The movement of nutrients into the vascular stele requires energy, unlike the movement of water that does not require energy. This energy is provided by the phloem. Sugars are produced in the leafs, as part of the photosynthesis process, so whenever a new root is growing, or a fruit needs to grow, the phloem gets in action. With the help of the water the sugars get in the phloem, which can most simply be seen as an elevator inside the plant, that is moving nutrients up and down, on the places in which they are most needed.
