Early ontogeny of vascular cambium III

Observations were made on structural changes from the procambium to cambium in the developing shoots ofRobinia pseudo-acacia andSyringa oblata, both of which are characterized by relatively short fusiform initials. In both species, the procambium in transverse view shows radial seriations of cells as a result of repeated tangential divisions, and there is an almost continuous procambial cylinder in the young stem in the earlier stage. The procambium in tangential view has initially a homogeneous structure and later develops into two systems, one made of long cells, the other of short cells. Some of the short cells elongate to intrude among neighbouring cells and some long cells divide radially as well as tangentially. InRobinia, long cells have transverse or tapering end walls at a relatively earlier stage and mainly tapering end walls in the subsequent stages. Although some of the short cells develop into long cells, the height of axial files of the short cells decreases only a little, because of subsequent transverse divisions and growth of cells. InSyringa, long cells have mainly transverse end walls at a relatively earlier stage and usually tapering end walls in the subsequent stages. Short cells in axial files have predominantly transverse end walls. A number of additional long cells are derived from elongating short cells in the later stages. Therefore, the height of axial files becomes apparently lower than that of earlier stages. Radial divisions in short cells occur to some extent. Results are discussed in relation to the structure of the vascular meristem inGinkgo, Aucuba, Weigela, and others.