φλοιός

Ancient Greek φλοιός (phloiós) meant bark, the outer covering of a tree. German botanist Carl Nägeli coined the term Phloem in 1858, choosing the Greek root because the tissue he described—the sugar-conducting vascular tissue—is located near the bark, on the outer side of the cambium. Phloem and xylem (from Greek xylon, wood) together form the vascular system of plants: xylem carries water up; phloem carries sugars down.

Phloem transport is one of the least understood processes in plant biology. The leading theory, the Münch pressure-flow hypothesis proposed by Ernst Münch in 1930, suggests that sugar loading in the leaves creates osmotic pressure that pushes sap through sieve tubes to sugar-consuming tissues like roots, fruits, and growing tips. The mechanism has never been fully replicated artificially—plants manage a hydraulic engineering feat that human technology cannot yet match.

The sieve tubes that compose phloem are remarkable cells. They are alive but have lost their nuclei, surviving only because companion cells adjacent to them provide essential proteins and energy. A sieve tube is a cell that has given up its brain in exchange for becoming a better pipe. The trade-off is bizarre and effective.

Aphids discovered phloem millions of years before scientists did. These tiny insects insert their needle-like stylets directly into phloem sieve tubes and drink the sugar-rich sap. Entomologists have used aphid stylets as natural phloem taps—severing the aphid from its embedded stylet and collecting the sap that continues to flow. The insect's feeding apparatus became a scientific instrument.