Today everyone knows the answer. But before being able to push back the appearance of wood by over ten million years with certainty, Philippe Gerrienne and his team had to work hard. First step: dating the fossils. This was the job of his colleague, Philippe Steemans. How can you tell the exact 'age' of a stone that seems to have travelled through time? Some would hazard a guess at carbon-dating. Wrong answer. 'This procedure does not work on this type of rock. At best, carbon-dating can help us to go back 40,000 years. We needed to go back ten thousand times further!' So the team used a comparative dating method. Having gone through different acid baths, the rock was dissolved in order to compare the spores it contains with other spores from sites that have already been dated. Verdict: the plant in question had grown one fine day in the Early Devonian period, some 407 million years ago.

A (very distant) ancestor of pine and birch trees

All that remained to be done was to analyse these fossils... painstaking work to which Philippe Gerrienne devoted himself over many months. After a few successive acid baths, he was able to start peeling the rocks, a method which made it possible to 'attach' a fine layer of fossil to a small strip, in order to observe it under the microscope. 'We removed barely three or four thousandths of a millimetre at a time! I had to do over 150 peels.'

The researcher spent a lot of time observing these sections under his microscope and ended up detecting traces of wood. 'In the central zone we noticed a set of small cells arranged haphazardly. It was  the primary xylem which is produced vertically by the top of the plant, explains Philippe Gerrienne. All around we can see strands of cells, generated by the division of peripheral cells. These peripheral cells have the capacity to divide parallel AND perpendicularly to the surface of the stem. These strands divide into two, then again into two....and thus allow an indefinite increase in the diameter of the plant. This is what we call secondary xylem, i.e. wood.'

There they are, those famous traces of wood! Traces that were quite surprising as the diameter of the stemwas no bigger than two millimetres. As such, this plant seems to be the oldest ancestor of seed plants, the only ones to have this “perpendicular to the surface” type of division in their wood. Birch, oak, conifers? They are probably all descendants of, or at least related to, this frail and tiny grass-like plant!