The Planters' Guild
Module I · How a Plant Works
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Lesson 4 of 10 · ~6 min

The stem's two pipelines

Xylem carries water up. Phloem carries sugar in more than one direction. Knowing the difference is a real tool.

Besides holding the plant up, the stem carries water and minerals up from the roots and moves the sugars made in the leaves out to the rest of the plant (Clark et al., 2018). Two conducting tissues run alongside each other in the same bundles (Michaels et al., 2022).

Xylem : the water pipeline

  • Its water-conducting cells are no longer alive once mature: they're hollow, with thick walls stiffened by lignin, the compound that makes wood rigid. (Wood is made of xylem.)
  • The cells link up, through small pits in their walls, into a network of tubes running from the finest roots to the leaf veins. Water moves as a continuous column.
  • Flow is one-directional: up. It doesn't reverse.

Phloem : the sugar network

  • Its sieve-tube cells are still alive at maturity, and the companion cells beside them give them metabolic support.
  • Sugar moves from sources (usually leaves) to sinks (growing tips, roots, fruits, storage) depending on what the plant is doing. Water in the xylem moves one way only, from root to stem to leaf and out through the stomata (Michaels et al., 2022). Flow in the phloem runs in several directions among leaf, stem and root (Michaels et al., 2022). The highest leaves send sugar up to a growing shoot tip while the lower ones send it down to the roots, and leaves in between send it in both directions (Clark et al., 2018).
Labeled vascular bundle: xylem (large hollow tubes) at left, cambium band in the middle, phloem (living cells) at right. Xylem Cambium Phloem

Fig. 5. A vascular bundle. Xylem, whose conducting cells are hollow, lignified and no longer alive, carries water up; living phloem carries sugar in more than one direction. In woody stems a cambium between them lays down new tissue each year, which is what tree rings are.

Because both systems run side by side through the stem, a deep enough wound can cut through both at once. A string trimmer that cuts all the way around a broadleaf tree's trunk, through the tissue just inside the bark, can be more than the tree can recover from; a palm shrugs off the same wound because its bundles are scattered through the whole trunk (Moore & Bradley, 2022).

From memory Tier 1

Strengthen your understanding

Try these in your head first, then open to see.

Someone snaps a plant's main stem near the base. What happens to the water and sugar supply for everything above the break?
The stem connects the roots to the leaves: water and minerals travel up through it, and the sugars the leaves make travel out through it to everything else. A break near the base cuts both routes at once for everything above it.
Xylem vs. phloem: which has living conducting cells, which doesn't, and which way does each move?
Xylem: water and minerals, one way, from the roots up to the leaves; its water-conducting cells are no longer alive at maturity. Phloem: sugars, moving in several directions among leaves, stem and roots; its sieve-tube cells are still alive at maturity, though they've lost their nucleus.
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