Models of Cell Tetrads and Clones in Meristematic Cell Layers

Abstract

Regular repetitions of new wall orientation with respect to the orientation of older walls were analyzed. Wall patterns in four-cell groups (tetrads) in meristematic cell layers appear to be most suitable for such an analysis. The distribution of the five main tetrad types was studied in epidermal layers of Aponogeton crispus leaves, Selaginella kraussiana rhizophores, Tropaeolum peregrinum leaf primordia, and Angiopteris lygodiifolia shoot apices. Relationships were found between the growth directions of the organs and the lineages giving rise to different tetrad types, which point to mechanisms that regulate cell division orientation, possibly by positioning of microtubule-organizing sites. Our analysis of tetrads and larger cell groups (clones) is based on mathematical formalization in terms of double-wall map OL-systems. We use these systems to express and test biological hypotheses concerning similarities in cell division orientations and their effects on tissue growth. In histological studies, only the most recent division wall orientations have been analyzed until now, and there have been few attempts to identify lineages in cell groups with several wall generations. Even if consecutive division structures can be observed, it has proved to be too difficult to recognize unambiguous organizing principles for them. The recognition of such principles has become manageable since we can now perform iterative computations on cell wall networks.