A type of plant cell specialized for the short-distance high-volume transport of materials. Transfer cells have numerous knobs and ridges on the inner surface of the primary cell wall. These greatly increase the surface area of the plasma membrane, which follows the contours of the protuberances. The expanded plasma membrane can accommodate a large number of transport proteins, which are responsible for transporting materials in or out of the cell. Transfer cells are found mainly in salt-secreting glands and in regions where photosynthate (sugar) is loaded or unloaded into sieve elements of phloem.