The stem's two pipelines

The stem's job is transport: water and nutrients up from the roots, sugars down from the leaves to wherever they're needed. Two separate systems run in parallel inside every stem.

Xylem (ZY-lem): the water pipeline

• Made of dead cells: they mature, hollow out, reinforce their walls with lignin (the stiffening compound that makes wood woody), and die. What's left is a stack of rigid hollow tubes. (That's what wood is: accumulated dead xylem.)
• The tubes stack end-to-end into continuous pipes from root tip to leaf vein. Water moves as a continuous column.
• Flow is one-directional: up. It doesn't carry sugar and doesn't reverse.

Phloem (FLOH-em): the sugar network

• Made of living cells (sieve tube elements), each kept alive by an adjacent companion cell.
• Sugar moves from sources (usually leaves) to sinks (growing tips, roots, fruits, storage) depending on what the plant is doing. Unlike xylem, which only ever runs upward, phloem can run either way: sap may be traveling up to a flower and down to the roots at the same moment in different bundles. Within any one sieve tube, though, the flow goes one direction at a time (De Schepper et al., 2013).

Xylem Cambium Phloem

Fig. 5. A vascular bundle. Xylem (dead, hollow, lignified) carries water up; phloem (living) carries sugar in any direction. In woody stems a cambium between them lays down new tissue each year, which is what tree rings are.

Because both systems run through the single stem, a wound that severs it cuts off the water going up and the sugar coming down at once. A leaf can be lost and regrown. A stem severed at the base strands everything above the cut, with no water arriving and nowhere for sugar to go, and it rarely recovers.