VASCULARIZATION OF DEVELOPING LEAVES OF GLEDITSIA TRIACANTHOS L. II. LEAFLET INITIATION AND EARLY VASCULARIZATION

Abstract

Leaflets of Gleditsia triacanthos are vascularized identically, whether they develop from the rachis of a once‐pinnate leaf or the rachilla of a bipinnate leaf. The vascularization process of a leaflet is also similar to that of a rachilla departing the rachis of a bipinnate leaf. Vascularization in all cases involves a rotation and reorientation of the participating bundles to form either a new midvein or a rachilla axis. During this process, the vasculature of the parent axis is progressively depleted. The ventral ridge bundle, however, is partially depleted when each new organ is vascularized and reconstituted after it departs. Observations suggest that the ridge bundles also contribute to formation of the ventral trichome glands that accompany the departure of each new organ. Young, expanding leaflets are vascularized by secondary veins that depart in an acropetal sequence from a consolidated midvein. Major secondary veins that progressively deplete the midvein vasculature are interspersed with minor secondary veins. The lamina expands laterally by anticlinal divisions in the plate meristem between secondary veins. Periclinal divisions that contribute to new cell layers also occur in the plate meristem in association with the secondary veins. No evidence of a recognizable marginal meristem was found. A marginal vein was always present in the leaf margin and cell divisions appeared to occur in association with this vein. Bridge bundles uniting adjacent veins were frequent in the plate meristem. Bridge bundles also occurred between the midvein and the nearest secondary vein and between the marginal veins and leaf margin.