American Scientist

It may be time to rewrite a passage of the story of plant evolution—again. Paleobotanists have unearthed evidence that wood developed in plants earlier than was thought, but not for structural support.

Two scientists and their students found secondary xylem in fossils of small herbaceous plants dating as far back as 407 million years, to the early Devonian. That makes them about 10 million years older than the woody, middle-Devonian plant remains in which the oldest traces of wood previously were found.

These older plants had stems just 12 centimeters tall and 3 to 5 centimeters wide, which were built in part with thick-walled cortical cells that likely allowed them to hold themselves up. Still, wood cells, arranged in a typical ring-growth pattern, were found in the core of the stems, radiating outward from the middle.

What looks like a simple type of wood was probably most involved in aiding the movement of water from the plants’ roots to their tips, says University of North Carolina at Chapel Hill biologist Patricia Gensel, one author of an article on the discovery published in August in Science. Environmental pressure to improve water conduction was present during the early Devonian, Gensel says, because atmospheric carbon-dioxide concentrations were declining. Plants leave their stomata open longer when carbon dioxide is less abundant, which results in more water escaping from their interiors.

These wood cells “added additional cells for conduction over what a plant without them would have,” Gensel says. “In conjunction with greater water loss through stomata, this would improve conduction.”

The advent of wood, still important in water conduction in some small plants today, was a major event in Earth’s history. Wood was instrumental in the appearance of the large perennial plants that helped transform Earth’s surface and atmosphere. Woody plants, of course, still play a linchpin role in our planet’s ecosystems. Scientists are eager to get the facts of their origin straight.

Gensel published the findings with Philippe Gerrienne of the University of Liege in Belgium. Her research team had observed what she thought were wood traces in fossils she had retrieved from the Campbellton Formation in northern New Brunswick, Canada, in the 1990s. Years later, Gerrienne approached her after she gave a talk on her observations, saying he had seen the same thing in fossils recovered from the Chalonnes Formation in the Armorican Massif in France.