Early ontogeny of vascular cambium

Abstract

Observations were made on structural changes of procambium and the subsequent appearance of cambium in the developing shoot. The procambium in early stages shows radial seriation of cells as a result of repeated tangential divisions. In tangential view the procambium has initially a rather homogeneous structure and later becomes organized into two distinct systems, one composed of long cells and the other of short cells. The latter cells are arranged tangentially in axial files and transversely in radial files. They show repeated transverse divisions. Some of the short cells elongate to intrude between neighboring cells. Therefore, long cells may be derived both from cells of homogeneous structure in the first stage and from elongation of some of the short cells in axial files. Long cells have mostly tapering end walls and elongate actively. In the subsequent stages, the frequency of transverse divisions of short cells in axial files decreases and these cells expand radially. Short cells in axial files are separated from one another vertically by the elongation of neighboring long cells which break up the axial files as seen in tangential view. The vascular meristem in this stage is believed to initiate the cambium. Eventually, short cells remain mostly single, or form files of two or three cells in height in tangential view. The observations are discussed in relation to the structure of the vascular meristem in other plants.