Image caption Researchers said the identification of wood in the 400-million-year-old samples was "unexpected"

A study of fossilised plant samples has shown that woody plants probably first appeared about 10 million years earlier than previously thought.

The 400-million-year-old samples revealed rings of cells characteristic of wood, a team of scientists observed.

They also suggested that the woody substance appeared to be a mechanism to transport water rather than acting as a support to allow plants to grow taller.

The findings have been published in the journal Science.

"The previous earliest woody plants are of Middle Devonian age (roughly 390 million years old). Our plants are of Early Devonian age, [so about] 400 million years old," explained co-author Phillipe Gerrienne, a geologist from the University of Liege, Belgium.

Dr Gerrienne added that the samples were the first and, to date, only samples of woody plants that had been placed in the Early Devonian period.

"The Middle Devonian plants with wood are shrubs or trees of very small stature. Our plants are much smaller, herbaceous and probably 20-40cm (8-16in) tall ," he told BBC News.

"I would even say that our plants are smaller than some other contemporaneous plants. In fact, all Early Devonian plants were herbaceous, so externally, you would not be able to tell which had wood and which had not.

'True wood'

Dr Gerrienne went on to explain that the team thought that the samples were "early representatives" or ancestors of lignophytes, which is the largest group of plants on Earth today, and includes gymnosperms (such as conifers) and angiosperms (flowering plants).

"Lycophytes (seed-free vascular plants), some bizarre early ferns or early horsetails could produce some wood, but the wood of our plants shows a precise feature (divisions of the cells perpendicularly to the stem surface - see photo above) that is typical of lignophytes," he observed.

He added that the ancient plant samples featured in the study would help researchers understand the first steps of "true wood" evolution.

"For example, our plants show that the rays (horizontal cells) most probably evolved after the other cells in wood (longitudinal cells)."

In addition, Dr Gerrienne said the findings also helped shed light on the initial biological role of the woody substance in early plants.

"Our plants are very small; they have thickened cells just below their epidermis (skin). These two facts suggest that wood was not necessary for support," he concluded.

"This is why we suggest that wood was probably used to enhance the flow of water in the stem. It is only later in evolution that wood was used to improve support.

"The idea that wood first evolved because it improved water conductance had already been suggested by others, but on a theoretical basis only.

"It is nice to have two different plants that illustrate this theoretical inference."