Development of dorsiventrality in seedlings of Azara serrata R. & P. (Flacourtiaceae)

Azara spp. generally have dorsiventral shoots with the appearance of a large and a small ‘leaf at each node. On morphological grounds the small ‘leaf is usually considered to be derived from an upper stipule, while the lower stipule is reduced. This interpretation is reinforced by the changes during seedling development. Seedlings usually pass through a phase where the shoot is radially symmetrical and trilacunar nodes with small, glandular, non-vascular stipular structures are formed. Then nodes become more asymmetrical with the diminution of stipular development and lateral leaf trace development on one side, and accentuation on the other, and this process proceeds until the adult state is reached. Dorsiventrality depends on alternation of asymmetry at successive nodes and alternation may appear later than asymmetry. The changes in the seedling indicate that a recent interpretation of the adult structure of Azara based on homoeosis is not useful. The seedlings provide an interesting case in continuum morphology since they show a continuum of stipular structures from ‘gland’ through to ‘leaf but the continuum does not quite extend to the original leaf blade, which remains distinctive.