The fact that cells in the embryonic protoderm and meristematic L1 layer undergo predominantly anticlinal divisions can give the impression that L1 cell-layer identity is maintained in a clonal fashion throughout plant growth. Indeed it is true that in many higher plants shoot epidermal cells are all clon-ally related to a few protodermal precursors in the developing embryo. The study of periclinal mosaics in which the cell-layers of the shoot meristem differ for visible or histological markers, and clonal analysis using similar markers has been widely used to follow cell fates during plant development (reviewed in Neilson-Jones 1969; Tilney-Basset 1986; Poethig 1987; Marcotrigiano 2001). Such studies have confirmed that several structures that appear to be composed only of epidermal cells, for example the ligules of maize leaves and the sepal margins in Arabidopsis (Jenik and Irish 2000) are indeed entirely derived from the meristematic L1-layer. Such studies have also, however, shown that in some dicotyledonous species (such as privet) the mesophyll, which in the leaves of most dicots is predominantly L2-derived, can be L1-derived at leaf margins due to periclinal cell divisions in the L1 during organ development (Stewart 1975). Indeed in monocotyledonous species such analyses have shown that leaf marginal tissue is almost always L1 derived, and that periclinal divisions of the L1 cell layer are a routine feature of organ inception. Moreover, in maize, periclinal divisions of the meristematic L1 layer have been documented during development (Sharman 1940), and in rare cases epidermal cells derived from L2 lineages have even been noted (Poethig 1986). In every case where incursions between lineages have been noted, cells develop according to their positions rather than to their lineage, leading to the realisation that plant cells talk constantly to one another, and that their developmental fate, at least in the meristem and during early organ development is dependent on their physical position (Stewart 1975). Another discovery made during the study of periclinal mosaics, which is of particular importance to this discussion, is the fact that making mosaics in which different cell layers have different developmental vigour can markedly change the contribution of different meristematic cell-layers to organs without changing the overall shape of the organ (Stewart 1974). This serves to underline the degree of plasticity, and compensatory mechanisms manifest during plant organ development.
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