7.1.27 · Transport in Plants

Xylem and Phloem Distribution — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 10 parts Lesson 4 of 15 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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Lesson summary

7.1.2 Xylem and Phloem Distribution — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe where xylem and phloem lie in roots, stems and leaves of herbaceous dicots and relate their positions to support, transport and resistance to stress.

Vascular bundles
Water entry
Woody plants
Annual rings
Orientation
Short paths

Part 1 of 8Two transport tissues

Xylem

Xylem it carries dissolved minerals and water up the plant from the roots and also provides structural support.

Phloem

Phloem it carries organic solutes, mainly sucrose and amino acids (assimilates), from sources to sinks.

Vascular bundles

Vascular bundles the two tissues run together in bundles that connect the whole plant, from root tips to young leaves.

Position

Position the location of bundles depends on the organ, because different organs are under different stresses.

Part 2 of 8Distribution in the root

1Layout

Layout the xylem forms a central star or X, with phloem in the gaps between its arms.

2Strength

Strength the central xylem lets the root resist the tension forces when the shoot is blown about, like a flexible, strong rope.

3Water entry

Water entry the xylem arms reach close to the endodermis, so water crossing the cortex enters the xylem after a short distance.

4Pericycle

Pericycle it surrounds the vascular tissue and gives rise to lateral roots.

Part 3 of 8Distribution in the stem

Layout

Layout the vascular bundles form a ring near the outside, with xylem on the inner side, phloem on the outer side and cambium between.
- (closest to the epidermis)

Strength

Strength strong lignified xylem at the periphery resists bending best, as a tube is stiffer than a solid rod of the same mass.

Woody plants

Woody plants the cambium produces secondary xylem (wood) each year inward and secondary phloem outward, so the stem thickens.

Annual rings

Annual rings they reflect seasonal differences in vessel size in spring and summer,
- which allows a tree to be dated, with the oldest wood in the centre.

Part 4 of 8Distribution in the leaf

Layout

Layout the midrib has a large vascular bundle, with smaller veins branching through the blade.

Orientation

Orientation xylem is on the upper side and phloem on the lower side of each bundle.
- (nearest the lower epidermis)

Short paths

Short paths veins are close to every mesophyll cell, so water has a short distance from xylem to cells and sucrose has a short path to the phloem.

Support

Support the network of veins also supports the thin blade.

Organ	Xylem	Phloem
Root	Central, X-shaped	Between xylem arms
Stem	Inner side of each bundle	Outer side of each bundle
Leaf	Upper side of bundle	Lower side of bundle

Part 5 of 8Identifying the tissues on a slide

Xylem

Xylem large, empty, thick-walled cells with no contents, whose lignin is easy to see when stained.

Phloem

Phloem smaller cells, with thin-walled sieve tube elements and companion cells with dense cytoplasm and a nucleus placed next to them.

Sieve plates

Sieve plates they are sometimes visible in a transverse section.

Part 6 of 8Evidence for the direction of transport

Ringing

Ringing removing a ring of bark including phloem makes sugars accumulate above the ring, which shows that phloem carries organic solutes downward.
The xylem inside is unaffected, so water still moves upward.

Radioactive tracers

¹⁴C in carbon dioxide can be used to follow assimilates in the phloem.

Aphid stylets

Aphid stylets they can be used to sample phloem sap.

Part 7 of 8Support, lignin and turgor

Support, lignin and turgor

Turgor

Turgor water-filled cells press outward against their cellulose walls and keep herbaceous stems rigid,
- and loss of water lowers turgor so the plant wilts.

Lignified tissues

Lignified tissues lignified xylem and thickened collenchyma and sclerenchyma add strength.

Position and function

Position and function xylem near the centre of a root resists pulling, peripheral stem bundles resist bending,
- and leaf veins sit close to the mesophyll for short diffusion paths.

Part 8 of 8More from the specification

Key terms

Term	Meaning
Xylem	Tissue that transports water and mineral ions upward and provides support.
Phloem	Tissue that transports assimilates such as sucrose throughout the plant.
Assimilates	Organic substances such as sucrose and amino acids transported in the phloem.
Source	A region where assimilates are loaded into the phloem, such as a photosynthesising leaf.
Sink	A region where assimilates are removed from the phloem and used or stored.
Cambium	A layer of dividing cells between xylem and phloem that produces new vascular tissue.
Pericycle	A layer of cells inside the endodermis of a root that can produce lateral roots.

Avoid these mistakes

Reversing the positions of xylem and phloem in the stem bundle.

Saying phloem carries water upward — xylem carries water up and phloem carries assimilates in either direction.

Saying only xylem is dead — phloem sieve tubes are living cells.

Forgetting the cambium between xylem and phloem in a stem bundle.

Stating that xylem carries only water — it carries mineral ions as well.

Last updated: 15 October 2026