IAWA Bulletin Ns, Vol. 4 (2-3),1983 79 INTERVASCULAR PIT
IAWA Bulletin n.s., Vol. 4 (2-3),1983 79 INTERVASCULAR PIT MEMBRANES IN ULMUS AND CELTIS NATIVE TO THE UNITED STATES· by E. A. Wheeler Department of Wood and Paper Science, North Carolina State University, Raleigh, North Carolina 27650, U.S.A. Summary Intervascular pit membranes in Ulmus ameri Studies to date have resulted in the generali cana L. resemble those of other dicotyledons as sation that the membranes in bordered pit pairs they are ofunifonn thickness, lack visible open of dicotyledons exhibit primary wall texture, ings, and have randomly arranged microfibrils. that is, they have randomly arranged micro In contrast, some of the intervascular pit mem fibrils, lack visible openings, and are of unifonn branes in U. alata Michx. , U. thomasii Sarg., thickness (Panshin & DeZeeuw, 1980; Schmid, Celtis laevigata Willd., C occidentalis L., and 1965). This generalisation is based on examina C reticulata Torr. have a central thickened re tion with the transmission electron microscope gion, a torus. Evidence from ultrathin sections of fewer than a dozen species of angiospenns. suggests such thickenings are fonned primarily In contrast, the intertracheary pit membranes in vascular tracheids. Microfibrils are clearly of conifers generally are differentiated into a visible in the membranes in the outer sapwood central thickened region, the torus, and periph of U. americana, U. alata, and C laevigata , while erally, a margo with radiating strands of micro in the inner sapwood, encrustives are present fibrils and often openings readily seen with the on some membranes. There is variation in how electron microscope. There exists variation in infilled and thickened the heartwood mem how well defined the torus is and how large the branes are.
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