ONTOGENY OF THE STEM-NODE-LEAF VASCULAR CONTINUUM OF AUSTROBAILEYA

Developmental study of the stem-node-leaf vascular continuum of Austrobaileya scandens White reveals that the vasculature within each leaf originates from a single procambial strand, that becomes separated into two strands only at the junction of leaf and stem. At lower levels in the stem the two strands become incorporated into independent portions of the stele. At later stages of development the solitary vascular bundle within the young leaf undergoes considerable lateral growth, resulting in an essentially continuous arc of vascular tissue. Ontogenetic evidence indicates that the vascular bundle in the midrib of the lamina should be regarded as a fundamentally single bundle and not interpreted as two bundles that have undergone various degrees of secondary fusion. A condition of two totally separate bundles extending the entire length of the leaf was not encountered. Our observations confirm the characterization of Austrobaileya as an example of "second rank" level of leaf vasculature. Nodal anatomy emphasizes the extremely isolated taxonomic position of Austrobaileya within the primitive dicotyledons. THE DISCOVERY ofthe nodal anatomical pattern in which a unilacunar node is associated with two leaf traces, each related to an independent portion of the stem vascular cylinder, brought about a reassessment of ideas relating to the evolution of nodal vasculature in dicotyledons. Suggestions were made that the unilacunar, twotrace nodal condition was primitive in dicotyledons (Canright, 1955; Marsden and Bailey, 1955; Bailey, 1956). This idea was subsequently accepted by a number of individuals (Eames, 1961; Cronquist, 1968; Foster and Gifford, 1974). One of the primary evidences used in support of the primitive nature of the 2:1 node is its occurrence (or modifications thereof) in various members of the primitive Ranalean complex, including the monotypic family Aus