LIFE × EARTH

The Invention of Wood

~393 to 383 million years ago

Phanerozoic · Paleozoic · Devonian

Wood, a water-conducting tissue stiffened by a substance called lignin, appeared in plants between 393 and 383 million years ago and lets a stem hold a column of water metres above the ground.

Wood is secondary xylem, the water-conducting tissue that a stem adds year after year. Lignin stiffens this tissue. Lignin is a polymer, a molecule made of a long chain of repeated units, which the plant lays down around the cellulose of its cell walls. Cellulose is the fibre these walls are made of. Lignin makes the wall rigid and waterproof. A stem can then hold a column of water metres above the ground and carry its own weight at the same time. Roots developed along with trunks: by the Middle Devonian, the tree Archaeopteris had a root system built essentially like that of a modern seed plant.

Why it matters

Wood lets a plant grow tall. Height is an advantage only because of shading: a plant that overtops its neighbours takes their light. This competition for light explains the height of forests and their stratification, which is their arrangement in layers of different heights. It also explains why a large share of the carbon on the continents is in trunks rather than in leaves.

Dating & uncertainty

Lignin is older than wood: its chemistry appears with the first vascular plants, the plants with internal vessels that conduct water. The tree appears later, in the Middle Devonian, a subdivision of the Devonian period, with root systems comparable to those of present-day seed plants.

Earth history as one day

21:55:202 h 5 min before midnight

if Earth’s 4.54 billion years were compressed into a single day