The first step in the long-distance transport of nutrients in plants. Phloem loading also provides the driving force for this transport. Why is long-distance transport needed? Although vascular plants produce their own organic molecules by photosynthesis, not all organs are self-sustaining. Any tissue that uses or stores more carbohydrate than it produces is a sink, including roots, fruits, tubers, and immature leaves. The sources of carbohydrate are generally mature leaves. This nutrient transport, from source to sink, takes place in the phloem, a highly specialized tissue that links all the organs of the plant and is found in close association with the water-conducting tissue, the xylem.