In the present study, morphology of hairs, flowers, and achenes of Polygonum sect. Echinocaulon are examined. Six types of hairs were found in the section; (1) simple unicellular hair, (2) filiform simple multicellular hair, (3) tufted hair, (4) sessile stellate hair, (5) stalked multicellular glandular hair, and (6) peltate multicellular glandular hair. The stamen number varies from four to nine, and shows intra- and interspecific variation. Stamens are inserted at the base of the perianth in one or two whorls, and nectaries are present near the base of the inner stamens. Achenes are trigonous or biconvex and this character is relatively constant within each taxa. However, in P. strigosum both types rarely occur on the same plant. Four types of achene surfaces were recognized. Achene surfaces are either (1) with randomly distributed tubercles, (2) with tubercles distributed along anticlinal walls forming reticulate pattern, (3) with thickened, sinuous anticlinal walls, or (4) smooth, depending on taxa. Hair types and their distribution, the number and arrangement of stamens, nectary shapes, and achene surface characteristics appear to be useful in delimiting the taxa in the section.
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