Fig. 6.3-7a. Longitudinal section through a cactus shoot apex (Oroya depressa). Oroya is one of the cacti that grows as a ball, so it has only a single shoot apical meristem. Also, because the shoot becomes very wide but not especially tall, the older leaf primordia are not below the younger ones but instead are to the side of them (four leaf primordia are visible [LP], and the two outermost ones have spine primordia on their axillary buds [arrows]). This species has a large apical meristem, and even at this low magnification, the files of the cells in the pith-rib meristem are striking, and the CMC zone is obvious due to its paleness (its cells are so vacuolate they do not stain strongly).