Plane trees are a hybrid, produced at least 300 years ago by crossing the American sycamore with the oriental plane tree, which grew in southern Europe and the Middle East. They were first planted in Paris in the 1860s to replace elms killed by a wave of Dutch elm disease; planes are ideal for urban environments because they propagate easily and grow quickly into tall, attractive, long-lived trees that are resistant to pollution. They are common in many cities, notably London, where in the 1920s they made up nearly two-thirds of all street trees.
Most of the time the trunks of these trees are a drab grey-brown, with very pale areas of yellow. When young, the outer bark contains chlorophyll, giving some trees a dark green colour. As the bark ages it becomes dry, dark and brittle, peeling off in scales and exposing the faintly tinted, lighter bark beneath. As it comes into contact with the air, this base bark gradually changes colour.
When it rains, the wood seems to glow, accentuating colours that are normally barely visible. This effect is most dramatic when the sun comes out after a downpour.
But, just as pebbles lose their jewel-like lustre out of water, when the rain stops the bark drains quickly back to its familiar greys and browns. By the time we venture out from our shelter, or fold our umbrellas and look around, the magic has gone.