In general, only those characters have been used in the keys which can be observed without a magnifying glass and without dissection of the flower. In several groups of plants, reference is made to the fuller descriptions to be found in the Manuals. The standard manuals for Michigan are Gray's New Manual of Botany, 7th edition (American Book Company, $2.50), and Britton and Brown, Illustrated Flora of the Northern States and Canada (Chas. Scribner's Sons, $13.50). These books may be consulted in most school or public libraries. All dimensions are expressed in the metric system. For convenience, it may be stated that 25 millimeters (mm.) are about equal to one inch; 1 centimeter (cm.) to two-fifths of an inch; 1 decimeter (dm.) to 4 inches; and 1 meter (m.) is a little more than 3 feet. 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. GROUP 1, WOODY PLANTS 1a. Trees, with erect stem and central trunk, attaining a height of 6 m. (20 ft.) or more --2. 1b. Shrubs or woody vines, without true tree habit, or attaining heights of less than 6 m. (20 ft.) --52. 2a. Key for use in earliest spring, for trees which have flowers but no leaves --3. 2b. Key for use with trees bearing leaves --21.