| 1a. Small brown leafless plants, growing as parasites on the tamarack or black spruce | LORANTHACEAE, p. 25. |
| 1b. Aquatic plants, with all or most of the leaves submerged, or leafless — 2. | |
| 1c. Aquatic plants, with the leaves or the whole plant floating on or near the surface — 20. | |
| 1d. Terrestrial or marsh plants, without floating or submerged leaves — 29. | |
| 2a. Submerged aquatics, without leaves — 3. | |
| 2b. Submerged aquatics, with the leaves linear or dissected — 4. | |
| 3a. Flowers showy, yellow or purple | LENTIBULARIACEAE, p. 105. |
| 3b. Flowers small and inconspicuous, sessile, purplish or greenish | 3a, in HALORAGIDACEAE, p. 79. |
| 4a. Leaves linear or lanceolate, not lobed or dissected — 5. | |
| 4b. Leaves more or less lobed or dissected — 13. | |
| 5a. Leaves all basal — 6. | |
| 5b. Stem-leaves present — 7. | |
| 6a. Flowers blue, 1 cm. long or more; leaves cylindrical, blunt, hollow, partitioned lengthwise | 1a, in LOBELIACEAE, p. 113. |
| 6b. Flowers yellow; leaves minute | LENTIBULARIACEAE, p. 105. |
| 6c. Flowers white, in clusters; leaves linear-lanceolate, acute, not hollow | 1b, in ALISMACEAE, p. 3. |
| 6d. Flowers greenish, solitary at the end of elongated peduncles; leaves very long and ribbon-like, flat or trough-shape | 1a, in HYDROCHARITACEAE, p. 3. |
| 6e. Flowers minute, whitish or lead-color, in heads | ERIOCAULACEAE, p. 9. |
| 7a. Leaves alternate — 8. | |
| 7b. Leaves opposite — 10. | |
| 7c. Leaves whorled — 12. | |
| 8a. Leaves with thin sheathing stipules | 1b, in NAJADACEAE, p. 2. |
| 8b. Leaves without stipules — 9. | |
| 9a. Flowers greenish, in a head | SPARGANIACEAE, p. 2. |
| 9b. Flowers pale yellow | 1b, in PONTEDERIACEAE, p. 10. |
| 10a. Leaves serrate | 2b, in NAJADACEAE, p. 2. |
| 10b. Leaves entire — 11. | |
| 11a. Leaves 2 cm. long or less | CALLITRICHACEAE, p. 68. |
| 11b. Leaves thread-like, 2-8 cm. long | 2a, in NAJADACEAE, p. 2. |
| 11c. Leaves linear to elliptical, more than 2 cm. long | 1b, in NAJADACEAE, p. 2. |
| 12a. Leaves in whorls of 3, abruptly widened at the base | 2b, in NAJADACEAE, p. 2. |
| 12b. Leaves in whorls of 3, widest near the middle | 1b, in HYDROCHARITACEAE, p. 3. |
| 12c. Stems straight and erect, at leaves the flowers emerged; leaves in whorls of 4 or more | 2a, in HALORAGIDACEAE, p. 79. |
| 13a. Leaves with numerous small bladders attached, each bladder 1-3 mm. long | LENTIBULARIACEAE, p. 105. |
| 13b. Leaves without bladders — 14. | |
| 14a. Leaves alternate — 15. | |
| 14b. Leaves opposite or whorled — 18. | |
| 15a. Delicate and rare plants growing attached to stones in running water | PODOSTEMACEAE, p. 48. |
| 15b. Plants 2 dm. high or more, with roots in mud or sand — 16. | |
| 16a. Leaves once-pinnate — 17. | |
| 16b. Leaves 2-3 times pinnate | 15b, in UMBELLIFERAE, p. 81. |
| 16c. Leaves palmately dissected | 1b, in RANUNCULACEAE, p. 35. |
| 17a. Taste peppery or mustardy | 34, in CRUCIFERAE, p. 45. |
| 17b. Taste not peppery or mustardy | 1a, in HALORAGIDACEAE, p. 79. |
| 18a. Leaves pinnately compound | 3b, in HALORAGIDACEAE, p. 79. |
| 18b. Leaves palmately compound — 19. | |
| 19a. Leaves opposite or in whorls of four | 119a, in COMPOSITAE, p. 123. |
| 19b. Leaves in whorls of 5-12 | CERATOPHYLLACEAE, p. 34. |
| 20a. Plants small, flattened, rounded or ovate, without distinction of stem and leaf; the whole plant floating on or near the surface | LEMNACEAE, p. 9. |
| 20b. Plant attached to the soil, with differentiated stem and leaves — 21. | |
