Permanent tissues: protect, support, carry

Cut a very thin slice straight across the stem or root of a soft, non-woody plant: a transverse section (T.S.). A leaf is usually cut lengthwise instead, a vertical section (V.S.). Under a microscope the slice is not a single kind of cell at all.

Here is our drawing of a sunflower stem T.S. Tap any region to see which tissue it is, or switch to the game and find each one yourself.

Cells of different shapes, sizes and wall thicknesses sit in separate zones, and each zone does a separate job. These are permanent tissues. When a permanent tissue is built from just one type of cell, it is simple; when more than one type of cell works together in it, it is complex. The book groups them by what they do: protecting, supporting and conducting.

A plant cannot run from heat, drought, insects or germs, so it needs a good outer guard. That guard is the epidermis, the skin on every part of the plant: just one layer of flat, box-like cells pressed tightly side by side. Switch between three views of it.

Cuticle. Over the epidermal cells lies a waxy coat made of cutin: the cuticle. It cuts down water loss and guards against injury and against parasites getting in. Plants of very dry places often have an extra thick cuticle.

Hairs. In many plants the epidermal cells grow hair-like outgrowths. On roots they are root hairs, thousands of them, which hugely increase the surface that takes in water and minerals from the soil.

Stomata. The epidermis of leaves has tiny pores called stomata. They let gases in and out, and water evaporates out through them as vapour: transpiration. Transpiration sets up a transpiration pull that helps draw water up through the xylem, and it also helps the plant get rid of some wastes.

What keeps a plant upright? Why are seed coats hard? How does a water plant stay afloat? Three simple permanent tissues share these jobs, each built differently. Pick one to see its cells.

Parenchyma: living cells with thin walls, packed loosely with spaces between them. Its main job is to store food, and in green parts it also carries out photosynthesis. Water plants have a special parenchyma riddled with big air spaces, which work like floats.

Collenchyma: living cells whose walls are thickened unevenly, mostly at the corners, by deposits of pectin, a substance that makes them flexible, a bit like rubber. It gives support with flexibility, so stems and tendrils bend without breaking.

Sclerenchyma: cells with very thick walls stiffened by lignin, which makes them hard and strong (the stuff of woody parts). Most of these cells are dead. You find it in stems, in leaf veins and in tough seed and nut coverings: think of coconut husk or a walnut shell. Now feel the difference.

Water enters at the roots but is needed in the leaves. Food is made in the leaves but is needed everywhere, including the roots. Two conducting tissues handle the traffic. Because each is made of several types of cell working together, they are complex permanent tissues.

Xylem carries water and minerals from the roots to the rest of the plant, and it also makes the plant stronger. It has four kinds of cells: tracheids and vessels, which are tubular with thick walls, xylem parenchyma, the only living part of xylem, and xylem fibres. Tracheids, vessels and fibres are mainly sclerenchyma.

Phloem is mostly made of living cells. Its sieve tubes are long tube-like cells joined end to end through walls full of holes (sieve pores), and they carry food from the leaves to other parts. Each sieve tube is managed by companion cells, specialised parenchyma that oversee the loading and unloading of sugars. Phloem parenchyma stores food, resin, tannins and latex, and phloem fibres, mainly sclerenchyma, give strength.

Protecting, supporting and conducting tissues never work on their own. The plant body bundles them into bigger teams, the tissue systems. There are three, and you find all three in a root, a stem and a leaf. Light each one up.

Dermal tissue system: the outer covering. It protects what is inside and cuts down water loss.

Ground tissue system: most of the plant body, packed in between the dermal and the conducting tissues. Its tissues are parenchyma, collenchyma and sclerenchyma.

Vascular tissue system: xylem and phloem, the plant's conducting tissues.