title: An anatomical assessment of branch abscission and branch-base hydraulic architecture in the endangered Wollemia nobilis.
abstract: OBJECTIVE
  The branch-base xylem structure of the endangered Wollemia nobilis was anatomically investigated. Wollemia nobilis is probably the only extant tree species that produces only first-order branches and where all branches are cleanly abscised. An investigation was carried out to see if these unusual features might influence branch-base xylem structure and water supply to the foliage.
  
  
  METHODS
  The xylem was sectioned at various distances along the branch bases of 6-year-old saplings. Huber values and relative theoretical hydraulic conductivities were calculated for various regions of the branch base.
  
  
  RESULTS
  The most proximal branch base featured a pronounced xylem constriction. The constriction had only 14-31 % (average 21 %) of the cross-sectional area and 20-42 % (average 28 %) of the theoretical hydraulic conductivity of the more distal branch xylem. Wollemia nobilis had extremely low Huber values for a conifer.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Institute for Land, Water and Society, Charles Sturt University, Locked Bag 588, Wagga Wagga, NSW 2678, Australia. gburrows@csu.edu.au
authors.forename: G E
authors.initials: GE
authors.lastname: Burrows
authors.forename: P F
authors.initials: PF
authors.lastname: Meagher
authors.forename: R D
authors.initials: RD
authors.lastname: Heady
mesh_terms: D053476:Extinction, Biological; D008855:Microscopy, Electron, Scanning; D064028:Tracheophyta / Q000033:anatomy & histology* / Q000254:growth & development / Q000378:metabolism; D014867:Water / Q000378:metabolism*; D052584:Xylem / Q000033:anatomy & histology / Q000254:growth & development / Q000378:metabolism
chemical_list: D014867:Water