Scientists have been left mystified by fossils from a 374-million-year-old tree found in north-west China, which shows that early trees were more complex structures than those we have today.
Writing in the journal Proceedings of the National Academy of Sciences, the scientists from Cardiff University, Nanjing Institute of Geology and Palaeontology and State University of New York said the fossils revealed an interconnected web of strands that is much more intricate than that of the trees we see around us today.
“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,” said Dr Chris Berry, co-author of the study and senior lecturer in the School of Earth and Ocean Sciences at Cardiff University.
“This raises a provoking question: why are the very oldest trees the most complicated?”
In trees, strands, known as xylem, are responsible for conducting water from a tree’s roots to its branches and leaves.
In most trees, the xylem forms a single cylinder to which new growth is added in rings year by year just under the bark. Xylem can also be formed in strands, like in palms, which is embedded in softer tissues throughout the trunk.
But in the earliest trees, xylem was dispersed in strands in the outer 5 cm of the tree trunk only, while the middle of the trunk was completely hollow.
The narrow strands were arranged in an organised fashion and were interconnected to each other like a finely tuned network of water pipes.
The development of these strands allowed the tree’s overall growth.
But 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 shows conclusively that the connections between each of the strands would split apart in a curiously controlled and self-repairing way to accommodate the growth.
“There is no other tree that I know of in the history of the Earth that has ever done anything as complicated as this,” said Berry.
“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.”