Thursday, 18 August 2011

The Oldest Wood On Earth

The fossilised cells under a microscope,
displaying the layers of wood around each cell
The first ever land based plants had fleshy stems supported by a network of rigid tubes called xylem, which are used to transport water up and down the plant. Then, during the middle Devonian around 370 million years ago, plants developed a tough and complex molecular structure which we call wood. It would have provided support for the plant, protected it from parasites and helped transport water up and down the stem in a similar fashion to xylem.

The oldest known plants that used wood were 390 million years old. However a geologist called Dr Phillipe Gerrienne from the University of Liege, Belgium has discovered fossilised wood that is even older than these ancient samples. The fossils in question are 400 million years old and are the only woody plant specimens known that come from the early Devonian. They would have been far smaller than their middle Devonian counterparts, at just 20 to 40 centimetres tall.

They would also have been herbaceous, meaning that they would have definite 'flowering' and 'hibernation' seasons. This makes Gerrienne believe that they are the oldest known and possibly basal forms of the plant group known as the lignophytes. Lignophyta is the largest plant group on Earth, containing both the gymnosperms (such as conifers) and the angiosperms (flowing plants). The wood consisted of a very thin layer just below the skin of the plant.

This fact helps shed light on the initial biological role of wood in plants. Gerrienne suggests the wood was originally used as a means of transporting water up and down the plant, rather than as a means of support. Devonian soil quality was very poor and could not have sustained or even supported large trees and shrubs. These samples have possibly solved the great debate between palaeobotanists about why plants evolved wood.