Architecture and development of the phyllode–stipule whorls of Acacia longipedunculata: controversial interpretations and continuum approach

Abstract

The whorls of Acacia longipedunculata consist of 15–27 terete phyllodes and as many setaceous stipules. Their four most surprising features are as follows. (i) Only five to eight traces leave the stele within a whorl node; branches of these supply all appendages (phyllodes, stipules), (ii) The development of a new whorl starts with a polygonal bulge around the shoot apex. On this girdling primordium all whorl appendages are initiated centrifugally. (iii) Occasionally there are intermediates between phyllodes and stipules. (iv) The seedlings change from spiral to whorled phyllotaxis, with fascicles that fit into the generative helix. Four perspectives are proposed to interpret the phyllode–stipule whorls morphologically, based on different sets of assumptions such as the relative value of early developmental events as opposed to vascular patterns. All of these interpretations approximate nature to some degree, but in all probability none of them reflects nature fully. As the most adequate interpretation we propose a heterogeneous continuum in which pinnate juvenile leaves, phyllodes, stipules, and even clubshaped multiseriate hairs (inside and between the phyllodes) are parts of a morphocline linked by rare intermediates. A corollary of this interpretation is that stipules need not necessarily be considered as subunits of leaves. Such a modified leaf–stipule concept seems to be appropriate for various angiospermous taxa, especially Leguminosae, Rubiaceae, and Cunoniaceae. The whorl architecture of Acacia longipedunculata is compared with that of various other acacias (e.g., A. verticillata) and other taxa of angiosperms. Pertinent botanical and philosophical terms are explained in a detailed glossary.