RESUMEN
Analysis of cell length, cross-sectional area and nuclear length of pericycle cells in adventitious roots of Allium cepa L. has revealed the existence of three cell types whose topographical location is related to the vascular pattern. Cells opposite the xylem are short and narrow with an elongated nucleus. The intervening cells are also short but their cross-sectional area is much greater than that of cells at the xylem pofes, while their nucleus is relatively short. Cells opposite the phloem are long, with an elongated nucleus, and their cross-sectional area is intermediate between that of the other two cell types. Lateral root primordia are always initiated from the population of short cells opposite the xylem. However, the different cell types appear in the pericycle before primordial initiation and not as a result of this event.