ULTRASTRUCTURE OF STOMATAL DEVELOPMENT IN GINKGO BILOBA

We present the first ultrastructural study of stomatal development in the living fossil gymnosperm species Ginkgo biloba. The Ginkgo lineage occupies a pivotal position in seed-plant phylogenies, but as most representatives are known only from fossils, the developmental pattern in G. biloba is critical for inferring patterns of stomatal evolution among seed plants. The distinctive fan-shaped leaf morphology of Ginkgo is related to its unusual petiolar vasculature. Our results show that both cell lineages and cell interactions control stomatal patterning in this species. Both perigenous and mesoperigenous patterns of stomatal development are present, contrary to earlier reports of exclusively perigenous development. Ginkgo resembles grasses in possessing asymmetric divisions in perigenous neighbor cells but resembles Arabidopsis in possessing amplifying divisions within the stomatal lineage, though these divisions are relatively chaotic in Ginkgo. The degree of asymmetry in meristemoidal divisions is less marked in Ginkgo than in Arabidopsis, a factor that influences the relative orientations of stomata in mature epidermal tissue. The role of asymmetric divisions represents a crucial question in determining stomatal classification.