Xylem vessels carry water and mineral ions from the roots up to the leaves, in one direction only. A xylem vessel is made of dead cells joined end to end. The end walls between the cells have broken down, so the vessel forms a continuous hollow tube with no cytoplasm to get in the way of the water. The side walls are thickened with a strong substance called lignin, often in rings. Lignin makes the vessel strong enough to stay open and to support the plant, and it stops the walls collapsing inwards.
Phloem carries dissolved sucrose in both directions, up and down the plant. It is made of living cells. Sieve tube cells have end walls with holes in them called sieve plates, so the sap can flow from one cell to the next. They have very few organelles, which leaves more room for the sap to move.
Each sieve tube cell has a companion cell next to it. Companion cells have many mitochondria. These release the energy needed to move sucrose into the sieve tubes by active transport.
