7.1.4—Structure and function

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.