Structure of transport tissues
Introduced| English | Meaning |
|---|---|
| xylem/ˈzaɪləm/ | Plant vascular tissue that transports water and mineral ions and helps support the plant. |
| phloem/ˈfləʊɪm/ | Plant vascular tissue that transports sugars and other organic substances between sources and sinks. |
| vessel/ˈvesl/ | A water-conducting xylem tube formed from vessel elements joined end to end. |
| lignin/ˈlɪɡnɪn/ | A strong organic material deposited in some plant cell walls to provide support and waterproofing. |
| sieve tube/sɪv tjuːb/ | sieve tube |
| companion cell/kəmˈpænɪən sel/ | A living phloem cell that supports a sieve-tube element and helps load or unload transported sugars. |
| sieve plate/sɪv pleɪt/ | A perforated end wall between sieve-tube elements through which phloem sap can pass. |
Transport tissues in plants
- A plant must move water up and food around — two different jobs.
- So it has two transport tissues: xylem 木质部 and phloem 韧皮部.
- Their structure is beautifully matched to what they carry.
Xylem vs phloem
| Xylem | Phloem | |
|---|---|---|
| carries | water + mineral ions | dissolved sugars (assimilates) |
| direction | up from roots | to wherever needed (source → sink) |
| cells | dead, empty | living |
- In a stem they sit in bundles near the outside (xylem on the inside); in a root the xylem is a central star; in a leaf both are in the veins.
Xylem (water, up) and phloem (sugars) side by side
Which statement is correct?
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Xylem moves water and mineral ions upward; phloem moves dissolved assimilates (mainly sucrose).
Xylem vessels 导管
- Xylem tubes are vessels: dead, empty cells joined end to end with the end walls gone — one long open pipe.
- The walls are thickened and waterproofed with lignin 木质素.
- So: the hollow tube lets water flow fast, and the lignin gives strength and support.
A stem vascular bundle: xylem and phloem grouped together
Xylem vessels are strengthened and waterproofed by ______ in their walls.
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Lignin makes a strong, waterproof, hollow pipe — and the cells die, leaving an open tube.
A mature xylem vessel is a living cell, full of cytoplasm and a nucleus.
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Xylem vessels are dead and empty — that is what makes them a fast, open pipe for water.
Phloem sieve tubes 筛管 and companion cells 伴胞
- Phloem tubes are sieve tubes: living cells joined end to end. Their end walls become sieve plates 筛板 with holes for sap. To leave room, the cell loses most contents and has no nucleus.
- Each sieve tube has a companion cell beside it — it keeps its nucleus and many mitochondria, and does the living work, loading sugars in.
Xylem and phloem side by side
Tap each part. Xylem is a dead, hollow pipe for water; phloem is a living tube for sugars, helped by its companion cell.
The companion cell beside a sieve tube:
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The sieve tube loses most contents, so the companion cell (with nucleus and mitochondria) does the living work.
Match each structure to its description.
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Xylem = dead lignified pipe; sieve tube = living with sieve plates; companion cell = nucleus + loads sugar.
Sieve plates are:
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Sieve plates are perforated end walls between sieve-tube cells, allowing sap to pass along the phloem.
You've got it
- xylem: water + minerals up; dead, empty vessels with lignin (fast flow + support)
- phloem: sugars to where needed; living sieve tubes with sieve plates and no nucleus
- each sieve tube has a companion cell (nucleus + mitochondria) that loads sugar
- stem: bundles near outside; root: central xylem star; leaf: in the veins