Ovule development in Ginkgo biloba L., with emphasis on the collar and nucellus

Ovule development in Ginkgo biloba is examined. Ovulate stalks are initiated in leaf axils, and each stalk primordium dichotomizes to produce two ovule primordia. Integument development appears circumferential and during micropyle formation becomes slightly lobed. Primary nucellar tissues are derived from periclinal divisions in the hypodermal layer, which is limited to several apically located cells of the nucellus and remains active through pollen chamber formation. One spore mother cell differentiates in the sporangial tissues; others differentiate as tapetum. The epidermal layer does not contribute directly to development of the primary nucellus but does contribute to secondary nucellar tissues. Secondary parietal tissues are divided into regions based on their growth patterns and location within the nucellus: the pollen chamber, including a distinct exothecium; the micropylar parietal tissue, including the distal and proximal tissues; the flanking parietal tissue; and the chalazal parietal tissue that lies between the flattened-cell layer and the enlarging sporogenous tissue. The collar develops from periclinal divisions of cells in the hypodermal layer proximal to the ovule after the integument and nucellus have differentiated. The collar is interpreted as a unique structure within ginkgophytes. In some ovules, an organized flap of tissue develops between the collar and the ovule. There does not seem to be any homology of the flap with other structures in gymnosperms based on position or timing of growth, and it is interpreted as a structure sui generis.