Leaf arrangement on stem called:
Phyllotaxy describes geometric arrangement of leaves, bracts, branches, or floral organs around shoot axis such as spiral arrangement with golden angle approximately 137.5 degrees, opposite decussate pairs, alternate distichous, or whorled patterns producing remarkable regularity. It emerges from positioning of new primordia in peripheral zone of shoot apical meristem where auxin maxima formed by polar PIN1 localization mark incipient organ sites. Auxin activates MONOPTEROS and represses CUC boundary genes, while mechanical buckling of epidermis and cytokinin fields refine periodic spacing, optimizing light interception, packing efficiency, and reproductive architecture for organism fitness.
Ref: Reinhardt et al., Nature 2003; Gilbert Plant Dev: phyllotaxy is leaf arrangement pattern generated by auxin-PIN1 maxima.