The embryonic protoderm gives rise, via anticlinal cell divisions, to the epidermal cells of embryonic organs such as the hypocotyl and cotyledons (in dicots) and the scutellum and coleoptile in monocots. In addition to strictly embryonic structures, the seedling shoot apical meristem (SAM), which will give rise to all shoot structures, is also specified during embryogenesis. In Arabidopsis, the SAM is composed of three layers of cells denoted L1 (epidermal), L2 (subepidermal) and L3. The L1 and L2 are a single cell thick and undergo principally anticlinal cell divisions. Together they form the tunica. The underlying L3 cells do not show particular restrictions in their division planes and form the corpus. In Arabidopsis, embryonic protoderm cells give rise to the L1 or outer cell layer of the shoot apical meristem, with underlying cell layers in the meristem originating from hypodermal embryonic cell layers (Barton 1993). A similar developmental ontogeny is probably applicable in the maize embryonic SAM (Poethig 1986), although there is often no easily distinguishable subepidermal tunica cell layer in maize vegetative tissues, and in maize "L2" is often used to denote the meristematic corpus.
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