Flange-type parenchyma cells: occurrence and structure in the haustorium of the dwarf mistletoeKorthalsella (Viscaceae)

Summary Flange cells are an unusual type of parenchyma cells with an open reticulate pattern of secondary wall thickenings. The cells superficially resemble tracheary elements but are otherwise fundamentally different. Flange cells were found in haustorial sucker tissue of the dwarf mistletoeKorthalsella. Such cells were previously unknown for a mistletoe, or other parasitic angiosperm. Flange cells are confined to the xylem of the sucker and occur as either diffuse aggregates amongst the ordinary parenchyma tissue lying between the tracts of vessels, or abut the vessels. Typical flange cells are absent at the parasite/host xylem interface. The cells contain a well differentiated protoplast, including chloroplasts with extensive granal stacks. Histochemical staining and fluorescence microscopy indicate lignification of the flange wall. In thin section, the flange wall is often stratified into dark and light staining layers. Flange cells inKorthalsella resemble contact cells, vessel associated cells and certain types of transfer cells reported in the literature. Based on morphological considerations, it is suggested that flange cells inKorthalsella are involved in absorption and transport between host and parasite. As host sap moves through the sucker apoplasm, substance might be selectively absorbed by the flange cell, before the remaining the sap passes into the vessels for long distance transport in the mistletoe.Dedicated to Prof. Dr. Rainer Kollmann on the occasion of his 65th birthday