Constant distance between leaf initiation sites permits non-destructive analysis of apical meristem activity during cactus shoot growth

Shoot apical meristem diameter in both species can be estimated non-destructively by simply counting the number of orthostichies/ribs the SAM is producing (or produced in the past). The growth rate of C. gigantea SAMs varies from plant to plant and can change with age. All C. gigantea SAMs stop increasing in diameter at some point, but that diameter varies from plant to plant.