Know the importance of the physical structure of (and constraints placed on) tracheids and xylem vessels. How do these structures assist in temporarily overcoming the situation when the rate of evaporation in the leaves exceeds the water uptake by the roots? Solution Evaporative demand is mainly depends on temperature and humidity The relation between the plant and water pressure range is from positive to negative that occur within different tissues and cells. Positive pressures (turgor) occur in all living cells and the negative pressures (tensions) occur in dead cells. xylem cells are walls are heavily lignified it can resist deformation. In water transport xylem play an important role. In Rapid evaporation plant needs high amount of water. so it can uptake large amount of water from soil, heavily lignified xylem cells play a crucial role in evaporation. Transpirational pull: The evaporation of water can creates a negative pressure at the top of a plant. So the surface tension can causes negative tension (pressure) in the xylem that pulls the water from the roots and soil. Transpiration pull requires that the vessels transporting the water are very small in diameter, otherwise cavitations would break the water column..