Transportation in plants

Xylem

• Structure:

  • A hollow tube​ made of dead cells​

  • No end walls​ between cells → forms a continuous column of water.

  • Walls are thickened with lignin​ — a carbohydrate that provides strength and rigidity, preventing the vessel from collapsing under tension.

• Function

  • Transports water and mineral ions up​ the plant (roots → stem → leaves) in xylem vessels.

  • The lignin also makes xylem waterproof​ and gives the plant mechanical support.

Phloem

• Structure

  • A tube made of living cells​ (sieve 筛子 tube cells, which doesn’t have a nucleus and many other organs).

  • End walls with perforations​ (small holes) called sieve plates, allowing substances to move through.

  • Sieve tube cells have gaps in their cell walls​ at the joining points.

  • Each sieve tube cell is supported by a companion cell, which supplies energy and maintains the sieve tube cell.

  
  

• Function

  • Transports sucrose​ (made from glucose + fructose) and amino acids.

  • These are transported from where they are made/entered (source) to where they are used or stored (e.g. sink, the growing parts of a plant)​ — e.g. from photosynthesising leaves​ to growing parts​ or roots.

  • Movement is two-way​ (can go up or down the plant), unlike xylem.

Transpiration 蒸发

• Transpiration​ = the evaporation of water vapour​ from the surface of a plant (mainly through open stomata in the leaves).

• Transpiration stream​ = the movement of water through the xylem vessels from roots to leaves.

• As water evaporates from the leaves, more water is pulled up​ the xylem to replace it — this is the main mechanism driving xylem transport.

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