Intervascular Pit Membrane Structure in Daphne and Wikstroemia - Systematic Implications

Intervascular pit membranes were investigated in species of Daphne, Wikstroemia , and other allied genera of the Thymelaeaceae. Results confirmed a previous study showing that, except for section Mezereum , all sections of Daphne had pit membranes with tori. Taxonomically isolated species D. aurantiaca and D. genkwa had tori, but lacked a G-layer. Tori similar in structure to those of D. aurantiaca and D. genkwa were observed in three species from the subgenus Diplomorpha of Wikstroemia . Tori of a slightly different morphology were noted in W. kudoi (subg. Daphnimorpha ). Tori appeared absent from species of the subgenus Wikstroemia (= Euwikstroemia of Domke), and from the genera Drapetes , Edgeworthia , and Eriosolena . These results suggest a close relationship between Daphne and Wikstroemia . The degree of torus development and the distinctiveness of helical thickenings suggest that smaller tracheary elements serve as a backup water-conducting system to larger vessel elements.