ICSE Class 8 Answered

The water deficit or suction force also called transpirational pull, developed in leaf cells due to transpiration is responsible for pulling the water column in the upward direction (ascent of sap). The magnitude of suction force is sufficient to lift the water column to any heights, even in tallest trees. However, it is not enough to break the continuity of water column. Thus, greater the transpiration, greater is the transpiration pull and hence higher the rate of ascent of sap. Similarly, when transpiration is low, the transpiration pull is also low and hence the rate of ascent of sap is also lower.