Amborella trichopoda: Relationships with the Dliciales and Implications for Vessel Origin1

Light microscopy was used to study leaf hypodermis, vein sclerenchyma, stomatal subsidiary cell types, and stem and root xylem in liquid-preserved material of Amborella trichopoda; oblique borders on tracheid pits, scalariform end walls on tracheids, and porosities in end-wall pit membranes were studied with scanning electron microscopy. Amborella shares stomatal configurations, nodal type (in part), ray types, and porose pit membranes in tracheary elements with llliciaies s.l., but differs from that order in lacking oil cells, vessels, and grouped axial parenchyma cells. These data are consistent with a basal position in angiosperms for Amborella, and for a close relationship with, but not inclusion in, llliciales; inclusion in a monofamilial order is conceivable. Both loss of pit membranes or pit membrane portions on end walls and increase in cell diameter are requisites for origin of vessels. Sarcandra and Illiciaceae show these early stages in origin of vessels; Amborella shows development of porosities in pit membranes. Vessel presence or absence may not be strictly bipolar, because some primitive vessel elements exhibit at least some tracheidlike characteristics and are thus transitional, and because changes in at least two characters define vessel origin. . THE SINGLE SPECIES OF Amborellaceae (Amborella trichopoda Baill., New Caledonia) is claimed to be the sister group to the remainder of angiosperms according to recent studies (Mathews and Donoghue 1999, Parkinson et al. 1999, Qiu et al. 1999, Soltis et al. 1999). These studies each analyzed more than a single gene site, and this expanded data base as well as the similarity of the cladistic results yielded by these studies have commanded attention by phylogenists. Branching from the cladogram just above Amborella are Nymphaeales (excluding Nelumbonaceae), and then an expanded llliciaies (llliciaceae, Schisandraceae, Austrobaileyaceae, Trimeniaceae). This treatment was foreshadowed by the results of the Angiosperm Phylogeny 1 Manuscript accepted 17 November 2000. 2 Santa Barbara Botanic Garden, 1212 Mission Canyon Road, Santa Barbara, California 93105. Pacific Science (2001), vol. 55, no. 3:305-312 © 200 I by University of Hawai'i Press All rights reserved Group (1998), who placed a similar roster of families in an unnamed grouping cited first (and thus basal) in their ordering of angiosperms. The phylogenetic significance of anatomical features of Amborella potentially becomes very considerable. In terms of vegetative anatomy, Amborella had already attracted attention because of the vesselless nature of its wood (Tieghem 1900, Bailey and Swamy 1948, Bailey 1957). Bailey regarded vessellessness as a primitive feature in woody dicotyledons, whereas recent cladists, beginning with Young (1981), regarded vessellessness as a derived condition in woody dicotyledons (for a discussion, see Baas and Wheeler 1996). The more recent cladistic work, cited above, by placing Amborella in a basal position in angiosperms, reopens the possibility that vessellessness is a primitive feature. If this possibility is valid, the nature of tracheids in Amborella becomes worthy of consideration. By means of scanning electron microscopy (SEM), we are attempting to show whether tracheids of Amborella are transitional to vessels in any respect, and, by inference, whether