Pipelines of a plant

A plant needs pipes: one set to carry water up from the roots, and another to carry food from the leaves. These are xylem and phloem. Each is a complex tissue: several kinds of cells working together.

Xylem: the water pipe. It carries water and minerals from the roots to the rest of the plant, and makes the plant strong. It has four kinds of cells:

• tracheids and vessels (long, tube-like, thick-walled);
• xylem parenchyma (the only living part);
• xylem fibres.

Phloem: the food pipe. It is mostly living. Long cells joined end to end, with walls full of holes between them, form sieve tubes, which carry food from the leaves to every other part. Beside them:

• companion cells manage the sieve tubes and check sugar being loaded and unloaded;
• phloem parenchyma stores food, resin, tannins and latex;
• phloem fibres give strength.

An easy way to remember: xylem, water up; phloem, food from the leaves.

All the plant's tissues fall into three tissue systems:

• Dermal: the outer covering, for protection and less water loss.
• Ground: the main body of the plant (parenchyma, collenchyma, sclerenchyma).
• Vascular: the pipes (xylem and phloem).

In older stems, cells under the epidermis become cork cambium. It makes cork cells: dead, tightly packed, and water and gases cannot pass through them. Together they form the bark.