VASCULARIZATION OF A MULTILACUNAR SPECIES: POLYSCIAS QUILFOYLEI (ARALIACEAE) I. THE STEM

Abstract

odd-pinnate leaves of Polyscias quilfoylei have a sheathing leaf base that completely encircles the stem. At each node, many traces depart the vascular cylinder and traverse an obliquely upward course through the leaf base before aggregating in the rachis. Lateral traces diverge from parent traces in the stem vasculature at variable times relative to the leaf they serve, from variable positions in the vascular cylinder and from parent traces of variable ages. stem vasculature is formed by the coalescing of leaf traces from as many as five leaves. All bundles departing the vascular cylinder at a node to serve a leaf are true leaf traces originating independently in the stem. Leaf traces develop acropetally from their positions of origin on parent traces. Primordial leaves are first served by the median trace and later by lateral traces. Many traces were recognized in the internodes subtending embryonic leaves, but they could not be related either to a specific leaf or to a specific position within a leaf. Because these traces had not yet achieved contact with a primordial leaf site, they were assumed to be in the process of developing acropetally at the time of sampling. Observations suggest that the multiple traces in this species might perform a similar function of integrating the vascular cylinder that subsidiary bundles perform in certain uniand trilacunar species. IN A REVIEW OF the stem-node-leaf vascular continuum in the Dicotyledoneae, Howard (1974) wrote, The continuity of the vascular tissue from the stem through the petiole to the apex of the leaf is evident, yet the nature of its path and the variations of pattern of bundle arrangement and position along its length have not received much consideration. A similar review ten yrs later (Larson, 1984a) revealed that our knowledge of the course of vascular bundles through the stem-node-leaf continuum had advanced very little. Many investigators have examined the vasculature in either the stem, the node, or the leaf, and some have speculated regarding the course of bundles throughout the continuum. However, one specific question remains unanswered-are all of the bundles that depart through a multilacunar node true leaf traces originating independently in the stem, or are some of them subsidiary bundles of leaf traces? present study was designed to examine this and related questions in Polyscias quilfoylei (Araliaceae), a species with well-developed multilacunar nodes. objectives of the study i Received for publication 24 January 1986; revision accepted 21 May 1986. I thank Dr. J. B. Fisher, Fairchild Tropical Gardens, Miami, FL for kindly supplying the cuttings of Polyscias quilfoylei, and Drs. T. D. Pizzolato, Univ. of Delaware, Newark, DE and W. A. Russin, Carleton College, Northfield, MN for technical reviews of the manuscript. were to: 1) analyze the stem vasculature through a series of nodes by following each leaf trace from its origin to its departure in a leaf base; 2) to determine if the leaf traces conformed to a predictable pattern; and 3) to speculate on the relation of multiple traces of multilacunar species to the subsidiary trace bundles of previously examined uniand trilacunar species. MATERIALS AND METHODS Stem cuttings of Polyscias quilfoylei (Bull.) L. H. Bailey were rooted, planted in soil in 8-inch pots, and grown in a greenhouse under ambient conditions. Shoots were occasionally cut back, both to maintain a workable plant size and to promote vigorous lateral shoot growth. Apical buds having different numbers of subtending leaves and internodes were collected for microscopic examination from lateral shoots differing in their states of elongation. One apical shoot was fixed with 3% glutaraldehyde in Pipes buffer (Sigma Chemical Co.2), post-fixed with 1% osmium tetroxide, and embedded in Spurr's (1 969) resin for sectioning on an ultramicrotome. entire shoot (9.1 mm long) was sectioned transversely at 2 ,um and the sections were analyzed with an optical shuttle microscope (Zimmermann and Tom2 Mention of trade names is for the information of the reader and does not constitute endorsement by the U.S. Dept. of Agriculture or the Forest Service.