A slice of eucalyptus a tenth of a millimeter thick is placed under a polarized light microscope, and the wood suddenly blooms: blue, yellow, and pink bands twist into spirals, forming rings and swirls. This is not paint but the play of light: the cellulose fibers that make up cell walls transmit light differently depending on their orientation. So by looking at the colored image, one can tell the angle of each fiber.

It turns out that in different wood cells, the fibers are arranged differently. In the long fibers that give the trunk its strength, the fibers twist like a spiral staircase: near the inner surface of the wall they lie almost along the cell, and toward the outer surface they gradually tilt more and more, up to forty-five degrees. In the vessels that carry water, the fibers form a spiral, but around the pits that connect vessels, this spiral breaks and curls into swirls. In the ray cells that transport nutrients across the trunk, the fibers cross each other like a woven basket.

These patterns are familiar to physicists: molecules in liquid crystals behave exactly the same way. Liquid crystals are substances that flow like a liquid but retain order like a solid. If a liquid crystal is cooled, it solidifies, and the pattern is preserved. It seems that the cell wall was also once liquid: cellulose fibers floated in it, arranged themselves into spirals and swirls, and then froze, leaving us their pattern.

To test this idea, scientists built computer models: virtual cells with different fiber arrangements, from simple spirals to complex swirls. Then they compared the colored patterns from the models with those from the microscope. The match was almost perfect, and the color even allowed them to measure the tilt angle of the fibers to within a degree.

So wood is no longer just a building material. It turns out to be a frozen record of phase transitions, those moments when a liquid becomes a solid. And now, looking at a tree stump, you can read that record: each colored band tells how a cell chose its shape.