concept

Pressure flow (the Münch mechanism)

How a plant ships sugar over distance without a pump, proposed by Ernst Münch in the 1920s. Loading sugar into the phloem at a sunlit leaf raises the concentration there, so water is drawn in osmotically and the pressure at that end climbs — to two or three MPa. At the far end, in a root, sugar is unloaded and consumed, the water leaves, and the pressure falls. That difference pushes the whole loaded column along the pipe. Its beauty is that the plant need only manage the two ends; the transport takes care of itself. The trap is geometric: a tree twice as tall means twice the distance with half the push behind each metre, so travel time grows with the square of height.