Stem and Leaf Anatomy of the Alseuosmiaceae

The stem and leaf anatomy of five species representing all three genera of the small Southwest Pacific dicotyledonous family, Alseuosmiaceae, were studied. Salient anatomical features common to all genera include: a trilacunar, three-trace leaf-node structure in which petioles are supplied with three separate vascular bundles; rosoid teeth bearing hydathodes; anomocytic stomata; unicellular, bicellular, and multicellular, unbranched, living trichomes; a superficial origin of periderm; leafmesophyll composed of a uniseriate palisade region and a loosely constructed spongy zone; and the presence of a stem and foliar endodermis. Sclerenchyma occurs in stem tissues of species belonging to all three genera and Crispi/oba disperma contain numerous elongate, filiform or fibrosclereids in the stem, petiole, and leaf mesophyll. Leaves are pinnately veined or, in the case of Wittsteinia vacciniacea, with acrodromous venation. Secondary venation is brochidodromous or semicraspidodromous. As presently constituted the family has a distinctive combination of derived wood and stem anatomical characters, and provides an excellent example of the successful application of vegetative anatomical evidence in solving problems of family relationship and generic affinities. The presence of an endodermis in the aerial organs provides additional support to the suggestion that the family evolved from a small, shrubby ancestor, subsequently became semiherbaceous, followed by the development of a secondarily woody body in some taxa. The totality of anatomical characters reinforces the view that the Alseuosmiaceae have their nearest relatives within the rosalean-saxifragalean complex.