The rotated-lamina syndrome. VIII. Lamina rotation and anisophylly in two species of Elatostema (Urticacae), and early development of seedlings of E. sessile

Abstract

Elatostema rugosum A. Cunn. and Elatostema sessile J.R. Forst. & G. Forst. have dorsiventral shoots in which the leaves are arranged in anisophyllous pairs forming four ranks: two on the dorsal and two on the ventral side of the shoot. The ventral leaves have expanded asymmetrical leaf blades with lamina rotation, i.e., the lamina develops in the bud facing towards the upper side of the shoot rather than towards the shoot apex. Each ventral leaf has an asymmetrical intrapetiolar stipule. Lamina rotation occurs by asymmetrical development at the base of the leaf blade above the attachment of the stipule. The dorsal leaves are small, scale-like, approximately symmetrical exstipulate structures. The development of the ventral leaf is generally similar to that found in previous studies of related anisophyllous cases, but the dorsal leaf is reduced to a scale and is simpler than in related cases. Seedlings of E. sessile have been examined to investigate the way in which the combination of unusual features of shoot structure is produced, and also to cast light on the homology of the dorsal leaf. Anisophylly and asymmetry appear in the first pair of leaves formed above the cotyledons in seedlings and become accentuated in subsequent pairs. Events in seedlings suggest the dorsal leaf is derived by reduction from an initial state with blade, stipule, and basal region to the basal region alone. The rapid establishment of lamina rotation and asymmetry in the seedling shoot differs from most other cases described and adds to previous arguments that lamina rotation has arisen independently in a number of taxonomic groups.