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

Abstract

We present a preliminary description of the morphology and anatomy of contrasted axes in the recently discovered conifer Wollemia nobilis, based on clonally propagated material. The novelty of this discovery and the tree's size and rarity has led to global interest and a comprehensive and successful conservation program. Our results should serve as a model on which future studies of this tree and other members of the Araucariaceae can be based. The specimens studied are mimics of the architecture of the mature tree, with precise distinction between orthotropic (= trunk) axes, which have radial symmetry (spiral phyllotaxy) and plagiotropic (= branch) axes with dorsiventral symmetry (as a modified decussate phyllotaxy). Trunk axes develop irregular pseudowhorls of branches that originate within the terminal bud by syllepsis, their extension coincident with that of the parent axis. The two kinds of axes show considerable anatomical differences, but are still based on the common feature of a single stelar-derived trace to each leaf that becomes subdivided in the cortex, a feature of the whole family. Trunk axes include extended cortical leaf and branch traces associated with abundant sclerenchyma, but branches have short cortical leaf traces, no branch traces, and limited sclerenchyma. Reiteration is limited and largely involves the formation of basal suckers on the trunk. Branches normally remain unbranched, but can do so most often when damaged. This study thus emphasizes the phenomenon of axis differentiation in conifers, which has been little investigated anatomically, but could be very important in the identification of fossils.

Partial shoot reiteration in Wollemia nobilis (Araucariaceae) does not arise from ‘axillary meristems’

“…As has been reported in some detail, orthotropic (trunk) and plagiotropic (branch) axes are sharply contrasted in both morphology and histology (Tomlinson and Murch, 2009). Orthotropic (trunk) axes grow erect ( Fig.…”

“…Consequently, the cortex can include traces to at least two successive nodes in branch axes and many sequential nodes in the radially symmetrical trunk axes. Other histological features include the system of interconnected resin canals, but a welldeveloped series of sclereids is present only in trunk axes (Tomlinson and Murch, 2009). In the present description we refer only to branch axes.…”

“…We surveyed sequential sections using methods of Huggett and Tomlinson (2010) in which microphotographs were converted into movie sequences allowing easy viewing and a clear understanding of vascular topography. Additional information was obtained from the same series of paraffin sections used in Tomlinson and Murch (2009).…”

“…The most common contact made with this system of bud traces is with the cambium of the vascular bundles that frame the leaf gap next below on the same orthostichy. Stages in this linkage are illustrated in Tomlinson and Murch (2009;their fig. 5C -E).…”

Diversity of phyllotaxis in land plants in reference to the shoot apical meristem structure

“…It is worth mentioning that in newly discovered Wollemia (Araucariaceae) spiral phyllotaxis can only develop at the main orthotropic axis, whereas in the lateral plagiotropic branches, a strict decussate pattern occurs [67].…”