The Wrap-Tension Ladder — and What Is Invariant at Its End

The dry seed ends the plant's ladder of boundary pressures two decades past the xylem — and where drying stops buying life, the tension holds fixed while the water content scatters with chemistry.

A  The plant's boundary pressures, one log axis tension magnitude |ψ| (MPa, log) 0.01 0.1 1 10 100 1000 wet soil  −0.03 plant-cell turgor  +0.5 to +1 phloem  +1 to +2 xylem  −1 to −3 cohesion ceiling — the tree stops climbing two decades dry orthodox seed −256 (15% RH store)  ·  −304 (glass limit) B  The same seven seeds, at the point drying stops helping deviation from the mean — Ellis, Hong, Roberts & Tao (1990), n = 7 species +60% +30% mean −30% −60% critical moisture content (% w/w) groundnut 2.0% onion 3.5% cowpea 5.6% ±41% critical water potential ψ (MPa) −312  onion, sugar beet −292  wheat mean −304 MPa ±3% the tension is ~12× tighter than the water ψ = (RT/Vₜ) ln h,  h = 9.9–11.5% RH at 20°C
push (positive pressure) pull (negative potential) chemistry's variable (water content)