monday.
Writing in the journal Proceedings of the National Academy of Sciences, researchers from China, Britain and the United States said the trunk of the tree, about 70 cm in diameter, has an interconnected web of woody strands that is much more intricate than that of the trees we see around us today.
The strands, known as xylem, are responsible for conducting water from a tree's roots to its branches and leaves.
In the most familiar trees, the xylem forms a single cylinder to which new growth is added in rings year by year just under the bark.
In other trees, notably palms, xylem is formed in strands embedded in softer tissues throughout the trunk.
The new fossils, found in Xinjiang, showed that the earliest trees, belonging to a group known as the cladoxlopsids, had their xylem dispersed in strands in the outer five cm of the tree trunk only, whilst the middle of the trunk was completely hollow.
The narrow strands were arranged in an organized fashion and were interconnected to each other like a finely tuned network of water pipes, they said.
"It is a chance event to find our silicified fossil," Hong-He Xu of the Nanjing Institute of Geology and Paleontology, part of the Chinese Academy Of Sciences, recalled in an email of his first discovery of the fossils in 2012.
"When I firstly saw the block, which is black and quite heavy and not looking as tree, only a few small cellular structures showing something of a plant."
Xu went back to Xinjiang for more fieldwork in 2015 and found in the next year the whole trunk in a remote and open desert area, a discovery he described as "new, special and unique."
The team, which also included researchers from Cardiff University in Britain and State University of New York, showed that the development of the strands allowed the tree's overall growth.
Rather than the tree laying down one growth ring under the bark every year, each of the hundreds of individual strands were growing their own rings, like a large collection of mini trees.
As the strands got bigger, and the volume of soft tissues between the strands increased, the diameter of the tree trunk expanded.
The new discovery showed conclusively that the connections between each of the strands would split apart in a curiously controlled and self-repairing way to accommodate the growth.
At the very bottom of the tree there was also a peculiar mechanism at play -- as the tree's diameter expanded the woody strands rolled out from the side of the trunk at the base of the tree, forming the characteristic flat base and bulbous shape synonymous with the cladoxylopsids.
"There is no other tree that I know of in the history of the Earth that has ever done anything as complicated as this," co-author Chris Berry from the Cardiff University, said in a statement.
"The tree simultaneously ripped its skeleton apart and collapsed under its own weight while staying alive and growing upwards and outwards to become the dominant plant of its day."
Berry, who has been studying cladoxylopsids for nearly 30 years, has previously helped uncovered a previously mythical fossil forest in Gilboa, New York, where cladoxylopsid trees grew over 385 million years ago.
"By studying these extremely rare fossils, we've gained an unprecedented insight into the anatomy of our earliest trees and the complex growth mechanisms that they employed," he said.
"This raises a provoking question: why are the very oldest trees the most complicated?"