Carpology, seed anatomy and taxonomic relationships of Tetracentron (Tetracentraceae) and Trochodendron (Trochodendraceae)

Abstract

This study of the anatomy and morphology of fruits and seeds of Trochodendron aralioides Sieb. and Zucc. and Tetracentron sinense Oliv. has revealed significant new data in the understanding of their systematic and phylogenetic relationships. The correction of the seed coat type for Trochodendron as (endotestal-) exotegmic (instead of endotestal) and for Tetracentron as 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 between Trochodendron and fossil forms such as Trochodendroides, Nordenskioldia, Trochodendrocarpus and Trochodendrospermum is also found untenable in light of the carpological survey of modern forms presented here.