Morphology and genome weight of Symphyotrichum species (Asteraceae) along rivers in The Netherlands

Abstract The characters used in separating Symphyotrichum species naturalised along rivers in Western Europe (leaf bases more or less clasping, length of outer phyllaries relative to inner ones, hairiness of leaves) hardly apply to the plants found in the wild. We present morphological and cytometric data on 88 plants of Symphyotrichum from riversides in The Netherlands and adjacent Germany, in order to understand better the taxonomy of these plants. Ploidy was inferred from 2C-values obtained by flow cytometry. Inferred tetraploid plants with relatively deeply lobed disc florets (mean lobe/limb ratio >50%), relatively few florets per head (<40), disc florets usually <4 mm, and small flowering heads are referred to S. ontarionis (Wiegand) G.L. Nesom. Inferred hexaploids and octoploids with relatively deeply lobed disc florets (mean lobe/limb ratio >50%) and relatively many florets per head (>50) are referred to S. aff. lateriflorum (L.) Á. Löve & D. Löve. Inferred hexaploids and octoploids with a disc floret lobe/limb ratio <50% are named S. lanceolatum (Willd.) G.L. Nesom. Usually these plants have large flowering heads (to 3·6 cm in diameter) with >45 florets. Putative hybrids, comprising inferred pentaploids and heptaploids, are morphologically more or less intermediate between their parent species.