The plant is so well preserved that it has allowed scientists from the Carnegie Institution in Washington DC, US, to detect the presence of lignin, a key chemical in the development of virtually every land plant.
It is this chemical, a woody polymer, which gives the stems and roots of plants the stiffness needed to grow higher.
The find in Rhynie, Aberdeenshire, is thought to be the first time that scientists have been able to determine lignin in a plant fossil of this age.
Dr George Cody, who led the study, said: "We think this is a rather significant result from an amazing locality. There's nothing else like it in the world."
Stem evolution
There is evidence of land-based plants dating from approximately 475 million years ago.
However, it was not until the Devonian Period, at approximately 400 million years ago, that they are thought to have somehow evolved to produce more rigid stems and roots capable of supporting a larger plant.
"
We think this is a rather significant result from an amazing locality
"
Dr George Cody, Carnegie Institution
Most plant fossils can only be examined in two dimensions, essentially because they are formed when plants are compressed flat in sediment.
Rhynie is one of the most important locations in plant fossil history because it was the site of hot springs.
These occasionally bubbled over and entombed surrounding plants in silica deposits, preserving them instantly in three dimensions.
The preservation is so perfect that individual plant cells can be viewed.
'Earliest find'
Dr Charles Wellman, a lecturer in paleobotany from the University of Sheffield, UK, told BBC News Online: "The find tells us what we would have assumed about plants of this period.
"Without lignin, you could never get these big stems.
"We know this happened during the Devonian Period - by the end of the Devonian, you had trees, and you could never have had these without lignin.
"This is likely to be the earliest material evidence of lignin."
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