Structure of transport tissues

1. Overview

Plants move two main things over long distances: water with dissolved mineral ions, carried upwards in xylem, and dissolved organic assimilates such as sucrose, carried in phloem. These transport tissues are grouped together as vascular tissue and are arranged in distinct patterns in the stem, root and leaf of a herbaceous dicotyledonous plant.

At A-Level you must be able to:

• draw plan diagrams of transverse sections to show where the tissues lie;
• draw and label the individual cells (vessel elements, sieve tube elements, companion cells); and
• relate the structure of each cell to its function.

This topic is mostly about careful observation and clear drawing, so the technique advice below matters as much as the biology.

Key Definitions

• Vascular tissue: the transport tissue of a plant, made up of xylem (water and mineral ions) and phloem (dissolved organic assimilates such as sucrose).
• Xylem vessel element: a dead, hollow, lignified cell joined end to end with others to form a continuous tube that carries water and mineral ions and supports the plant.
• Phloem sieve tube element: a living cell, with little cytoplasm and no nucleus, joined end to end through sieve plates to form tubes that transport assimilates.
• Companion cell: a living cell with a nucleus and many mitochondria, linked to a sieve tube element by plasmodesmata, that carries out the metabolic work the sieve tube element cannot.
• Lignin: a strong, waterproof, woody substance deposited in xylem cell walls that provides support and prevents the vessels collapsing.
• Sieve plate: the perforated end wall between two sieve tube elements through which assimilates pass from cell to cell.
• Plan diagram: a low-power drawing that shows only the outlines and distribution of tissues, with no individual cells drawn in.
• Herbaceous dicotyledonous plant: a non-woody flowering plant whose seeds contain two cotyledons, used as the standard example for studying plant tissue arrangement.

Content

Drawing plan diagrams

A plan diagram (also called a low-power or tissue-level drawing) shows the outlines and distribution of tissues only — you do not draw individual cells inside them.

Good technique:

• Use a sharp pencil, draw clear continuous lines and add no shading.
• Keep the proportions of the tissues true to what you see down the microscope or in a photomicrograph.
• Draw label lines with a ruler, do not let them cross, and point exactly to the tissue named.
• Work outwards from the centre, marking the boundaries between the main layers (for example epidermis, cortex and the vascular tissue) before adding labels.

The fastest way to identify the organ in an unlabelled section is the arrangement of the vascular tissue, summarised here:

Organ	Arrangement of vascular tissue	Quick visual cue
Stem	Separate vascular bundles in a ring near the outside; xylem inside, phloem outside, cambium between	"Ring of bundles"
Root	A single central core; xylem in a central star (X), phloem between the arms	"Central star"
Leaf	Vascular tissue in the midrib and veins; xylem on the upper side, phloem on the lower side	"Vein with xylem on top"

Distribution of xylem and phloem in the stem

In the stem of a herbaceous dicotyledonous plant the vascular tissue is arranged as a ring of separate vascular bundles near the outside, just inside the cortex. Within each bundle:

• the xylem lies towards the inside (nearer the centre);
• the phloem lies towards the outside (nearer the epidermis);
• a layer of cambium sits between them.

Placing the supporting, lignified xylem in a peripheral ring gives the stem good resistance to bending.

Distribution of xylem and phloem in the root

In the root the vascular tissue forms a single central core rather than scattered bundles. The xylem is arranged as a star shape (an X or cross) in the very centre, with the phloem in the patches between the arms of the xylem star. This central position means the strong xylem is well placed to resist the pulling (tension) forces that act on a root as the plant is anchored and as water is drawn up.

Distribution of xylem and phloem in the leaf

In the leaf the vascular tissue runs through the midrib and the network of veins. In a section through a vein the xylem sits on the upper side and the phloem on the lower side — so on an unlabelled slide, xylem is on the side facing the upper epidermis and phloem on the side facing the lower epidermis. This makes sense functionally: water arriving in the xylem is delivered close to the upper, brightly lit palisade mesophyll where most photosynthesis occurs, and the sucrose made there is loaded into the nearby phloem for export.

Xylem vessel elements and their functions

Xylem vessel elements are dead, hollow cells with no cytoplasm, no nucleus and no end walls between successive elements, so they join into a continuous open tube (rather like a drainpipe) through which water flows with little resistance. Their walls are thickened and waterproofed with lignin, deposited in rings, spirals or as a near-complete layer.

This lignin does two jobs:

• it gives mechanical support to the whole plant and stops the vessel collapsing under the tension of the transpiration stream;
• it is waterproof, so water cannot leak out sideways through the wall.

Water can still move into and out of the vessel through unlignified gaps called pits in the wall, and through the open ends. The absence of cell contents and cross-walls is the key feature that lets a continuous, uninterrupted column of water be pulled up the plant.

Phloem sieve tube elements and their functions

Sieve tube elements are living cells joined end to end to form sieve tubes. The end walls between them are perforated to form sieve plates with pores, allowing assimilates to pass from one element to the next.

To reduce the obstruction to flow, a mature sieve tube element loses most of its contents:

• it loses its nucleus, ribosomes, vacuole, tonoplast and most other organelles;
• it retains only a thin layer of cytoplasm pressed against the wall, giving the dissolved sucrose solution a low-resistance path;
• it does not become hollow like a xylem vessel — it stays alive and keeps that thin layer of cytoplasm.

