Background and Aims The shoot apical meristem (SAM) plays a key role in developmental biology by generating all aerial organs. Most research has focused on model plants, such as Arabidopsis thaliana, but studying plants in arid environments, such as cacti, can reveal evolutionary insights. Cacti have large SAMs that are correlated with stem size. This study examines the development of the SAM in Mammillaria san-angelensis and explores the relationship between SAM size and plant allometry during development.Methods We analysed plant morphology, specifically hypocotyl and stem dimensions, along with five SAM anatomical traits across eight developmental stages, from 1 day post-germination to a 4-year-old plant, using conventional microscopy and anatomical techniques. We compared the vegetative SAM, floral meristem and areole meristem and performed correlation analysis to link SAM with plant traits.Key Results We identified key developmental processes during the early stages of plantlet growth, including morphogenesis of the SAM and the initiation of dimorphic areole formation. We noted the increase in size and the change from two to three layers in the SAM, along with alterations in its dome formation. We found that the size of the SAM can explain the allometric growth in stem width and length.Conclusions The correlation of larger SAM with larger stem seen in adult cacti across the Cactaceae family was recapitulated during the development of a single species (M. san-angelensis). Altogether, we set the developmental framework for future genetic research to characterize the evolutionary bases of SAM size increase, the meristem variants of M. san-angelensis and the potential of their understanding to explain the origins of gigantism, succulence and the diversity of shapes and sizes in the cacti family.