| 21a. Leaves on long slender stalks, which bear also a cluster of slender tubers near the leaf-base; leaves heart-shape | 1b, in GENTIANACEAE, p. 88. |
| 21b. Leaf-stalks without a cluster of tubers — 22. | |
| 22a. Leaves all basal — 23. | |
| 22b. Stem-leaves present — 24. | |
| 23a. Leaves parallel-veined | 1b, in ALISMACEAE, p. 3. |
| 23b. Leaves net-veined | 1b, in NYMPHAEACEAE, p. 35. |
| 24a. Leaves opposite or whorled — 25. | |
| 24b. Leaves alternate — 26. | |
| 25a. Leaves less than 2 cm. long | CALLITRICHACEAE, p. 68. |
| 25b. Leaves more than 2 cm. long | 1b, in NAJADACEAE, p. 2. |
| 26a. Leaves attached by the center to the stalk | 2b, in NYMPHAEACEAE, p. 35. |
| 26b. Leaves attached by the margin — 27. | |
| 27a. Leaves parallel-veined — 28. | |
| 27b. Leaves net-veined, with a single mid-vein; lanceolate or elliptical in outline | 29b, in POLYGONACEAE, p. 28. |
| 28a. Leaves not over 2 dm. long | 1b, in NAJADACEAE, p. 2. |
| 28b. Leaves very long and grass-like | 14a, in GRAMINEAE, p. 5. |
| 29a. Brown, yellow, or white plants, without green color — 30. | |
| 29b. Plants with normal green color, at least in some parts — 33. | |
| 30a. Stemless and leafless plants, consisting of flowers only and partly underground | 4a, in ARACEAE, p. 8. |
| 30b. Stems climbing on other plants | 1b, in CONVOLVULACEAE, p. 91. |
| 30c. Stem and flower-stalks erect, not climbing — 31. | |
| 31a. Corolla regular; stamens 6-12 | 1a, in ERICACEAE, p. 84. |
| 31b. Corolla irregular — 32. | |
| 32a. Sepals and petals each 3; flowers in simple racemes | 5a, in ORCHIDACEAE, p. 16. |
| 32b. Sepals 5; corolla of united petals | OROBANCHACEAE, p. 106. |
| 33a. Stem thick and fleshy, leafless, thorny | CACTACEAE, p. 77. |
| 33b. Stem not thorny — 34. | |
| 34a. Leaves none — 35. | |
| 34b. Leaves reduced to small scales — 40. | |
| 34c. Leaves thick and fleshy — 44. | |
| 34d. Leaves hollow — 45. | |
| 34e. Leaves small, all basal, bearing large glandular hairs on the upper surface | DROSERACEAE, p. 48. |
| 35a. Stem none, the flowers appearing at or partially beneath the surface of the soil | 4a, in ARACEAE, p. 8. |
| 35b. Stem present — 36. | |
| 36a. Stem freely branched | 2a, in LILIACEAE, p. 10. |
| 36b. Stem unbranched, except possibly in the flower-clusters — 37. | |
| 37a. Flowers greenish or brownish, without obvious colored petals — 38. | |
| 37b. Flowers with conspicuous white or colored petals — 39. | |
| 38a. Each flower with 6 small chaffy petals | 4, in JUNCACEAE, p. 10. |
| 38b. Petals none; each flower in the axil of a single chaffy bract | 2a, in CYPERACEAE, p. 7. |
| 39a. Flowers regular | 34a, in LILIACEAE, p. 11. |
| 39b. Flowers irregular | 10a, in ORCHIDACEAE, p. 16. |
| 40a. Stem erect, unbranched or with one or two branches only — 41. | |
| 40b. Stem freely branched — 42. | |
| 41a. Plants of moist soil, with opposite scales; corolla regular, with 4 petal-like lobes | 1a, in GENTIANACEAE, p. 88. |
| 41b. Swamp plants, with a few alternate scales; corolla irregular | LENTIBULARIACEAE, p. 105. |
| 42a. Leaves numerous and close, concealing the stem | 2a, in CISTACEAE, p. 74 |
| 42b. Leaves spreading, not concealing the stem — 43. | |
| 43a. Leaf-scales in small clusters; flowers greenish, with 6 petals | 2a, in LILIACEAE, p. 10. |
| 43b. Leaf-scales distinctly opposite; petals 5, yellow | 7a, in HYPERICACEAE, p. 74. |
| 44a. Sepals 2 | 3, in PORTULACACEAE, p. 34. |
| 44b. Sepals 4 or 5 | CRASSULACEAE, p. 48. |
| 45a. Leaves pitcher-shape, open at the top | SARRACENIACEAE, p. 48. |
| 45b. Leaves tubular, closed at the end | 37b, in LILIACEAE, p. 13. |