Identification of hybrid formulae of a few willows (Salix) using ovule numbers

Abstract Salix is a genus of considerable taxonomic complexity, and accurate identification of its species and hybrids is not always possible. Quantification of ovules was used in this study to verify the parentage of a few hybrids of Salix. It has been shown that ovule numbers in willow hybrids are the mean of the ovule numbers of their parents. The ovule index of a prostrate specimen of S. ×cottetii affirmed that this was a hybrid of S. myrsinifolia Salisb. and S. retusa L., and the ovule index of the ornamental cultivar ‘The Hague’ affirmed that this was a hybrid of S. caprea L. and S. gracilistyla Miq. Finally, we also examined a confusing group, previously identified in North America as S. pentandra. The ovule indexes and other morphological characters indicated that there were four taxa among the studied specimens: S. pentandra, S. ×meyeriana, S. serissima Fernald, and a hybrid of S. serissima and S. fragilis that has not previously been described. It was concluded that quantification of ovules in willows is a reliable tool that can be used in willow taxonomy, genetics and population studies.