Xylem, phloem and water uptake

Last time

Before we start: last time, in Transpiration and translocation, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Pipes inside a plant

A tall tree lifts hundreds of litres of water a day from its roots to its highest leaves, with no heart to pump it. It uses two sets of tubes: xylem and phloem. Today: how the tubes are built, and how water gets into the roots.

Two transport tissues

Xylem carries water and mineral ions from the roots up to the leaves. Xylem also gives the plant support, because its walls are strengthened with lignin. Phloem carries sucrose and amino acids from where they are made to where they are needed, up or down.

Xylem structure

Xylem vessels are long tubes made of dead cells joined end to end. There are no cross walls and no cytoplasm, so water flows freely in a continuous column. The walls are thick and strengthened with lignin, so they do not collapse.

Phloem structure

Phloem is made of living cells called sieve tubes, joined end to end. The end walls have holes, forming sieve plates, so the sap can flow through. Phloem walls are thin, and there is no lignin.

Xylem or phloem?

Here is the comparison. Dead or living, lignin or not, what they carry, and which way. In a root, the xylem is in the centre. In a stem, the bundles form a ring with xylem on the inside.

Root hair cells

Water enters the plant through root hair cells near the tip of the root. Each root hair is a long, thin extension that gives a large surface area for absorbing water and ions. Water enters by osmosis, because the cell sap has a lower water potential than the soil water. Mineral ions enter by active transport, because they are more concentrated inside the cell.

The pathway of water

Water passes from the root hair cells across the root cortex cells by osmosis. It enters the xylem in the centre of the root. It travels up the xylem in the stem to the leaves, then into the mesophyll cells.

Showing the xylem

Stand a celery stalk in water with a coloured dye for a few hours. The dye moves up with the water. Cut across the stalk: the coloured spots show exactly where the xylem vessels are.