Companion cells and their functions

Because a sieve tube element has no nucleus and very few organelles, it cannot carry out its own protein synthesis or generate much ATP. Each sieve tube element is therefore paired with a companion cell, a fully living cell with a nucleus, dense cytoplasm and many mitochondria. The two are joined by many plasmodesmata (cytoplasmic bridges through the cell walls).

The companion cell:

• provides the ATP (from its many mitochondria) needed to load sucrose into the phloem by active transport;
• carries out the metabolism and protein synthesis that keep the sieve tube element alive.

Together, a sieve tube element and its companion cell act as one functional unit.

Comparing the three transport cells

The structure-to-function differences are easiest to revise side by side:

Feature	Xylem vessel element	Sieve tube element	Companion cell
Living or dead?	Dead	Living	Living
Nucleus?	No	No	Yes
Cytoplasm?	None (hollow)	Thin layer lining the wall	Dense cytoplasm
End walls?	None (open tube)	Perforated sieve plates	Normal walls; plasmodesmata to sieve tube
Wall feature	Lignified (rings/spirals), pits	Thin cellulose wall	Thin cellulose wall
Main function	Carry water + mineral ions; support	Carry assimilates (sucrose)	Support the sieve tube; supply ATP for loading

Drawing and labelling the cells (high-power)

When you are asked for a high-power drawing of the individual cells (from a slide, photomicrograph or electron micrograph), you draw the detail of single cells, not tissue outlines. Make sure your labels include the features that "earn" the structure-function marks:

• Xylem vessel element: the lignified wall (show the ring or spiral pattern), the wide hollow lumen with no cell contents, the absence of end walls, and any pits in the wall.
• Sieve tube element: the sieve plate with pores at the end wall, the thin layer of cytoplasm lining the inside, and the absence of a nucleus.
• Companion cell: the nucleus, dense cytoplasm, many mitochondria, and the plasmodesmata connecting it to the neighbouring sieve tube element.

Worked example

Exam-style question: A student examines a stained transverse section of a young herbaceous dicotyledonous stem and a separate slide of the root of the same species. Describe how the distribution of xylem and phloem differs between the stem and the root, and explain one way in which the position of the xylem suits each organ. [4]

Model answer:

• In the stem the vascular tissue is in separate bundles arranged in a ring near the outside, with xylem on the inner side and phloem on the outer side of each bundle.
• In the root the vascular tissue is a single central core, with the xylem in a central star (X) shape and phloem between its arms.
• In the stem the peripheral ring of lignified xylem helps the stem resist bending.
• In the root the central xylem is well placed to resist the pulling/tension forces acting along the root.

Worked example

Exam-style question: A phloem sieve tube element lies next to its companion cell. Three features are noted: (A) the sieve tube element has lost its nucleus and most organelles, (B) the end wall is a sieve plate with pores, and (C) the companion cell contains many mitochondria. Explain how each of features A, B and C suits the mass flow of assimilates through the phloem. [3]

Model answer:

• A — loss of the nucleus and most organelles: leaves a near-empty, low-resistance path through the sieve tube element so the sucrose solution can flow with little obstruction.
• B — sieve plate pores: provide cytoplasmic continuity between successive elements, letting the assimilate stream pass from one cell to the next along the tube.
• C — many mitochondria in the companion cell: release ATP by aerobic respiration, supplying the energy for the active loading of sucrose into the phloem (the companion cell passes materials to the sieve tube via plasmodesmata).

Key Equations

This topic is qualitative — it is assessed through accurate diagrams, descriptions of tissue distribution and structure-function reasoning, so there are no equations to apply here.

Common Mistakes to Avoid

• Drawing individual cells in a plan diagram. A plan (low-power) diagram shows tissue outlines and distribution only — no cells, no shading. Save the detailed cell drawings for the high-power task.
• Mixing up the xylem and phloem positions in the stem. In a dicot stem bundle the xylem is on the inside and the phloem on the outside, with cambium between them — a common slip is to reverse them.
• Saying sieve tube elements have no cytoplasm. Although they lose their nucleus and most organelles, sieve tube elements stay alive and keep a thin layer of cytoplasm lining the wall; only xylem vessels are truly hollow and dead.
• Forgetting why xylem must be dead and empty. State explicitly that the lack of cell contents and end walls gives a continuous, uninterrupted tube for water; do not just call it "hollow".
• Describing lignin only as "support". Mention both roles: it gives mechanical support / stops the vessel collapsing and it is waterproof, preventing sideways water loss.
• Treating the companion cell as just a "helper" with no detail. Name the features that make it useful — a nucleus, dense cytoplasm and many mitochondria that supply ATP for loading sucrose by active transport — and the plasmodesmata linking it to the sieve tube element.
• Using vague water language in transport answers. When explaining the movement of water, use water potential terms (water moving from a higher to a lower water potential) rather than "concentration of water".

Exam Tips

• For drawing questions, read whether you are asked for a plan diagram (tissues only) or a high-power drawing (individual cells) — the two are marked very differently.
• Quote the giveaway shapes: a ring of bundles for a stem and a central star of xylem for a root. These let you identify the organ instantly in unlabelled photomicrographs.
• When a question says "relate structure to function", pair each structural feature with the job it does in one sentence (for example, "no end walls, so water flows in a continuous column").
• Use the precise cell names — vessel element, sieve tube element, companion cell — rather than just "xylem cell" or "phloem cell".
• For "compare" questions on xylem and phloem, write comparative sentences (xylem is dead whereas phloem is living; xylem has no cytoplasm whereas sieve tubes keep a thin layer), not two separate descriptions.