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4.2.2Topic 4 · Biodiversity and Natural Resources

Plant Stems — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 8 parts Lesson 13 of 22 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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4.2.2 Plant Stems — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe the tissues in a herbaceous dicotyledonous stem and relate the structure of xylem and phloem to their functions.The stem of a herbaceous dicotyledonous plant supports the leaves and flowers, and carries wat…

  • vascular bundles
  • parenchyma
  • Xylem vessels
  • Sieve tube elements
  • sieve plates
  • Companion cells

Section 1 of 8Overview of stem tissues

The stem of a herbaceous dicotyledonous plant supports the leaves and flowers, and carries water, minerals and sugars between roots and shoots.

Vascular bundles
In transverse section the vascular tissue is arranged in a ring of separate vascular bundles near the outside of the stem.
Cambium
Each bundle contains xylem towards the centre and phloem towards the outside, with a thin layer of dividing cells, the cambium, between them.

Outside the bundles is the cortex and epidermis; in the middle is the pith. Both pith and cortex are mostly parenchyma: thin-walled living cells that store starch and fill space. Because they are turgid they also give some support to young stems.

Section 2 of 8Xylem

Xylem transports water and dissolved mineral ions upwards from the roots and also provides mechanical support. It contains several cell types:

Xylem vessels
Xylem vessels long tubes formed from cells stacked end to end.
  • The end walls break down, forming a continuous hollow tube with no cytoplasm, which gives little resistance to flow.
  • The walls are thickened with lignin, a waterproof, strong polymer, laid down in rings, spirals or networks.
  • This lets the vessel resist the negative pressure (tension) created by transpiration and stops it collapsing.
Pits
Pits in the lignified walls allow water to move sideways into neighbouring vessels or living cells, and gives an alternative route if a vessel is blocked by an air bubble.
Fibres
(sclerenchyma): elongated dead cells with thick lignified walls that give support.
Parenchyma
Parenchyma living cells that store food.

Lignin is impermeable, so the contents of a vessel cell die once the wall is complete. Water moves by mass flow along the tension gradient (cohesion-tension), with cohesion between water molecules and adhesion to the walls helping maintain the continuous column.

Section 3 of 8Phloem

Phloem transports organic solutes, mainly sucrose and amino acids, from sources (such as mature leaves) to sinks (such as roots, growing shoots, fruits and storage organs). This is translocation. Its main cells are:

Sieve tube elements
Sieve tube elements living cells joined end to end, with perforated end walls called sieve plates. They have very little cytoplasm, no nucleus and few organelles, which leaves more room for the flow of sap.
Companion cells
Companion cells each is linked to its sieve tube element by many plasmodesmata. The companion cell has a nucleus, dense cytoplasm and many mitochondria, and supplies the ATP needed for the active loading of sucrose into the sieve tube.
Phloem parenchyma and some fibres.

Section 4 of 8Support, sclerenchyma and collenchyma

Mechanical strength in stems comes from three sources.

Collenchyma
Turgid parenchyma pushes outwards and presses cells against each other. Collenchyma has unevenly thickened cellulose walls and supports growing stems while still allowing them to elongate. Sclerenchyma fibres have walls thickened with lignin and are dead at maturity; they give strong, permanent support.
Xylem vessels are therefore both a transport system and a skeleton, which is why wood is made mainly of xylem.
TissueMain functionKey feature
Xylem vesselWater transport and supportDead, lignified, hollow
Sieve tubeSugar transportLiving, sieve plates
Companion cellSupports sieve tubeMany mitochondria
Sclerenchyma fibreSupportThick lignified wall
ParenchymaStorage, packingThin walls, living

Section 5 of 8Observing and interpreting stem sections

Stem sections are commonly examined as a transverse section (TS) cut across the stem and a longitudinal section (LS) cut along it.

Observing and interpreting stem sections
In TS the xylem vessels are seen as large circles with thick walls, whereas in LS they appear as long tubes with spiral, ring or reticulate thickening.
Sieve plates are visible in LS as cross walls with pores.
Comparing the two views helps you understand three-dimensional structure, because a single section only shows one plane.

Growth of a stem in thickness (secondary growth) happens in woody plants.

1
The cambium in each bundle divides and joins with cambium between bundles to make a complete ring.
2
Cells produced on the inside differentiate into secondary xylem and those on the outside into secondary phloem, so the xylem accumulates as wood and forms the annual rings seen in tree trunks.
3
Herbaceous plants show little or no secondary growth, which is why they rely on turgor for support.
4
In a dicotyledon the bundles are in a ring; in a monocotyledon such as maize they are scattered.
5
Be ready to state where in a bundle each tissue lies and to explain why this arrangement near the outside resists bending forces effectively.

Section 6 of 8Exam-style questions

  1. Describe how the structure of a xylem vessel is related to its function. [4]4 marks

  2. Explain why companion cells contain many mitochondria. [2]2 marks

  3. State two ways in which sclerenchyma differs from parenchyma. [2]2 marks

Section 7 of 8Key terms

TermMeaning
Vascular bundleGroup of xylem, phloem and cambium running through the stem.
Xylem vesselDead, lignified tube transporting water and ions and giving support.
LigninStrong waterproof polymer that thickens the walls of xylem and sclerenchyma.
Sieve tube elementLiving phloem cell with perforated end walls that transports organic solutes.
Companion cellCell with dense cytoplasm and mitochondria linked to a sieve tube element.
ParenchymaThin-walled living cells used for storage and packing.
SclerenchymaFibres with thick lignified walls giving support.
TranslocationMovement of organic solutes in the phloem from source to sink.

Section 8 of 8Exam technique

1
Saying xylem transports foodxylem carries water and ions, phloem carries sucrose and amino acids.
2
Stating that sieve tube elements have no cell membrane or are dead: they are living but have reduced contents.
3
Believing lignin is found in all plant cell walls: it is mainly in xylem and sclerenchyma.
4
Saying phloem moves materials only downwardsflow is from source to sink in any direction.
5
Describing xylem as having sieve platesthose belong to phloem.

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