3. Gleditsia L.

Plants small to more commonly large trees, usually incompletely dioecious, the trunks and branches usually armed with simple or branched thorns, these often in clusters, the branches differentiated into short shoots with clustered leaves and elongate shoots with alternate leaves, the twigs of long shoots often somewhat zigzag, the winter buds inconspicuous and partially sunken into the twig; root nodules absent. Leaves appearing before the flowers, the petiole 1–3 cm long, the blade 1 or 2 times pinnately compound, the first leaves of the year evenly 1 time pinnately compound on the short shoots, the later-produced leaves 2 times pinnately compound on the long shoots. Stipules inconspicuous and scalelike, shed early. Leaflets alternate on the rachis, the margins sometimes minutely scalloped. Inflorescences spikelike racemes, solitary or clustered on the short shoots, arched or drooping; some trees with all staminate or all pistillate inflorescences, but often otherwise pistillate trees with some inflorescences having mixed imperfect and perfect flowers. Flowers perigynous, small, more or less actinomorphic. Hypanthium 2–3 mm long, cup-shaped to bell-shaped. Calyces of 3–5 sepals, these usually slightly unequal, similar in color and slightly shorter than the petals, not closing the flower in buds. Petals 3–5, slightly unequal, greenish white or occasionally slightly yellowish-tinged. Stamens 5–8, usually all fertile, the filaments 2–4 mm long, not fused, hairy toward the base, the anthers about 1.2 mm long, 0.5 mm wide, attached toward the midpoint. Style short, the stigma 2-lobed. Fruits legumes, relatively short or elongate, strongly flattened, short-stalked, straight or curved, sometimes twisted, 1- to many-seeded, indehiscent or dehiscing with age. Seeds elliptic to more or less circular, strongly flattened, brown to greenish brown; pleurogram absent. Twelve to 14 species, North America, South America, Asia.

Gleditsia is closely related to Gymnocladus (Gordon, 1966; Lee, 1976; Bruneau et al., 2001). Both are dioecious or incompletely dioecious trees with distributions disjunct between eastern Asia and eastern North America. The flowers of both genera are weakly differentiated into sepals and petals, and they share the unusual condition of sepals that do not cover the petals in the bud. These two genera were once considered primitive, ancient taxa dating back to the late Cretaceous (Polhill et al., 1981), but that idea is now changing with the advent of molecular data (Bruneau et al., 2001; Schnabel and Wendel, 1998) and reevaluation of the fossil record. It now appears that the simple, imperfect flower trait may be derived, and the disjunction may be of more recent origin, with the oldest reliable fossils from the Oligocene, approximately 25–35 million years ago (Schnabel and Wendel, 1998; Herendeen et al., 1992).

Gordon (1966) suggested that the two North American species of Gleditsia were each more closely related to different species occurring in ecologically similar regions of Asia than to each other. However, molecular studies have indicated that G. triacanthos and G. aquatica are sister species, and that they are in turn related to G. japonica Lodd. ex W. Baxter, a widespread Asian species (Schnabel and Wendel, 1998).