Sugar flows from source to sink in plants through a vital network of specialized tubes. There are sugar sources which also act as sinks, and sinks which act as sources.
A sugar source is anything that produces more sugar than it uses. Generally, the biggest sugar source in plants are the leaves, which convert water, CO2, and sunlight into sugar through photosynthesis. There are, however, other sugar sources in plants. The roots and storage organs of a plant may be used to to store starch, which can be broken down into sugar later.
Sugar sinks are anything that use more sugar than they produce. The main sugar sinks of a plant are growing tissue such as stems, roots, buds, and, counter-intuitively, young leaves. Storage organs like bulbs and tubers can also be sinks, when they absorb sugars for later use.
Sugar moves from source to sink through a network of vascular tissue. There are two different kinds of vascular tissue, called the xylem and the phloem. The xylem transports a thin solution of water and minerals up from the roots, called the xylem sap. The phloem transports a thick syrup-like solution of various large molecules, amino acids, and sugars, called the phloem sap. The phloem sap is pushed from source to sink, because there is a higher concentration at its source. Plants are able to cut off sugar supply to fruits, flowers, and other non-vital organs in times of scarcity to supply the vital organs.
It has been suggested that the phloem serves as a sort of primitive nervous system in plants. In addition to transporting sugars, the phloem also transports molecules used in cell signaling. The phloem is similar to the circulatory and nervous system in animals.
Some sugar sources, such as leaves, can also be sinks when they are young and still growing. Likewise, sugar sinks, like roots and storage organs, can also be sources when sugar is scarce. Sugar moves from source to sink through a vital system of vascular tissue called the phloem, which acts like a primitive nervous system as well. The phloem is a vital part of vascular plants which serves multiple purposes.