The four xylem cells

Xylem is a complex tissue, so the first question is not what it does but what it is made of. Four kinds of cell, each with a different share of the work.

Figure to be added

Figure 1.10 — a stem section under the microscope with the complex permanent tissues labelled: phloem as a darker patch near the outside of each vascular bundle and xylem as the large open cells inside it.

Both xylem and phloem lie within the vascular system of the root, the stem and the leaf. They are not scattered — they run the length of the plant together, in bundles, which is why a stem section shows them side by side in a repeating pattern.

Figure to be added

Figure 1.11 — xylem tissue: a longitudinal view of a xylem vessel made of vessel elements stacked end to end, with xylem parenchyma beside it; and the four cell types drawn separately — a vessel element, a long pointed tracheid, a xylem parenchyma cell and a narrow xylem fibre.

Cell type	Living or dead	What it does
Xylem vessel cells (vessel elements)	Dead	Joined end to end into a continuous tube that carries water
Tracheids	Dead	Long spindle-shaped cells that also carry water
Xylem fibres	Dead	Shorter and narrower than tracheids; provide support
Xylem parenchyma	Living	Thin-walled cells, important for storing food

Three of the four are dead, and that is not a defect

Lignin has been deposited in the walls of the vessel cells, the tracheids and the fibres, so all three have become non-living. Those are the cells that provide support. Only the xylem parenchyma is still alive — and it is the only one of the four whose wall is thin.