Transfer cells occur at the interface between tissues; they are specialized cells that facilitate transport (absorption or secretion) of soluble substances across tissue boundaries. For example, they can occur at the junction of the megagametophyte and mega-sporophyte, in companion cells in phloem tissue (especially at the node of a stem), in root nodules, in the haustoria of parasitic plants, and in the epidermis of water plants. Several cells of the embryo sac and seed, including synergids, antipodals and specialized endosperm cells, have been identified as transfer cells in different species. Transfer cells are typically characterized by numerous cell-wall ingrowths protruding into their protoplasts or those of adjacent cells; these ingrowths are sometimes visible using light microscopy. Secretory cells, such as those of glandular hairs and nectaries, also frequently possess wall ingrowths. The plasma membrane of the transfer cell follows the contour of the wall ingrowths, thus increasing the surface area.
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