| a. | Leaves simple. |
| | b. | Leaves needle-shaped, awl-shaped, strap-shaped or scale-like. |
| | | c. | Leaves in clusters of 2-many. |
| | | | d. | Leaves in clusters of 2-5, sheathed, persistent for several years. | PINUS, p. 4. |
| dd. | Leaves in fascicles of 8-many, on short, lateral branchlets, deciduous in autumn. | LARIX, p. 17. |
| cc. | Leaves solitary, not clustered. |
| d. | Leaves opposite. |
| | | | | e. | Twigs flattened; leaves all of one kind, scale-like, decurrent on the stem; fruit a small, pale brown cone. | THUJA, p. 31. |
| ee. | Twigs essentially terete; leaves of two kinds, either scale-like, or else awl-shaped, often both kinds on the same branch, not decurrent on the stem; fruit berry-like, bluish. | JUNIPERUS, p. 33. |
| dd. | Leaves alternate or spirally-whorled. |
| e. | Leaves flattened, soft to the touch. |
| | | | | | f. | Leaves 1/2-1-1/4 inches long, sessile, aromatic; cones 2-4 inches long; bark of trunk with raised blisters containing resin. | ABIES, p. 27. |
| ff. | Leaves seldom over 1/2 inch long, short-petioled, not aromatic; cones about 3/4 inch long; bark of trunk without raised blisters. | TSUGA, p. 29. |
| ee. | Leaves 4-sided, harsh to the touch. | PICEA, p. 18. |
| bb. | Leaves broad and flat. |
| c. | Leaves alternate or clustered, never opposite nor whorled. |
| 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. | SASSAFRAS, p. 139. |
| hh. | Twigs not spicy-aromatic; leaves not of many shapes on the same branch. |
| i. | Leaves thick, abruptly pointed, very lustrous above, not clustered at the ends of the branches. | NYSSA, p. 209. |
| ii. | Leaves thin, long-pointed, not lustrous above, clustered 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, coarsely but singly serrate; fruit a globular drupe, ripe in autumn. | CELTIS, p. 131. |
| ii; | Leaves thick, coarsely and doubly serrate; fruit a samara, ripe in spring. | ULMUS, p. 122. |
| gg. | Base of leaves essentially symmetrical. |
| h. | Teeth coarse, 2-5 per inch of margin. |
| i. | Leaves very glabrous both sides; fruit a prickly bur. |
| | | | | | | | | | j. | Leaves 3-5 inches long, very lustrous beneath; bark close, smooth, steel-gray. | FAGUS, p. 93. |
| jj. | Leaves 6-8 inches long, not lustrous beneath; bark fissured, brownish. | CASTANEA, p. 95. |
| ii. | Leaves pubescent or white-tomentose, at least beneath; fruit not a prickly 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. |
| f. | Fruit an acorn. | QUERCUS, p.96. |
| ff. | Fruit not an acorn. |
| g. | Leaves fan-shaped, with many fine veins radiating from the base of the blade. | GINKGO, p. 3. |
| gg. | Leaves not fan-shaped, without many fine veins radiating from the base of the blade. |
| h. | Leaf-lobes entire. |
| i. | Leaf-petioles 5-6 inches long; leaves lustrous above; twigs not aromatic when bruised. | LIRIODENDRON, p. 137. |
| ii. | Leaf-petioles about 1 inch long; leaves dull above; twigs spicy-aromatic when bruised. | SASSAFRAS, p. 139. |
| hh. | Leaf-lobes sinuate-toothed to serrate. |
| i. | Leaf-lobes coarsely sinuate-toothed. | PLATANUS, p. 141. |
| ii. | Leaf-lobes serrate. |
| j. | Branches armed with stiff, sharp thorns; sap not milky. | CRATAEGUS, p. 151. |
| jj. | Branches unarmed; sap milky. | MORUS, p. 135. |
| cc. | Leaves opposite or whorled. |
| d. | Margin of leaves entire or only slightly undulate. |
| e. | Leaves 3-5 inches long; spray fine; fruit an ovoid, scarlet drupe. | CORNUS, p. 202. |
| ee. | Leaves 5-12 inches long; spray coarse; fruit a long, slender-cylindrical capsule. | CATALPA, p. 222. |
| dd. | Margin of leaves serrate to lobed. |
| e. | Margin of leaves finely serrate. | VIBURNUM, p. 229. |
| ee. | Margin of leaves distinctly lobed. | ACER, p. 172. |
| aa. | Leaves compound. |
| b. | Leaves alternate. |
| c. | Leaves simple-pinnate. |
| d. | Branchlets armed with short, sharp prickles. | ROBINIA, p. 169. |
| dd. | Branchlets unarmed. |
| e. | Leaflets entire with the exception of 2 or more coarse, glandular teeth at the base. | AILANTHUS, p. 171. |
| ee. | Leaflets serrate the entire length. |
| f. | Upper leaflets less than 1 inch broad. |
| g. | Trunk and large branches armed with stout spines; leaflets 3/4-1-1/2 inches long. | GLEDITSIA, p. 165. |
| gg. | Trunk and large branches unarmed; leaflets 2-3 inches long. | PYRUS, p. 142. |
| ff. | Upper leaflets 1-5 inches broad. |
| g. | Leaflets 5-11; pith homogeneous. | CARYA, p. 66. |
| gg. | Leaflets 11-23; pith chambered. | JUGLANS, p. 60. |
| cc. | Leaves bi-pinnate. |
| d. | Trunk and large branches armed with stout spines; leaflets 3/4 - 1-1/2 inches long. | GLEDITSIA, p. 165. |
| dd. | Trunk and large branches unarmed; leaflets 2 - 2-1/2 inches long. | GYMNOCLADUS, p. 163. |
| bb. | Leaves opposite. |
| c. | Leaves pinnately compound; fruit a samara. |
| d. | Leaflets 3-5; samaras paired. | ACER, p. 172. |
| dd. | Leaflets 7-11, exceptionally 5; samaras not paired. | FRAXINUS, p. 210. |
| cc. | Leaves digitately compound; fruit a prickly bur. | AESCULUS, p. 194. |
