Fruits of Polygonaceae have a basically similar construction of indehiscent nuts or achenes. Sections of fruits, coupled with surface patterns were studied with SEM and LM in all genera of the tribes Persicarieae and Polygoneae (Polygonoideae-Polygonaceae). The outer layer of the pericarp is usually thickened and its anatomy can be used consistently to delimit genera more than any other character of the fruit. Cells are most often puzzle piece-shaped in surface view, but the shape of the cells may become polygonal with straight anticlinal walls towards the endocarp The primary sculpture of the cells is highly variable and has value at the specific level, rarely at the generic level. No strict correlation exists between the external surface patterns and the anatomy. Two main cell types can be recognized in cross- and longitudinal section, correlated with the straight or undulating outline of the anticlinal walls. No distinction can be made between sections Persicaria, Tovara, Echinocaulon, and Cephalophilon of the genus Persicaria; all share narrow rectangular cells with undulating anticlinal walls. Aconogonon and Bistorta can be delimited by the square to rectangular cells with a narrow dichotomously branching lumen and straight anticlinal walls; both genera are best grouped as a single genus with two sections. A similar arrangement is found occasionally in species of Polygonum s.s., Polygonella, Atraphaxis, Fallopia and Calligonum. Fruit anatomy of Pteropyrum is distinctive. The genus Polygonum s.s. shows a wide range of integrating patterns, ranging from straight to undulating anticlinal walls and cannot be separated from Polygonella. Fagopyrum is aberrant in having a parenchymatic exocarp and a thickened mesocarp; other evidence supports its isolated position. Different fruit anatomical patterns have arisen several times in evolution and have a limited value at tribal level but are useful at generic level. It is suggested that an arrangement with Straight anticlinal walls and a broad lumen, eventually with dendritic branching towards the periphery, is ancestral. (C) 2000 The Linnean Society of London.
