STEM FORMATION FROM A SUCCULENT LEAF: ITS BEARING ON THEORIES OF AXIATION

The radial symmetry of shoots and roots arises from a center of symmetry within the apical meristem. When a lateral axis forms at a distance from the tip, a new center of radial symmetry must arise. We have studied the biophysics of this kind of transformation in the epidermal layer of the succulent Graptopetalum where a stem “regenerates” from organized leaf tissue. Study of the epidermal cell pattern (with scanning electron microscopy) shows that reorganization involves neither a cellular pre-pattern blocked out by oriented cell divisions nor a callus-like stage where cell files, expansion direction, and primary cell wall cellulose orientation are randomized throughout. Rather, developmental events are a function of initial position. In regions of geometrical compatibility between parent axis and prospective lateral, there is little or no modification of files, expansion, or cellulose. In regions requiring 90° changes in orientation, cellulose orientation (studied with polarized light) conforms to the new symmetry first. This is followed later by changes in the surface growth pattern and in the cell division pattern. The early establishment of a circumferential cellulose pattern in the epidermal layer could account for both the cylindrical shape of the new axis and the subsequent rearrangement of directional growth and cell file pattern.