Meristem Development at the Sporophyll Pinna Apex inCeratopteris richardii

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Abstract
Sporophylls of the homosporous fern Ceratopteris richardii have naked pinna meristems that allow surface growth at the pinna apex to be observed nondestructively over time. Sequential scanning electron microscopy was combined with conventional anatomical approaches to determine the patterns of development in the abaxial epidermis. There were consistent themes in the spatial arrangement of clonal cell patches in the epidermal layer, but the exact anatomical pattern was variable within and between pinnae. Two sporangial cell lineages that run down the abaxial flanks of each pinna always appeared to originate from subapical positions just behind the pinna tip. During pinna growth, the principal direction of tip elongation varied occasionally, independent of the existing cell‐tiling patterns near the pinna apex. These local changes in growth direction resulted in protodermal cells just behind the apex changing their fates to correspond with the new growth vector, independent of their recent cell lineage histories. In particular, the position of sporangial initials shifted to follow the prevailing direction of growth. A model for fern pinna development is proposed to fit these observations, one that emphasizes the existence of developmental parameters that control organogenesis above the cellular level.