Ontogeny of xylem transfer cells inHieracium floribundum

Abstract

The ontogeny of transfer cells associated with tracheary elements in the rhizome of the rosette plant,Hieracium floribundum is described. The events preceding the initiation of wall ingrowths in parenchymatous cells associated with tracheary elements determining them as transfer cells vary with the type of tracheary element and the position within the rhizome. Parenchyma cells adjacent to protoxylem and metaxylem elements close to the rhizome apex usually divide before wall ingrowth formation whereas this tendency is decreased in the basipetal direction in the rhizome. When parenchyma cells do not divide before wall ingrowth formation, the wall ingrowths are poorly developed and the cells remain vacuolated. If cell division does occur, the daughter cell next to the tracheary element always becomes a transfer cell and the other develops as a xylem parenchyma cell. Within a vascular bundle there is a gradient in intensity of wall ingrowth formation, with the transfer cells associated with protoxylem elements having the most elaborate wall ingrowths, and the transfer cells associated with the late metaxylem elements having few or no wall ingrowths. Parenchyma cells associated with secondary xylem elements lack wall ingrowths. At the end of primary growth and the establishment of secondary growth within a bundle, successive cytological changes occur in the xylem transfer cells, leading ultimately to the loss of wall ingrowths, the thickening of cell walls and the complete vacuolation of the once-richly cytoplasmic cells. Although the thickened walls of other primary xylem parenchyma cells become lignified at this time, the transfer cell walls remain unlignified until the rhizome is very old.