The cells, which gave rise to the roots of an ancient plant, were found in a fossilised root tip held in the Oxford University Herbaria.

As well as revealing the oldest plant root stem cells identified to date, the research also marks the first time an actively growing fossilised root has been discovered – in effect, an ancient plant frozen in time.

The study is published in the journal Current Biology.

Oxford Plant Sciences PhD student Alexander (Sandy) Hetherington, who made the discovery during the course of his research, said: 'I was examining one of the fossilised soil slides held at the University Herbaria as part of my research into the rooting systems of ancient trees when I noticed a structure that looked like the living root tips we see in plants today.

'I began to realise that I was looking at a population of 320 million-year-old plant stem cells preserved as they were growing – and that it was the first time anything like this had ever been found.

'It gives us a unique window into how roots developed hundreds of millions of years ago.'

Stem cells – self-renewing cells responsible for the formation of multicellular organisms – are located in plants at the tips of shoots and roots in groups called meristems. The 320 million-year-old stem cells discovered in Oxford are different to all those living today, with a unique pattern of cell division that remained unknown until now. That tells us that some of the mechanisms controlling root formation in plants and trees have now become extinct and may have been more diverse than thought.

These roots were important because they comprised the rooting structures of the plants growing in the Earth's first global tropical wetland forests with tall trees over 50m in height and were in part responsible for one of the most dramatic climate change events in history. The evolution of deep rooting systems increased the rate of chemical weathering of silicate minerals in rocks – a chemical reaction that pulled CO2 out of the atmosphere, leading to the cooling of the Earth and thus one of the planet's great ice ages.

The fossils studied during this research are the remains of the soil from the first giant tropical rainforests on Earth. The rock in which the soil is preserved formed in the Carboniferous swamps that gave rise to the coal sources spanning what is now Appalachia to central Europe, including the coal fields in Wales, northern England and Scotland.

Sandy has named the stem-cell fossil Radix carbonica (Latin for 'coal root').

Professor Liam Dolan, Head of the Department of Plant Sciences at Oxford University and senior author of the paper, said: 'These fossils demonstrate how the roots of these ancient plants grew for the first time. It is startling that something so small could have had such a dramatic effect on the Earth's climate.

'This discovery also shows the importance of collections such as the Oxford University Herbaria – they are so valuable, and we need to maintain them for future generations.'