SHOOT INITIATION ON A GRAPTOPETALUM LEAF - SEQUENTIAL SCANNING ELECTRON-MICROSCOPIC ANALYSIS FOR EPIDERMAL DIVISION PATTERNS AND QUANTITATION OF SURFACE GROWTH (KINEMATICS)

A non destructive mold and cast procedure allows repeated imaging of growing plant surfaces by scanning electron microscopy. Individual cells and their clonal progeny can be recognized in succesive images. This allows comprehensive analysis of cell lineage on the epidermis. It also provides data for the quantitative charcterization of surface growth (kinematics). The technique and both types of analysis were applied to a small meristematic surface of a Graptopetalum leaf as it develops from a flat field of parallel cell files into a stem and leaves, all with radial symmetry. New organ surfaces formed from groups of cells that were not clones of the cells initially present. Hence cell image was not important. In the revision of symmetry some regions retain the original cell file polarity, others change it by 90-degrees. Areas that do not change have mainly tranverse anticlinal divisions. Areas that change maximally show repeated longitudinal divisions to make a rib meristem pattern with new cross walls normal to the direction of future growth. Growth direction soon changes. This occurs in regions known to undergo a 90-degrees shift in cellulose reinforcement direction. It is concluded that distinctive anticlinal division behavior accompanies lateral formation and may be significant in the mechanism.