Sporophyll crosiers in Ceratopteris richardii have naked pinna meristems that can be observed non-destructively over time by sequential scanning electron microscopy (SSEM). In addition, clonal analyses of the meristematic region of the pinna surface can be straightforward because developmental relationships among cells in the primordium are often easy to infer. Anatomical study of paradermal sections of the abaxial side of sporophyll pinna tips was combined with SSEM of developing organs to characterize cellular and morphological aspects of organ growth. There were common themes in the spatial arrangement of clonal cell patches at the abaxial pinna surface, although exact patterns varied within and between pinnae. The initiation of sporangia from single superficial cells provided discrete phenotypic markers to analyze patterns of cell fate determination in the epidermis. Sporangial initial cell lineages could be traced to two sub-apical positions at the flanks of each pinna apex. During ontogeny, the principal direction of growth at the pinna tip occasionally changed its location. These changes in growth direction occurred independent of previously existing cell tile patterns. Consequently, cells near the pinna apex (including putative sporangial initials) shifted their fates to correspond with the new local growth zone. This result suggests that the fates of epidermal cells including sporangial initials are determined in a non-cell-autonomous fashion in C. richardii. A model for pinna development that is consistent with these observations is proposed, stressing the existence of morphological parameters controlling fern leaf ontogeny above the cellular level.

Key words: cell fate, Ceratopteris, development, fern, leaf