INTERVASCULAR PIT MEMBRANE-STRUCTURE IN DAPHNE AND WIKSTROEMIA - SYSTEMATIC IMPLICATIONS

Intervascular pit membranes were investigated in species of Daphne, Wi kstroemia, and other allied genera of the Thymelaeaceae. Results confi rmed a previous study showing that, except for section Mezereum, all s ections of Daphne had pit membranes with tori. Taxonomically isolated species D. aurantiaca and D. genkwa had tori, but lacked a G-layer. To ri similar in structure to those of D. aurantiaca and D. genkwa were o bserved 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 Wiks troemia (= Euwikstroemia of Domke), and from the genera Drapetes, Edge worthia, and Eriosolena. These results suggest a close relationship be tween 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 el ements.