Xylem vessels carry water and mineral ions from the roots to the leaves. Xylem is made of dead cells. The cells have lost their cytoplasm and their end walls, so together they form a continuous hollow tube that water can flow through easily. The walls are thickened and strengthened with lignin, which stops the vessels collapsing and also helps to support the plant. Water and mineral ions move in one direction only, upwards.
Phloem carries dissolved sucrose around the plant, and it is made of living cells. The sieve tube cells have perforated end walls called sieve plates, which let the sucrose solution pass through. Each sieve tube has a companion cell beside it. The companion cell has many mitochondria that release energy for moving sucrose into and along the phloem. Phloem moves sucrose in both directions, up and down the plant.
