d. Margin of leaves entire or only slightly undulate.

e. Leaves heart-shaped or rounded; fruit a legume. CERCIS, p.

167.

ee. Leaves oval, ovate or obovate; fruit not a legume.

f. Branches armed with stout, straight spines; fruit large, orange-like. MACLURA, p. 133.

ff. Branches without spines; fruit small, not orange-like.

g. Fruit an acorn. QUERCUS, p. 96.

gg. Fruit a drupe or berry.

h. Twigs spicy-aromatic when bruised; leaves of many shapes on the same branch. Sa.s.sAFRAS, p. 139.

hh. Twigs not spicy-aromatic; leaves not of many shapes on the same branch.

i. Leaves thick, abruptly pointed, very l.u.s.trous above, not cl.u.s.tered at the ends of the branches. NYSSA, p. 209.

ii. Leaves thin, long-pointed, not l.u.s.trous above, cl.u.s.tered at the ends of the branches. CORNUS, p. 202.

dd. Margin of leaves serrate, toothed or lobed.

e. Margin of leaves serrate to toothed.

f. Branches armed with stiff, sharp thorns. CRATAEGUS, p. 151.

ff. Branches not armed.

g. Base of leaves decidedly oblique.

h. Leaf-blades about as long as they are broad, heart-shaped.

TILIA, p. 201.

hh. Leaf-blades 1-1/2 - 2 times as long as they are broad, oval to ovate.

i. Leaves thin, coa.r.s.ely but singly serrate; fruit a globular drupe, ripe in autumn. CELTIS, p. 131.

ii; Leaves thick, coa.r.s.ely and doubly serrate; fruit a samara, ripe in spring. ULMUS, p. 122.

gg. Base of leaves essentially symmetrical.

h. Teeth coa.r.s.e, 2-5 per inch of margin.

i. Leaves very glabrous both sides; fruit a p.r.i.c.kly bur.

j. Leaves 3-5 inches long, very l.u.s.trous beneath; bark close, smooth, steel-gray. f.a.gUS, p. 93.

jj. Leaves 6-8 inches long, not l.u.s.trous beneath; bark fissured, brownish. CASTANEA, p. 95.

ii. Leaves p.u.b.escent or white-tomentose, at least beneath; fruit not a p.r.i.c.kly bur.

j. Leaves 2-4 inches long, broadly ovate to suborbicular; fruit a very small capsule, falling in spring. POPULUS, p. 44.

jj. Leaves 4-7 inches long, oblong-lanceolate to obovate; fruit an acorn, falling in autumn. QUERCUS, p. 96.

hh. Teeth fine, 6-many per inch of margin.

i. Leaf-petioles laterally compressed; leaves tremulous.

POPULUS, p. 44.

ii. Leaf-petioles terete; leaves not tremulous.

j. Leaf-blades at least 3 times as long as they are broad.

k. Twigs brittle; fruit a very small capsule, falling in spring. SALIX, p. 34.

kk. Twigs tough; fruit a fleshy drupe, falling in late summer or autumn. PRUNUS, p. 152.

jj. Leaf-blades not more than twice as long as they are broad.

k. Leaf-blades about twice as long as they are broad.

l. Margin of leaves singly serrate; fruit fleshy.

m. Lenticels conspicuous; pith whitish or brownish; bark easily peeled off in papery layers; buds ovoid. PRUNUS, p.

152.

mm. Lenticels inconspicuous; pith greenish; bark not separable into papery layers; buds narrow-conical. AMELANCHIER, p. 149.

ll. Margin of leaves doubly serrate; fruit not fleshy.

m. Trunk fluted; fruit inclosed within a halberd-shaped involucre. CARPINUS, p. 83.

mm. Trunk not fluted; fruit not inclosed within a halberd-shaped involucre.

n. Bark of trunk gray-brown, broken into narrow, flattish pieces loose at the ends; fruit in hop-like strobiles. OSTRYA, p. 81.

nn. Bark of trunk white, yellow or dark brown, platy or cleaving off in papery layers; fruit not in hop-like strobiles. BETULA, p. 84.

kk. Leaf-blades almost as broad as they are long.

l. Lower side of leaves more or less downy; sap milky; leaves not crowded on short, spur-like branchlets; fruit berry-like, black. MORUS, p. 135.

ll. Lower side of leaves glabrous; sap not milky; leaves crowded on short, spur-like branchlets; fruit a large, green pome. PYRUS, p. 142.

ee. Margin of leaves distinctly lobed.