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

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. GLEDITSIA TRIACANTHOS (honeylocust) is a heteroblastic species. The first 10 to 12 leaves on a seedling are once-pinnate and a mature rachis may bear 20 or more leaflet pairs. Subsequent leaves are bipinnate with a lesser, but variable, number of leaflets occurring on each of the 3 to 8 rachilla pairs. Although rachillae and leaflets are usually considered to be paired, they seldom occur exactly opposite one another (Neville, 1970a). The species is highly heterophyllous. Not only are aberrant transitional leaf forms common in nature, but they can be easily induced by morphological experimentation (Neville, 1970b, 1974). Apparently, such leaf forms can be induced at any leaf age, unrelated to ontogeny. In Acacia Kaplan (1980) observed that pinnate leaf size was related to apex size during ontogeny; large leaves with many pinnae arose from large apices. The leaflets of honeylocust are initiated acropetally on each rachis or rachilla axis. This pattern is typical of most pinnatifid species with many pinnae per leaf, such as Carya buckleyi (Foster, 1 9 3 5), Sorbus spp. (Merrill, 1 9 7 9), and Fraxinus excelsior(Jeune, 1982). A similar pattern is found in juvenile leaves of Acacia. 1 Received for publication 2 December 1983; accepted 7 March 1984. In those species of Acacia examined by Kaplan (1980) a terminal leaflet was initiated first, but it atrophied early during leaf ontogeny. Subsequent leaflets were then initiated acropetally. Other species of Leguminosae, including honeylocust (Larson, 1984), also exhibit an atrophied rachis tip that may represent a terminal leaflet arrested in development (Watari, 1934; Stein, 1978). In less complex leaves of some Leguminosae species, such as Trifolium spp., the terminal leaflet is initiated first followed by the lateral leaflets in a basipetal sequence (Lersten, 1965; Denne, 1966). The vascularization of pinnate leaves has been described for a number of species (Hilger, 1978, 1982). In general, vascularization of a leaflet is similar to that of a rachilla, with the leaflet vasculature diverging progressively from the ventral to dorsal bundles of the rachis vasculature. Hilger (1982) commented on the similarity in structure of the stem and rachis in pinnatifid species. However, most studies of leaflet vascularization were either descriptive or comparative and limited in scope. I found no detailed description in the literature of leaflet vascularization during ontogeny. Venation of the pinnate leaflets of those species examined also proceeds acropetally (Foster,1935,1952; Merrill,1979). Secondary