Carpology, Seed Anatomy and Taxonomic Relationships ofTetracentron(Tetracentraceae) andTrochodendron(Trochodendraceae)

Abstract

This study of the anatomy and morphology of fruits and seeds ofTrochodendron aralioidesSieb. & Zucc. andTetracentron sinenseOliv. has revealed significant new data in the understanding of their systematic and phylogenetic relationships. The correction of the seed coat type forTrochodendronas (endotestal-) exotegmic (instead of endotestal) and forTetracentronas exotegmic (instead of endotestal) documents a large morphogenetic gap between these putatively advanced types of seed coat and those of other Hamamelididae. Among primitive angiosperms, the combination of a few-layered testa with a markedly differentiated endotesta and a relatively multilayered tegmen with a sclerified tracheidal exotegmen occurs only in the seeds of the family Dilleniaceae (Dilleniales), which occupies a somewhat isolated position within the subclass Dilleniidae. The former features, as well as characteristics of node anatomy, anther dehiscence, pollen, floral and fruit morphology, suggest a close relationship between Trochodendrales and Dilleniales. The intermediate position of Trochodendrales between Magnoliidae and Hamamelididae is rejected, and Trochodendrales are taxonomically transferred from Hamamelididae to Dilleniidae in the rank of a distinct superorder: Trochodendranae. It is considered necessary to reform further the systematics of the artificial complex of the so-called ‘lower’ Hamamelididae and Dilleniidae. The suggested relationship betweenTrochodendronand fossil forms such asTrochodendroides, Nordenskiöldia, TrochodendrocarpusandTrochodendrospermumis also found untenable in light of the carpological survey of modern forms presented here.