Fig. 6.3-7c. Magnification of a cactus apical meristem (Oroya depressa). This shows the peripheral zone, a leaf primordium and a developing axillary bud. In this large apical meristem, the peripheral zone is, relatively speaking, huge. It is many layers thick and extends for dozens of cells between the CMC zone and the leaf primordium. Because this apical meristem is so large, many leaf primordia can fit on its margin, so the leaf primordium visible here (the same one shown in Fig. 6.3-7a) may actually be quite old, even though it is the uppermost primordium visible in the median longitudinal section -- the apex may have many younger primordia that were microtomed away on the back or front sides. The axillary meristem here is still organizing itself, and has not yet started to produce its own leaf primordia, but when it does, they will develop as spines rather than as ordinary leaves or bud scales.

            In such large apical meristems, the production of small leaf primordia scarcely change the size and shape of the meristem; the typical cycling between a maximum and minimum phase is greatly reduced; see page 91 in Plant Anatomy (Mauseth).