The transition from sink to source status is one of the key events in leaf development. When a leaf is about half grown, it stops importing phloem-mobile nutrients from the rest of the plant and begins to export its own products of photosynthesis. This shift in transport direction, which is largely irreversible, involves major changes in the way metabolites are transported to and from the mesophyll through plasmodesmata and via transporters. The import of nutrients ceases when plasmodesmata in large veins are lost or narrowed, preventing phloem unloading. Export begins when the minor veins mature and begin to load sugars and other compounds into the phloem. The unidirectional nature of loading is a consequence of sucrose transporter orientation in the plasma membrane of phloem cells, or of the trapping of raffinose-family sugars in those species that load through plasmodesmata.