From dividing to specialised

Meristem cells are built for dividing. They are:

• small, with thin walls;
• with a large, clear nucleus and thick cytoplasm full of organelles;
• usually without vacuoles;
• tightly packed, with almost no space between them.

Everything about them suits fast, non-stop division.

Some new cells stay meristem cells. Others stop dividing, change their shape and job, and become permanent tissue, each with one fixed duty: support, transport or storage. This change is called differentiation. Think of it like a trainee who is finally given one permanent job.

Permanent tissues are of two kinds:

• Simple: made of one type of cell.
• Complex: made of more than one type of cell.

A thin slice across a sunflower stem shows them, from outside in: epidermis, ground tissue, xylem and phloem, and the lateral meristem.

The epidermis is the plant's outer skin: a single layer of flat, rectangular, tightly packed cells. On top is a waxy cuticle made of cutin. It protects against injury, water loss, germs and parasites. Plants of very dry places have a thicker cuticle, to hold on to water.

Some epidermis cells grow hair-like outgrowths. On roots these are root hairs: they give a much bigger surface for soaking up water and minerals.

On leaves, the epidermis has tiny pores called stomata. Gases pass in and out through them, and water vapour escapes: this is transpiration. Picture a chain: water leaves the leaf, the top of the plant runs short, so water is pulled up from the roots through the xylem. This transpiration pull helps move water up the plant, and also helps remove wastes.