Symmetry in plants: phyllode and stipule development in Acacia longipedunculata

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Abstract
Both phyllodes and so-called stipules develop on a whorl platform that surrounds the shoot apex. Phyllodes usually are initiated as radial primordia. During their development, although they become acrovergent and show a slight differentiation gradient from the abaxial towards the adaxial side, they retain a circular outline in transection until maturity. Longitudinally they differentiate into a basal pulvinus, a photosynthetic zone, and a terminal mucro. While only one concentric vascular bundle is formed in the pulvinus region, a cylindrical vascular network differentiates in the photosynthetic zone and only one collateral bundle surrounded by sclerenchyma develops in the mucro. In terms of both external symmetry and internal organization, phyllodes show a striking resemblance to stems, whereas so-called stipules appear more leaf-like because of their dorsiventral symmetry. After a detailed discussion of the role symmetry plays in various complementary interpretations of phyllodes and so-called stipules, symmetry transformations in angiosperms in general are illustrated. It is concluded that symmetry, growth pattern, and position do not constitute criteria that allow us to assign all known developmental pathways to mutually exclusive categories such as caulome or phyllome. Transformational analysis is suggested as a more adequate and dynamic tool for understanding the diversity of plant form.