Answer: Root pressure cannot fully explain the ascent of sap because:
- The magnitude of root pressure is small (about 1–2 atm, rarely more), which can raise water only up to a few metres, not to the top of tall trees (over 100 m).
- Root pressure is not found in all plants; it is absent or negligible in gymnosperms such as conifers, which are among the tallest trees.
- Ascent of sap continues even when the roots are removed (in cut stems placed in water).
- Root pressure is generally observed only at night or early morning (when transpiration is low) and in certain seasons, while the rate of ascent of sap is highest during the day when transpiration is high and root pressure is negligible.
- Xylem sap is usually under tension (negative pressure), not under positive pressure.
