EPIDERMAL DERIVATIVES AS XYLEM ELEMENTS AND TRANSFER CELLS - A STUDY OF THE HOST-PARASITE INTERFACE IN 2 SPECIES OF TRIPHYSARIA (SCROPHULARIACEAE)

Haustoria of Triphysaria pusilla and T versicolor subsp. faucibarbata from a natural habitat were analysed by light and electron microscopy. The keel-shaped edge of the secondary haustorium generally splits the epidermis and cortex of the host root parallel to the root axis, and penetrates to the host vascular tissue. Anticlinally elongated epiderm al cells of the haustorium constitute most of the host/parasite interf ace. Some of these epidermal cells are divided by oblique cell walls. Some of their oblique daughter cells as well as some undivided epiderm al cells differentiate into xylem elements. Single epidermal cells occ asionally intrude into the vascular tissue of the host and individual host cells can be invaded. The surface area of the plasmalemma in para sitic parenchymatous interface cells is increased by the differentiati on of wall labyrinths characteristic of transfer cells and by the deve lopment of membrane-lined cytoplasmic tubules or flattened sacs which become embedded in the partly lignified interface cell-wall. Mycorrhiz al fungal hyphae enter the xylem bridge in some haustoria. Implication s of these observations for the function of the haustorium are discuss ed.