Are shoot features of actual diagnostic value in European bladderworts (Utricularia L., Lentibulariaceae)?

In central Europe, seven native species of Utricularia occur, mainly distinguished by flower features. They can be subdivided in three aggregates according to shoot morphology: (1) Utricularia intermedia aggr., also comprising Utricularia ochroleuca and Utricularia stygia, (2) Utricularia vulgaris aggr., also comprising Utricularia australis and (3) Utricularia minor aggr., also comprising Utricularia bremii. We tested some shoot features as identification tools and investigated quadrifid digestive glands in all the studied species, using geometric morphometrics. For U. intermedia aggr., in most cases, the tooth number per ultimate leaf segment may be diagnostic. The three species of this aggregate overlap in terms of gland shape, but averaging measurements by traps and individuals, a decrease of overlapping among species was found. The shape and the centroid size of quadrifid glands in U. minor aggr. is unsuitable for discriminating species. Concerning U. vulgaris aggr., U. vulgaris bears a higher ratio of setula length to tooth length than U. australis (which is more toothed). This feature may help to distinguish species, but conflicting results were found in one studied population. The relevance of quadrifid gland features as identification tool and the puzzling systematic circumscriptions of the target species are discussed.