Answer
Phloem sap is moved by a push from the positive pressure built up in living sieve tubes, while xylem sap is usually moved by a pull from the negative pressure which results from transpiration. However, xylem sap sometimes moves due to a push from positive root pressure.
Work Step by Step
Review the mechanisms for driving sap movement (whole chapter 35) and make a list of basic features for the two driving forces for xylem sap movement and the one for phloem sap movement. Use these lists as the basis for your answer.
