Distinct localization of growth- and size-regulatory mechanisms. (a) Cell-autonomous signals act (positively or negatively) on the proliferating cell pool in the meristems or young tissues. (b) Non-cell-autonomous signals, exemplified by KLUH and its expression domain in differentiated margin cells, act at a distance to determine the proliferation potential of meristematic cells or to restrict the transition between proliferation and differentiation. Organ expansion beyond a critical area would result in the KLUH signal reaching a critical low value, insufficient to maintain proliferation and thus allowing differentiation to take place. Signals from mature organs or from environmental inputs also act non-cell-autonomously.