And plant created wood
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.
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.