Plant Biology
Xylem and Phloem
Plant vascular tissues — water and sugar transport throughout the plant
Xylem and phloem are the two vascular tissues that transport materials in vascular plants. Xylem: transports water and dissolved minerals from roots upward to leaves. Composed of dead, hollow cells (tracheids and vessel elements). Driven by transpiration pull (water evaporating at leaves). Cohesion-tension mechanism. Phloem: transports sugars (and other organic compounds) bidirectionally — from sources (leaves) to sinks (roots, fruits, growing tissues). Composed of living cells (sieve tubes + companion cells). Pressure-flow hypothesis. Together: vascular bundles in stems, roots, leaves. Critical for plant function.
- Xylem functionWater + minerals; roots → leaves
- Xylem cellsTracheids, vessel elements (dead at maturity)
- Phloem functionSugars; sources → sinks
- Phloem cellsSieve tubes + companion cells (living)
- Xylem mechanismTranspiration pull (cohesion-tension)
- Phloem mechanismPressure flow
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Why xylem/phloem matter
- Plant biology. Foundation of vascular function.
- Forestry. Wood is xylem.
- Agriculture. Crop water management.
- Climate. Trees as water cycle.
- Diseases. Vascular wilt diseases (e.g., Dutch elm disease).
- Trees. Understanding tree height limits, water logistics.
- Botany. Plant anatomy.
Common misconceptions
- Xylem cells are alive. Dead at maturity.
- Phloem cells are dead. Living.
- Plants pump water up. Actually pulled by transpiration.
- Sap is just water. Xylem: water + minerals. Phloem: sugars + nutrients.
- Phloem flow always one direction. Bidirectional possible.
- Xylem makes wood. Specifically secondary xylem.
Frequently asked questions
How does water move up trees?
Cohesion-tension (transpiration pull). (1) Water evaporates from leaves through stomata (transpiration) → tension. (2) Water molecules pulled up by cohesion (H-bonds between water molecules). (3) Continuous water column from roots to leaves. (4) Adhesion to xylem walls helps. (5) Roots take up water. Doesn't need active pumping. Water column up to 100+ m (tall trees). Pure physics + chemistry.
What's xylem made of?
Two main cell types. (1) Tracheids: long thin cells; tapered ends; pitted walls. Found in all vascular plants. (2) Vessel elements: shorter, wider; perforated end walls forming continuous tubes. Found in angiosperms (more efficient). Both: dead at maturity (just walls remain); rigid; hollow tubes. Plus: parenchyma cells (storage), fibers (support).
How does phloem work?
Pressure-flow mechanism. (1) Source (e.g., leaves): sucrose pumped into phloem (active transport) → high osmolality → water enters → high pressure. (2) Pressure pushes phloem contents toward sink. (3) Sink (e.g., roots, fruits): sucrose unloaded → osmolality drops → water exits → pressure drops. (4) Pressure gradient drives bulk flow. Bidirectional — different sources to different sinks at different times.
What's transpiration?
Water vapor loss from plants, mostly through stomata. Generates tension that pulls water up xylem. Amounts: significant — large tree can transpire 100+ liters/day. Cools plant. Side effect of needing CO₂ for photosynthesis (stomata must open). Drives the water cycle locally — moisture cycling between vegetation and atmosphere.
What about translocation?
Movement of organic compounds (especially sugars) in phloem. From sources (where made or stored) to sinks (where used). Sources: photosynthesizing leaves, storage organs (potatoes mobilizing in spring). Sinks: growing points, fruits, seeds, roots. Direction depends on time of year; can be bidirectional. Provides nutrients to all parts of plant.
What's a vascular bundle?
Group of xylem and phloem in stems and other organs. Arrangements vary: dicots (eustele) — bundles in ring around stem; monocots — scattered throughout. Can include vascular cambium (lateral meristem; produces secondary growth = wood). Stems thus have vascular tissue in specific arrangements characteristic of plant group.
How is wood structurally organized?
Wood = secondary xylem. From vascular cambium dividing inward. Annual rings: differences in wood produced spring vs summer (latewood denser). Heartwood (dead, central): structural support. Sapwood (functional outer): water transport. Hardwood (angiosperms): vessel elements + tracheids. Softwood (conifers): only tracheids. Tree growth measured in rings; dendrochronology dates events.