Developmental and Structural Section
Jansen, Steven , Dute, Roland R. , Gasson, P.E. , Smets, E. .
A preliminary survey of angiosperms with torus-bearing pit membranes.
Intervascular pit membranes with a torus are rarely noted in angiosperms. However, this feature might be present but easily overlooked in several families. It is clear that TEM-observations are most useful to investigate ultrastructural details of pit membranes. In addition, LM-observations provide a simple and adequate method for discovering tori in counterstained wood sections. Our observations of Ulmaceae and Cannabaceae illustrate that tori are present in ring-porous species of Ulmus, Zelkova and Celtis, but absent in diffuse-porous species. In particular, intervascular pit membranes connecting relatively wide vessel elements are of uniform thickness, but pit pairs in narrow (incomplete) vessel elements or vascular tracheids are frequently characterized by tori. Torus-bearing pit membranes were also observed in Thymelaeaceae, Oleaceae, and new records include Rosaceae and Elaeagnaceae. Tori have evolved several times in angiosperms and there seem to be significant differences in their chemical composition and ontogeny. Characteristics of torus-bearing angiosperms indicating that the development of tori is inherited as a functional complex include: (1) a relatively small pit aperture that is circular to elliptical in outline, (2) an indistinct pit canal, and (3) the prominent occurrence of helical thickenings throughout the body of narrow tracheary elements. Although functional adaptations are not fully understood, we suggest that a pit membrane with a torus would be more effective than one of uniform thickness in sealing off a pit aperture. Moreover, our observations indicate that tori are more common in cold temperate climates than in warm (sub)tropical environments, suggesting that narrow tracheary elements with torus-bearing pit membranes provide an auxiliary conducting system, which is of low conductivity, but offers greater resistance to freezing-induced cavitation.
homepage R.R. Dute
International Association of Wood Anatomists
1 - Royal Botanic Gardens, Kew, Jodrell Laboratory, Richmond, Surrey, TW9 3AB, UK
2 - Auburn University, Department of Biological Sciences, Life Sciences Building, Aubrun, Alabama, 36849-5407, U.S.A.
3 - Laboratory of Plant Systematics, K.U.Leuven, Institute of Botany and Microbiology, Kasteelpark Arenberg 31, Leuven, B-3000, Belgium
Presentation Type: Paper
Location: Alpine A (Snowbird Center)
Date: Wednesday, August 4th, 2004
Time: 3:15 PM