With the aid of a non-destructive replica method and computational protocol, surface geometry and expansion at the reproductive shoot apex are analysed for pin-formed 1 (pin1) Arabidopsis thaliana and compared with the wild type. The observed complexity of geometry and expansion at the pin1 apex indicates that both components of shoot apex growth, i.e. the meristem self-perpetuation and initiation of lateral organs, are realized by the pin1 apex. The realization of the latter component, however, is only occasionally completed. The pin1 apex is generally dome-shaped, but its curvature is not uniform, especially later during apex ontogeny, when bulges and saddle-shaped regions appear on its periphery. The only saddle-shaped regions at the wild-type shoot apex are creases separating flower primordia from the meristem. Surface expansion at the pin1 apex is faster than at the wild type. In both pin1 and wild type the apex surface is differentiated into regions of various areal strain rates. In the pin1 apex, but not in the wild type, these regions correspond to the geometrically distinguished central and peripheral zones. Expansion of the central zone of the pin1 apex is nearly isotropic and slower than in the peripheral zone. The peripheral zone is differentiated into ring-shaped portions of different expansion anisotropy. The distal portion of this zone expands anisotropically, similar to regions of the wild-type apex periphery, which contact older flower primordia. The proximal portion expands nearly isotropically, like sites of flower initiation in the wild type. The peripheral zone in pin1 is surrounded by a basal zone', a sui generis zone, where areal strain rates are low and expansion is anisotropic. The possible relationships between the observed regions of different expansion and the various gene expression patterns in the pin1 apex known from the literature are discussed. 10.1093/jxb/erh109
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