A MORPHOMETRIC STUDY OF THE ULTRASTRUCTURE OF ECHINOCEREUS ENGELMANNII (CACTACEAE). VI. THE INDIVIDUALIZED ULTRASTRUCTURES OF DIVERSE TYPES OF MERISTEMS

The structures of spine basal meristem cells were examined by stereological morphometric techniques and found to be different not only from shoot apical meristem cells but also from spine primordium cells in the relative volumes occupied by cellular organelles. Nineteen types of meristematic cells have been studied in E. engelmannii, and all are distinct ultrastructurally. This suggests that whereas they all appear to be primarily involved in mitosis and cytokinesis, there must be other important aspects to their physiology; possibly these cells have already begun differentiation toward their mature states even while they are parts of active meristems.