Growth behavior of single epidermal cells during flower formation: Sequential scanning electron micrographs provide...

A non-destructive method for scanning electron microscopy allows individual developing flower surfaces to be imaged sequentially. Kinematic analysis, the quantitative characterization of expansion behavior, can be applied to consecutive images of the same primordium. The individual cell, delimited as a polygon by its anticlinal walls, is the unit of analysis. Growth in two dimensions is characterized by a right-angle cross giving the maximal and minimal rates of extension relative to a known side of the cell. Methods have been developed here to make the analysis rapid and the results easy to portray. Data were obtained for the flower primordium of Anagallis arvensis L. from its origin as a smooth dome through the development of five small stamens and an incipient gynoecium. The outermost sepal whorl arises synchronously as a fivefold undulation. This maneuver is closely coupled to the formation of five stamen buttresses alternating with the small sepals (petals form later). The most characteristic kinematics occur as the stamen buttresses expand rapidly at their tips. The sides of the buttresses, and the regions between them, show highly directed extension following the five radii of the flower. These unique expansions are associated with the origin of the filament as a stalked structure and also correlate with the future bilateral symmetry of the anther. The region interior to the stamens grows slowly as circumferentially oriented anticlinal divisions initiate a radial cell-file pattern for the gynoecium. The developmental sequence has many features of "feed-forward" where the previous structure is important for the generation of structure to come (e.g. stamens alternate precisely with sepals). Some of these features fit plausible biomechanical explanations for morphogenesis.We wish to thank Mr. N. Rasmussen, Department of Biology, Stanford University, for providing Fig. 1A, B. Use of the SEM facility of Professor G. Goffinet, Institute of Zoology, University of Liège, is greatly appreciated. We thank Dr. R. Jacques, C.N.R.S., Le Phytotron, Gif-sur-Yvette, France, for providing the experimental material, and Mr. Philippe Ongena and Dr. Suresh Tiwari for expert photography. Support was from grants from the U.S. Department of Agriculture and National Science Foundation and as well as from the Fonds National de la Recherche Scientifique, Fonds de la Recherche Fondamentale et Collective and the "Action de Recherche Concertée" of Belgium.