Two transport tissues
Plants transport substances through two specialised vascular tissues: the xylem and the phloem. Together they run through the roots, stem and leaves as vascular bundles.
Xylem
Xylem transports water and dissolved mineral salts from the roots up to the rest of the plant. Its main conducting cells are vessels and tracheids. Important features:
- The cells are dead and hollow at maturity, with no cytoplasm to block flow.
- End walls break down in vessels, forming continuous tubes.
- Walls are strengthened with lignin, which also provides mechanical support to the plant.
- Transport is essentially one-way: upward.
Phloem
Phloem transports organic food (mainly sucrose, and some amino acids) made in the leaves to other parts of the plant. Its main cells are sieve tube elements and companion cells:
- Sieve tube elements are living but lose their nucleus and most organelles; they are joined end to end by perforated sieve plates.
- Each sieve tube has a companion cell with a nucleus and many mitochondria that provides energy and controls the sieve tube's activity.
- Transport can occur in both directions (up or down) — this is called translocation.
Key idea
Xylem is made of dead, lignified cells and carries water upward only. Phloem is made of living cells and carries food in either direction.
Example
In a young stem the vascular bundles are arranged in a ring, with xylem toward the inside and phloem toward the outside, separated by a strip of cambium that can produce new cells.
Remember
A quick memory aid: Xylem = water going up (think "x-tra water"); Phloem = food flowing, both ways. Xylem cells are dead; phloem cells are alive.