Xylem and phloem are the two transport tissues in a plant, and each has a structure that suits its job. They run together in bundles through the roots, stems and leaves.
Xylem carries water and mineral ions from the roots up to the leaves, in one direction only. Xylem is made of dead cells joined end to end. The end walls have broken down, so the cells form a long hollow tube with no cell contents to get in the way of the flow of water. The walls are thickened with a strong, waterproof substance called lignin, which stops the tube collapsing under the pull of the transpiration stream and also helps to support the plant.
Phloem carries dissolved sugars in both directions. It is made of living cells called sieve tube cells, joined end to end. The end walls have small holes in them, called sieve plates, so the sugar solution can flow through. Sieve tube cells have very little cytoplasm and no nucleus, which leaves more room for flow. Each is helped by a companion cell, which has many mitochondria to release the energy the sieve tube cell needs.
