Tori in species of Diarthron, Stellera and Thymelaea (Thymelaeaceae)

Abstract

Torus thickenings have been found previously in intervascular pit membranes of species of Daphne and Wikstroemia (Thymelaeaceae). A search for tori was undertaken in the closely related genera Diarthron, Stellera and Thymelaea. Tori were observed in five of the seven species of Diarthron that were investigated. Presence of tori was associated with commonly occurring imperforate conducting elements and with perennial growth habit. Tori of a different morphology from that of Diarthron were present in two of the three specimens of Stellera chamaejasme that were studied. This study suggests torus presence to have systematic value; specifically, tori are present in species of the subgenera Dendrostellera and Stelleropsis within Diarthron but absent in the subgenus Diarthron. Of 19 species of Thymelaea investigated, only two of four specimens of T. villosa contained torus-bearing pit membranes. It is suggested that the original classification of this species as Daphne villosa be reconsidered.