Axillary Meristem Ontogeny in Araucaria cunninghamii Aiton ex D Don

Abstract

The presence of axillary meristems in apparently blank leaf axils from the main stem of 2-year-old Araucaria cunninghamii is demonstrated. These meristems are groups of cells of meristematic appearance, which possess neither a bud-like organisation nor vascular or pro-vascular connections with the central vascular cylinder. They are first discernible in the axils of the recently initiated leaves, where each meristem is delimited from the vacuolating cortex by a shell zone. The axillary meristems then persist indefinitely in an inhibited, undifferentiated state, unless stimulated to bud formation by decapitation of the terminal shoot apex. They are exogenous in origin but are subsequently buried beneath the stem surface by the formation of localised periderms, the bark patches, and are not abscissed when extensive periderm formation begins. A. cunninghamii is apparently unique amongst conifers in possessing distinct, long-lived, exogenously initiated meristems or bud primordia in macroscopically blank leaf axils. The meristems are found in most leaf axils not occupied by branch buds. In this respect A. cunninghamii differs from most conifers and approaches the typical condition of angiosperms. The axillary meristems are inter- mediate in form between previously described vegetative axillary structures in gymnosperms and angiosperms.