Water is held in spaces inside the cells of the stem. If the plant has plenty of water the stem will stand straight up but if it does not have enough water it will sag or wilt. A stalk also has a second means of support. These are strands of tough fibres. They may form a sheath just underneath the surface of the stalk. When the stalk dies at the end of the growing season these fibres may be so strong that they hold up the stalk for a while. The stalks of grass plants behave like this. You may see their long yellowed stalks sticking up long after the flowers they were supporting have released their seeds.

Stems conduct water up the plant through long tiny pipes. These form from columns of cells which die. When the cells die their horizontal walls break down so each cell is connected directly to the ones below it and above it. The water is pushed a little way up the pipes by the root but it is mainly pulled up the stem by the leaves. Leaves constantly lose water to the air and suck up water from the stem to replace it.

There is a second set of pipes. They are made from columns of living cells. These cells transport food from the leaves to all the other parts of the plant. While the pipes carrying water can only move the water upwards or out along branches the pipes of living cells can carry the food upwards to flowers higher on the stem and carry food downwards to the roots.

In stalks the pipes which carry water and food are grouped together with the fibres which give support. You can see the pattern of these bundles in a celery stalk if you dip it in coloured water for a while. The coloured water passes up the stalk and marks the position of the water carrying pipes. Surrounding these water pipes are the food carrying pipes and supporting fibres.

The arrangement of pipes and fibres is different in most woody stems. The pipes and fibres form in rings. On the surface of a woody stem is the bark. This protects the inside of the stem from the harsh weather of winter so the stem can survive and continue growing in the following year. Beneath the bark is a ring of materials where the pipes and are fibres are made. Just below the bark are the pipes which carry the food. Next to these pipes is a ring of water pipes. A woody stem makes two kinds of water pipes. In the spring wide pipes are made to transport large amounts of water to the growing leaves and flowers. In the summer smaller pipes are made to supply the leaves as they make food. Each year the stem makes new water pipes and the old ones fill with food (turn in to harder wood) and give the stem extra support.