VASCULARIZATION OF A MULTILACUNAR SPECIES: POLYSCIAS QUILFOYLEI (ARALIACEAE) II. THE LEAF BASE AND RACHIS

Multiple leaf traces depart the stem vasculature at a node to form the rachis vasculature of the odd-pinnate leaf of Polyscias quilfoylei. The median (M) trace departs the vascular cylinder directly, but the converging lateral traces follow acropetal, oblique pathways through the ensheathing leaf base. The leaf traces are partitioned into sub-bundles within the leaf base, and some sub-bundles merge with others to form composite bundles. Most mergers in the leaf base occur among sub-bundles of adaxial and lateral traces; cross-overs of bundles from opposite sides of the M-trace are infrequent. The rachis vasculature distal to the leaf base is similar in structure and appearance to that of the stem. Vascularization of the leaflet pair at each rachis node involves partitioning and reaggregating of the trace sub-bundles. However, these processes are more extreme and bundle cross-overs are more frequent at rachis nodes than at the leaf base. At the more distal rachis nodes, lateral traces situated nearer the M-trace contribute to leaflet vascularization. The M-trace contributes sparingly to vascularization of lateral leaflets; it forms that part of the midvein that extends to the extreme lamina tip of the terminal leaflet. A PREVIOUS STUDY (Larson, 1986) examined the stem vasculature of Polyscias quilfoylei (Araliaceae), a multilacunar species. It was concluded that all bundles departing the vascular cylinder to serve a leaf were true leaf traces originating independently in the stem. Each median (M) trace diverged from a parent leaf trace about five nodes below the leaf it served. Lateral traces diverged from parent traces at variable times relative to the leaf they served, from variable positions in the vascular cylinder, and from parent traces of variable ages. No predictable pattern in lateral trace divergence could be detected, other than an approximate balance between the number of traces to the right and left of the M-trace and a tendency to distribute the lateral traces throughout the sheathing leaf base of most leaves. The primary objective of the present study was to examine the departing leaf traces as they enter the leaf base, traverse the rachis, and vascularize the leaflets of the compound, oddpinnate leaf. A secondary objective was to speculate on the relation of the leaf trace bundles in the leaf base to the acropetal components of subsidiary leaf trace bundles of previously examined uniand trilacunar species. MATERIALS AND METHODS-Stem cuttings of Polyscias quilfoylei (Bull.) L. H. Bailey were I Received for publication 24 January 1986; revision accepted 21 May 1986. rooted and grown in a greenhouse (Larson, 1986). One apical shoot was fixed with 3% glutaraldehyde in Pipes buffer, post-fixed with 1% osmium tetroxide, and embedded in Spurr's (1969) resin for sectioning on an ultramicrotome. The entire shoot (9.1 mm long) was sectioned transversely at 2 ,m; this shoot was referred to as the PLAS-bud in the previous paper of this series (Larson, 1986). The apical shoot consisted of three embryonic leaves and one leafprimordium. All leaves were sectioned in their entirety; the basal embryonic leaf (L4), approx 6 mm long, bore five leaflets. These sections, analyzed with an optical shuttle microscope (Zimmermann and Tomlinson, 1966), were used to follow leaf traces from the stem, in the leaf base, and throughout the length of leaf L4. Leaf traces of leaf L4 were numbered sequentially from the median (M) trace. Left lateral traces were assigned odd numbers (1 through 11) and right lateral traces were assigned even numbers (2 through 10). When leaf traces were partitioned and sub-bundles of several traces contributed to a composite rachis bundle, they were listed in descending order according to their relative size contributions, e.g., (5,7,1) indicates that left trace 5 was the major contributor. Eight leaf nodes at different states of development were fixed in Craf III under vacuum at 0 C, dehydrated in t-butanol, embedded in 61 C paraffin, serially sectioned transversely at 10 to 15 ,um and stained with safranin 0 and