Agathis, Araucaria and Wollemia All Possess Unusual Meristems in Their Leaf Axils

In most angiosperms a bud can be observed in almost every leaf axil, while in most conifers the numerous leaf axils are usually described as 'blank' or 'empty'. Consequently conifers are considered to have limited powers of vegetative regeneration. The Araucariaceae provide an exception with most species able to form coppice and epicormic shoots. However, buds can not be seen on the stem surface and when buds do form they appear to come from deep seated tissues ' both very unusual observations. Thin sections were prepared of a range of araucarian leaf axils to provide a structural explanation for the above peculiarities.The main feature of leaf axils from near the shoot apex of Hoop Pine is a small group of cells of meristematic appearance, the axillary meristem. In intact plants the axillary meristems remain in a relatively undifferentiated form ' they do not form an apical dome, leaf primordia or possess vascular connections with the central vascular cylinder. Through the action of small, localised phellogens the axillary meristems are transferred from their initial position on the stem surface to be buried a small distance below the surface. When extensive bark formation occurs commences the epidermal layers and the leaves are abscised, while the axillary meristems remain within the stem. As the trees grow the cells of the meristems divide occasionally to compensate for increase in girth but they still do not form into buds. When plants are decapitated the meristems receive hormonal signals to develop into buds that push their way through the overlying layers of cells. The buds establish vascular connections by dedifferentiation of the cortical parenchyma.Thus, in contrast to most other conifers, all the investigated members of the Araucariaceae possess long-lived bud-forming potential in all their leaf axils. This explains the pronounced ability of the family to form epicormic and coppice shoots.