Key to Groups
| 1a. Trees, shrubs, or woody climbers, with stems which last from year to year | Group 1, WOODY PLANTS, p. ix |
| 1b. Herbaceous plants, with stems which live above ground only a single season — 2. | |
| 2a. Plants with unusual habits or structures, including leafless, colorless, submerged, floating, parasitic, or hollow-leaved plants | Group 2, UNUSUAL PLANTS, p. xxiii. |
| 2b. Ordinary terrestrial or swamp plants, without unusual structural peculiarities — 3. | |
| 3a. Leaves parallel-veined (or net-veined in a few species); parts of the flower usually in threes or sixes, never in fives; wood-fibers scattered through the stem; seed with one cotyledon. All plants with definitely parallel-veined leaves may be identified through this division, unless the parts of the flower are distinctly in fives. | Group 3, MONOCOTYLEDONES, p. xxvii. |
| 3b. Leaves net-veined (or parallel-veined in a few species); parts of the flower usually in fours or fives; wood-fibers arranged in a circle in the stem; seeds with two cotyledons. All plants with definitely net-veined leaves may be identified through this division. | Group 4, DICOTYLEDONES, p. xxx. |
Note:—In order to avoid possible chances of error, many plants have been treated under both of the above groups. The following hints may also be useful in distinguishing Groups 3 and 4:
All herbaceous plants with deeply lobed, dissected or compound leaves may be sought under the Dicotyledones.
All herbaceous plants with five stamens in each flower, or with seven or more stamens and one ovary in each flower, may be sought under Dicotyledones.