Fig. 6.3-6. Longitudinal section of shoot tip of cocklebur (Xanthium). This is a rather large shoot apical meristem, and it has only weak zonation. The CMC zone is not easy to see, and even the pith-rib meristem does not have any obvious files of cells. Notice that the tunica is somewhat irregular near the very apex (arrows). In many flowering plants, if enough samples of shoot apical meristems are examined, occasional periclinal divisions can be found, but as long as they are rare, we continue to call the layer a tunica, not a mantle.