Bark wings are related to the primary vascular system: the case of Piptadenia gonoacantha (Leguminosae)

Bark wings are projections commonly interpreted as plant protective structures against herbivory. Their location in stems has been typically thought to be random, something that here we dispute, using Piptadenia gonoacantha as a case study. We collected stem samples from several specimens in different developmental stages from natural populations in the Brazilian Atlantic rainforest. These stems were studied in detail under light microscopy to determine the formation of bark wings and their possible correlation with other anatomical features. Stems of P. gonoacantha are deeply lobed during primary growth, with each lobe containing a large vascular bundle, alternating with smaller vascular bundles in the interlobes. On top of these lobes, prickles develop, being composed of epidermal tissue and the cortex beneath. Some of the eight lobes merge and a phellogen is installed on top of them, which starts to produce a large sheath of phellem, which will become the conspicuous bark wings of Piptadenia. The phellogen switches on and off, leaving marks resembling growth rings. The bark wings in Piptadenia have very specific positioning, always in the front of five to eight major vascular bundles, indicating that bark wings appear always in very precise locations.