On the developmental analogy of the shoot apex and the leaf margin and the genesis of L1-derived mesophyll areas in the leaf margin

The present theory describes the developmental analogy of the shoot apical meristem and the meristematic leaf margin. The paper follows the question why L1-derived mesophyll can be found in the leaf margin of some plant species, while it is missing in others. It is explained that the absence of L1-derived mesophyll is based upon suppressed periclinal divisions of epidermal cells. That means the epidermis forms a histologically independent mantle-like tissue layer around the mesophyll. The conditions and the crucial mark from which such a tissue mantle is formed are described. This mantle structuring point of 2.44 cell units is naturally constant and is analogous to the tunica structuring point. It is shown that besides the epidermis, additional mantle-like layers may develop in leaf margins of particularly large diameter. A mathematical formula is given to determine the existing number of these mantle layers in an idealized leaf margin. In recent years, a number of genes encoding a large variety of traits could be identified. However, the assumption that the development of mantle-like layers and hence the existence/absence of L1-derived mesophyll in the leaf is also genetically fixed can be refuted with this theory. (C) 2003 Elsevier Science Ireland Ltd. All rights reserved.