Structure and function of tracheary elements in Amborella trichopoda

Recent phylogenetic analyses have placed the root of flowering plants near Amborella trichopoda, a woody plant restricted to cloud forest habitats in New Caledonia. A distinctive feature of A. trichopoda is its reported lack of xylem vessels. Here we present observations of pit membrane structure and end wall morphology for primary and secondary tracheary cells of A. trichopoda as well as held measurements of stem hydraulic properties of A. trichopoda compared with five cloud fforest species from New Caledonia. Observations of stem radial sections revealed that the primary wall material in the protoxylem and metaxylem elements was intact. No large porosities (such as those that have been observed in the pit membranes of Nymphaeales) were observed. However, a few elliptical pits of tracheary cells in the secondary xylem appeared to lack pit membranes. These observations are consistent with our measurements of functional conduit length, which indicate that the longest open conduits are equal to the length of two secondary xylem elements joined end to end. Thus, the xylem of A. trichopoda appears to be functionally vesselless, with the caveat that connections between individual vascular elements may occasionally be open (i.e., lacking in at least one pit membrane). Sapwood area and leaf area-specific hydraulic conductivities of A. trichopoda are similar to those of conifers and angiosperms, with and without xylem vessels, growing in understory cloud forest environments. These findings bear on discussions of the morphology and ecology of the first flowering plants as well as on the possible causes of their diversification.