7.1 Structure of Transport Tissues

A plant transverse-section plan diagram is a large, simple map of tissue boundaries and the requested transport tissues. It records the evidence visible in the section without copying every cell or inventing structures that cannot be seen.

1. Orient and inspect: Identify whether the section is a stem, root or leaf, then locate the vascular bundle(s) and the relative position of xylem and phloem. Use visible boundary, size, shape and staining/contrast evidence; treat a label as an inference only when the pattern supports it.
2. Map the tissue outline: Draw a large, clean outline of the section and the main tissue regions with continuous lines. Keep the relative proportions and positions of the regions; at low power, do not draw individual cells.
3. Add only useful detail: Add a few simple features requested by the question or needed to distinguish tissues—for example, the characteristic xylem region and the phloem region in a vascular bundle. Do not shade or sketch a high-power image as if it were a plan diagram.
4. Label and check: Use a ruler for clear label lines that end at the correct tissue, avoid crossing lines, and check spelling. Add magnification or a scale line only when supplied or required.

The evidence-to-diagram chain is: visible tissue pattern → vascular bundle location → supported xylem/phloem identification → simplified labelled plan. Xylem transports water and mineral ions from roots, while phloem transports substances from source to sink; these functions explain why their positions matter, but the drawing itself must stay within what the section shows.

A plan diagram is not a photograph, a shaded cell drawing or a guess based only on the organ name. Preserve tissue boundaries and proportions, label only requested/identifiable structures, and separate direct observations (shape, position, contrast) from functional interpretation. Staff visual brief: show a before/after transverse-section micrograph and simplified plan with vascular bundle, xylem, phloem and clean label lines; do not generate or bind an image in this card transaction.

Xylem and phloem occur together in vascular bundles, but their relative positions change with the organ. Read the organ layout first, then identify the xylem and phloem positions rather than assuming one universal bundle pattern.

• Root: The vascular tissue is concentrated in the centre. Xylem forms the central transport region; phloem lies around the edge of the central core. This arrangement places conducting tissue where water and mineral ions entering from the roots can join the upward pathway.
• Stem: Vascular bundles are arranged around the outside of the stem. Within each bundle, xylem is on the inner side and phloem is on the outer side, nearer the epidermis. The distribution fits a stem that must conduct while also resisting bending and supporting the plant.
• Leaf: Vascular bundles form the midrib and veins and spread through the leaf. Xylem is above phloem within the bundle, closer to the upper epidermis; phloem is on the lower side, closer to the lower epidermis. The vein network supplies the leaf and carries substances away from source tissue.

For a transverse section, use a fixed read-out: (1) name the organ from the overall pattern, (2) locate the vascular bundle or central vascular region, (3) compare inner/outer or upper/lower position, then (4) label xylem and phloem only where the evidence supports the identification. Xylem carries water and dissolved mineral ions upward and also contributes structural support; phloem transports substances from source to sink.

Do not draw roots, stems and leaves with the same bundle layout. “Xylem is inside” is useful for roots and stems, but in leaves the key comparison is that xylem is above phloem. These are relative positions in herbaceous dicot transport tissues, not a complete map of every plant organ or an invitation to infer anatomy beyond the section.

Xylem vessel elements and phloem sieve-tube elements are specialised conducting cells, while companion cells maintain and support the associated sieve-tube elements. Their mature structures differ because water/mineral transport and assimilate transport impose different constraints.

Xylem vessel elements — water and mineral-ion pathway

• Large vessel elements join end to end; when mature they have no protoplasm and no end plates, leaving a relatively open path for upward mass flow.
• Thick lignified walls strengthen the vessel and help it resist collapse under the pressures involved in water transport; pits allow lateral water movement between conducting regions.
• The mature conducting element is therefore structurally strong and low in internal obstruction, matching one-way transport from roots towards the rest of the plant.

Phloem sieve-tube elements — assimilate pathway

• Sieve-tube elements join end to end to form a continuous tube. Sieve plates with pores connect neighbouring elements, allowing organic assimilates to move through the tube.
• Mature sieve-tube elements lack a nucleus, ribosomes and a large vacuole; thin cytoplasm leaves more space for assimilate flow. Their cellulose wall supports the tube without the lignified xylem pattern.
• Phloem transport can occur in either direction in the plant, according to source-to-sink relationships, rather than being a fixed upward stream.

Companion cells — sieve-tube support

• Each sieve-tube element is associated with a companion cell containing a nucleus and other organelles, including many mitochondria.
• Plasmodesmata link the companion cell to its sieve-tube element; transport proteins and ATP supply support loading and unloading of assimilates and the metabolism of the conducting partner.

Read the structures as an aligned comparison: conducting cell state — mature xylem vessel elements are non-living and open, whereas sieve-tube elements remain living but have reduced internal organelles; wall/connection — xylem uses lignified walls, no end plates and pits, whereas phloem uses cellulose walls and sieve plates; transport — xylem carries water and dissolved mineral ions mainly upward, while phloem carries organic assimilates up or down according to source and sink; support — xylem structure directly resists collapse, while companion-cell organelles and plasmodesmata support sieve-tube metabolism and loading/unloading.

Do not call companion cells the conducting tube or treat sieve plates as xylem end plates. Do not infer that every xylem cell has the same appearance: this objective centres on vessel elements, sieve-tube elements and their companion cells. Staff drawing brief: compare labelled longitudinal/transverse sketches of a vessel element, sieve-tube element and companion cell, showing only syllabus-supported walls, pores, organelles and connections; do not generate or bind an image.

Transport tissues work because their cell structures are matched to the materials moved and the support required. Xylem provides a strong, low-obstruction pathway for water and dissolved mineral ions; phloem combines sieve-tube conducting cells with companion-cell support for assimilate translocation.

• Xylem: structure → transport/support: Vessel elements join to form a continuous conducting path. When mature they are hollow, with no protoplasm and no end plates, so water and dissolved mineral ions can pass with little internal obstruction. Lignified walls add strength and resist collapse; pits allow lateral movement between conducting regions. Together, these features support mainly upward xylem transport and the plant's structural framework.
• Phloem: structure → assimilate translocation: Sieve-tube elements join end to end and are connected by sieve plates with pores, allowing organic assimilates to move through the tube. Their reduced mature contents leave space for flow, while the cellulose wall supports the tube.
• Companion cells: structure → support: Companion cells retain a nucleus and organelles, including many mitochondria, and connect to their associated sieve-tube elements through plasmodesmata. Their metabolism, membrane transport proteins and ATP support sieve-tube maintenance and the loading/unloading of assimilates.
• Direction boundary: Xylem is treated here as the water/mineral pathway from roots towards the rest of the plant, while phloem movement follows source-to-sink demand and may be up or down. Direction alone is not evidence for identifying a tissue; use structure and organ context together.

Synthesis cue: lignified, hollow, pitted vessel structure → water/mineral conduction plus support; sieve tubes + companion cells → livingly supported assimilate translocation.

This card explains how transport-tissue structure supports transport and support functions. It does not teach transpiration, cohesion-tension, water-potential gradients or other 7.2 transport mechanisms. Do not claim that phloem always moves downwards or that all xylem tissue is one identical cell type. Staff visual brief, if later commissioned: use two causal mini-panels (xylem and phloem) with structure→function arrows; do not generate or bind an image in this transaction.