In contrast to roots and monocot leaves, leaves of dicotyledonous plants possess growth zones that extend with comparable intensity in two dimensions and that do not show a clear distinction between dividing and expanding tissue. For Arabidopsis leaves, it was recently shown that the transition from the "proliferative" growth phase, in which the organ grows by cell division only, to the "expansive" growth phase, in which the individual cell area increases is accompanied by the onset of endoreduplication by which the ploidy level is increased (Beemster et al. 2005). It has been a central axiom of di-cot leaf growth research for decades that cells of the leaf tip are initiated earlier than cells located at the leaf base (Foster 1936). This developmental lag is then retained until the latest stages of leaf development, leading to pronounced base-tip gradients of growth distribution that reflect the differential developmental status of the two regions (VanVolkenburgh 1987). In Nicotiana tabacum for example, a developmental time lag of 4 days was identified (Walter et al. 2003b). Nicotiana tabacum has been the model system for dicotyledonous leaf growth for a long time (Avery 1933; Hannam 1968). Similar base-tip gradients were identified for Ricinus communis (Walter et al. 2002a) and for the molecular model system Arabidopsis thaliana (Donnelly et al. 1999; Wiese et al. 2007). Yet, compelling evidence has been reported recently that base-tip gradients of dicot leaf growth and connected cellular developmental lags between base and tip are not a uniform pattern within dicotyledonous plants. Other plants such as Glycine max (Ainsworth et al. 2005) and Populus deltoides (Walter et al. 2005; Matsubara et al. 2006) do not show pronounced base-tip gradients but an almost homogeneous distribution of growth rates within the leaf lamina, connected to uniform cell sizes during the entire post-emergent phase of leaf development. This leads to the conclusion that either there is no developmental lag in the initiation of leaves from the different tunica layers of the shoot apical meristem or that development of those tissue regions is synchronized in a later stage of leaf development, possibly under the influence of external signals such as light.
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