The question of whether the growth and morphogenesis of plant organs is controlled at the whole organ level (the organismal theory), the level of the combined action of single cells (cell theory), or by a less simplistic mechanism governed by cell-cell communication and co-ordination (the neo-cell theory) has been of interest for decades (Tsukaya 2002). Integrally linked to this question, is whether the mechanisms regulating organ growth and morphogenesis are active in any particular cell-layer of the developing organ. In 1933 George Avery carried out a detailed study of leaf development in tobacco (Avery, 1933). He suggested that in young leaf primordia the mesophyll provides the "impetus for development", placing the epidermis under tension at "the marginal meristem" [similar to the situation proposed by Wegner (below)]. However, he also remarked that although the epidermis ceases to divide before the mesophyll, its cells expand parallel to the lamina surface for much longer than underlying cells and are responsible for "pulling" mesophyll cells apart and thus play a potentially important role in determining final leaf shape. His study serves to elegantly illustrate the intuitively attractive idea that as a continuous planar monolayer of relatively uniform cells which encases every organ, the epidermis appears the simplest, most logical tissue in which to express the information regulating organ growth, at least in organs with an extended lamina. In the last 70 years however, many studies have sought to clarify the roles of different tissue layers in organ development by analysis of periclinal chimeras or clonal sectors in which different cell-layers have different developmental characteristics, studies of the effects of genetically or physically ablating the epidermis, and the study of developmental mutants. Despite this considerable body of work, it still remains unclear whether any one cell-layer can be thought of as the prime expression site for information regulating general organ growth and form. However, in organs with an extended lamina (such as leaves, petals and sepals), a certain body of evidence has been accrued suggesting that the epidermis can play a very important role in regulating organ shape.
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