Xylem Transport and the Cohesion-Tension Mechanism
Trace water and mineral movement through xylem and explain how transpiration, hydraulic resistance, cavitation, roots, and environment shape supply to the canopy.
Educational reference · evidence, sources, and limits shown below
Trace water and mineral movement through xylem and explain how transpiration, hydraulic resistance, cavitation, roots, and environment shape supply to the canopy.
Terms to know
- Xylem
- Vascular tissue conducting water and dissolved minerals and contributing structural support.
- Cohesion-tension
- Mechanism in which evaporation from leaves pulls a continuous, cohesive water column under tension.
- Cavitation
- Formation or expansion of gas within water-conducting xylem, interrupting hydraulic continuity.
- Hydraulic conductance
- Water flow per unit driving force through a defined segment or whole plant.
Core science
Mature xylem vessels and tracheids are largely dead conduits connected from roots through stems and leaf veins. Evaporation from moist cell walls in leaves creates tension that is transmitted down cohesive water columns. Adhesion to conduit walls and xylem anatomy help maintain the pathway. This transpiration-driven pull explains most daytime upward flow in actively growing plants.
The pathway has resistance at soil-to-root contact, root tissues, stem conduits, branch junctions, petioles, and leaves. Water supply can fail even when the reservoir is full if roots are hypoxic, saline, cold, diseased, physically disconnected, or too small for atmospheric demand. Narrower or embolized conduits increase resistance.
Cavitation can follow severe tension, freezing, injury, or pathogen effects. Plants can route around some blocked conduits and grow new xylem, but recovery is not guaranteed. Guttation or root pressure under low transpiration is not evidence that daytime hydraulic capacity is adequate.
Why this matters in cultivation
- Support heavy branches without crushing stems, and distinguish superficial bending injury from vascular collapse. Track wilt above and below an injury point.
- Avoid interpreting immediate post-irrigation recovery as proof of root health. Repeated cycles of midday wilt indicate a supply-demand mismatch requiring root, hydraulic, and environmental review.
Measure and record
Hydraulic path
Root system, stem diameter, branch order, injury points, support ties, and vascular discoloration.
Demand
Leaf area, PPFD, leaf temperature, VPD, airflow, CO2, and time of day.
Supply
Root-zone water distribution, EC, temperature, oxygen risk, and irrigation uniformity.
Plant response
Wilt position, timing, recovery, leaf water potential if measured, and tissue death.
Intervention
Irrigation, environmental change, pruning, support, sampling, and follow-up.
Common misconceptions
Correction: Transpiration-driven tension supplies most daytime xylem flow; root pressure is context-dependent.
Correction: Roots, conduits, junctions, leaves, and embolism all resist flow.
Correction: Water must remain available, oxygenated, connected, and hydraulically deliverable.
Evidence limits
Direct hydraulic measurements are uncommon in cannabis cultivation. Stem diameter changes, pot mass, and wilt are useful proxies but cannot by themselves identify cavitation.
Related encyclopedia topics
- THC-ENC-010, THC-ENC-043–045, THC-ENC-068, THC-ENC-081–097, and vascular-disease lessons.
Source notes
- Taiz L, Moller IM, Murphy A, and Zeiger E. Plant Physiology and Development. 7th ed. Oxford University Press, 2022. Publisher record
- THC – Teaching Healthy Cultivation. Cannabis Plant Science Source Packet v1. May 2026. Project source packet.
- Pena MM et al. Genotypic Variation in Photosynthesis and Biomass Partitioning Underlies Agronomic Performance and Cannabinoid Profile in Cannabis sativa Under Drought. Plants. 2025. Open source
This lesson summarizes the source material and its evidence limits for education. Use direct measurement, controlled comparison, and the cited sources when conditions differ or a decision carries meaningful risk.