Wood for breathing, not resisting

Indeed this discovery indirectly confirmed the hypothesis explaining the selection of the 'wood' character. Specialists were able to observe that, during the early Devonian period, a significant drop of the content of CO₂ in the atmosphere. 'Plants need CO₂ for photosynthesis; to absorb CO₂, plants must sweat, that is evaporate water which is replaced by the air absorbed by the plant. This air contains a small amount of CO₂. When the quantity of CO₂ in the air drops, the plants that sweat the most, therefore which have more cells to conduct the sap, are favoured. This is what happened to the very first woody, very small, plants: they used wood to increase the conduction of water and the mineral salts that it contains. So, the early success of wood was not because it increased resistance but because it improved water conduction. Through time, their descendants built on the other advantage provided by wood: resistance. They became bigger in size, allowing them to capture light more effectively. And, consequently, they had  more chance of reproducing, dispersing their spores onto bigger surfaces and increasing their chances of being deposited in favourable biotopes.' This is probably how the tree was born. From then on, the secondary function overtook the primary function and today only 5% of the wood in a tree still carries out conduction.

Since their first appearance, trees as we know them have changed profoundly, some having become very large specimens. According to this fact, would it be possible for trees to carry on evolving and become even higher? According to Philippe Gerrienne, trees have reached their maximal developmental stage at least in terms of size. 'Some giant sequoias are almost 100 metres high. We think that it is most probably themaximum height that a tree can reach. For a technical reason: the circulation of water is made possible within cells due to the polarity of water molecules which act like small magnets. When one water molecule escapes and evaporates from the top of the plant, it draws a new water molecule into the root. This process only works up to a height of 130 metres. Beyond this, the attraction between water molecules is no longer sufficient and the molecule chain is broken. '

A fossil in a haystack

However, one thing is certain: the tree has an ancestor which is even older than that discovered by Philippe Gerrienne. By observing different sections, the researcher noticed that certain cells of secondary xylem were different, having a thin wall (see illustration). Those cells probably could be used to store food (starch). These structures still exist today; they are called rays. A characteristic that is still 'subtle' on the transverse sections of this 407 million years old fossil but that can be found systematically in younger woody plants.

The characteristics of the plants described by Philippe Gerrienne suggest that they are probably the ancestors of seed plants, but not those of the close relatives, the  ferns. Which leads to another question: where is the ancestor of ferns? No doubt buried underground, most probably somewhere in the vestiges of the Gondwana, that 600 million year-old continent of the southern hemisphere which, before breaking away included, among other things, Africa, Antarctica, Australia and India, and which is probably the place where most plant groups first evolved. That's all. You might as well look for a fossil in a haystack!

Philippe Gerrienne will continue to explore. One never knows. But he doubts that he will be lucky enough to discover the ancestor of ferns: 'Discoveries are often made by chance. Everything was such a question of chance! It will probably be someone else who finds it. '