SummaryNuclei of onion (Allium cepa L.) bulb scale inner epidermal cells were found to increase in area, dry mass per unit area, and total dry mass with increasing age and distance of the scales from the centre of the bulb. The above nuclear characteristics reached maxima which were, respectively 4.1, 1.4, and 5.0 × those of the youngest scale studied (the oldest unemerged leaf). A sharp increase in these three characteristics in scale 2 is interpreted as reversible change due to senescence. The outermost turgid scale, i.e. the oldest, exhibited the increased nuclear dry mass concentration and decreased nuclear area and nuclear dry mass which characterize nuclear pyenosis. The outermost scale is therefore interpreted as exhibiting irreversible change due to senescence.