Road construction unearthed the plant fossils, which date to about 360 million years ago, this week along a site near Grahamstown in Eastern Cape province.
"Most of the plants that we're finding haven't been documented by scientists before," said Robert Gess, a researcher at Rhodes University in South Africa.
The fossils include Archaeopteris notosaria, an ancient tree from the Devonian period.
"Of course the Archaeopteris notosaria and the fact that it's got a scientific name, it's because we had described it before but we had described it from less complete material," Gess said. "We now have a far more complete picture of the plant."
The fossils were found during controlled rock cutting explosions along a highway between Grahamstown and Fish River.
Gess, who is also a consultant for the South African Roads Agency, said it's not uncommon to discover a collection of fossils during road construction.
"In a natural environment, fossiliferous rocks are weathered away into a white clay. But when we have occasional road works and we cut into the hills, we have the potential to find fresh black shales which contain the fossils."
It is the second major site of its kind in South Africa, Gess said. The first Devonian discovery was made in 1985 at a farm on the other side of Grahamstown, he said.
The Devonian period is when plant species evolved into the first trees. It dates to over 400 million years ago, when tectonic shifts brought parts of present day North America and Europe together to create a supercontinent known as Euramerica.
The period's name comes from where the first remnants of that collision were studied in Devon, England. The period is known as the "Age of the Fishes" for the number of species born during the era, but it also was a time when plants spread from wetlands to dry land -- giving way to the first forests.
"What is really significant about the site is that previously most of what we knew about the late Devonian was from rocks that during the late Devonian were in tropical waters," Gess said.
But the discoveries in South Africa are from higher latitudes, opening the door for the identification of even more species and a broader understanding of the Devonian period.