An Anatomical Assessment of Branch Abscission and Branch-base Hydraulic Architecture in the Endangered Wollemia nobilis

Abstract

The branch-base xylem constriction would appear to facilitate branch abscission, while the associated Huber values show that W. nobilis supplies a relatively large leaf area through a relatively small diameter 'pipe'. It is tempting to suggest that the pronounced decline of W. nobilis in the Tertiary is related to its unusual branch-base structure but physiological studies of whole plant conductance are still needed.

Wollemia nobilis (Araucariaceae): Branching, vasculature and histology in juvenile stages

“…2J). This pattern can be revealed by decorticating the axis, as used extensively by Burrows et al (2007) to demonstrate the resulting potential hydraulic constriction of the branch base.…”

“…3J). However, a fluted effect may be achieved distally, resulting in the “stranded” appearance described by Burrows et al (2007) in the abscission area, where the branch xylem bends through 45° and exits the stem into the morphological branch base. The vascular continuity from branch insertion into the branch base thus largely involves a considerable change in stem anatomy that is eventually obscured as secondary xylem is formed.…”

“…In Araucaria , it appears that the branch tier is initiated at the apex of the leader (still by syllepsis), just before the shoot goes into dormancy, and appears at the apex of the shoot so that the presence of branches is visible externally. This configuration may account for the contrasted methods of branch insertion in the two genera (Burrows et al, 2007). Furthermore, the branch tier in Araucaria can continue to extend as the trunk axis remains dormant, producing a characteristic flat top to the crown.…”

“…Perhaps the most distinctive feature we have described for Wollemia is the complexity of the vasculature of the primary cortex of the trunk axis because it includes an extended series of cortical leaf and branch traces that may be unique for conifers, especially in the axial orientation of the woody cylinder of the branch trace before it exits the stem (cf. Burrows et al, 2007). An analogous configuration occurs in the dicotyledonous Schefflera , in which branch traces have an extended cortical passage because they are largely derived from cortical leaf traces, which are numerous because of the encircling leaf base (Tomlinson et al, 2005).…”

“…Conservation strategies for the rare and threatened conifer Wollemia nobilis (Jones et al, 1995) have resulted in the wide distribution of clonal material that has been made freely available. Further work can now be added to the extensive studies that have been carried out (e.g., Burrows, 1999, Burrows and Bullock, 1999; Burrows et al, 2003, 2007; Heady et al, 2002; Hill, 1997; Offord et al, 1999). Much of this effort has concentrated on the establishment of the systematic position of Wollemia .…”