Plants can be divided into two classes:(A) those with evergreen leaves that usually last more than a single year, and (B) those with deciduous leaves that are shed annually. The evergreen class can be further subdivided into three groups: 1) leaves borne in clusters or bundles as found with the pines, 2) those with leaves opposite as found with the arborvitaes and junipers (often the individual leaves are very small and close observation is necessary to determine the arrangement), and 3) leaves alternating or scattered on the twigs as found with the spruces, hemlocks, and firs.
Deciduous plants can be divided into two groups: 1) those with leaves, leaf scars, or buds opposite (two) or in whorls (three or more leaves at a single point on the stem) and 2) those with leaves, leaf scars, or buds alternating along the stem.
Each group can be divided into four subgroups: a) leaves compound, those composed of two or more similar parts or leaflets (if these parts radiate from a single point, they are termed palmately compound; if they originate from different points along a leaf stem, they are termed pinnately compound), b) leaves lobed, c) leaves toothed, and d) leaves entire. This method of grouping is used in the key.
Branches and twigs are used extensively as identification points. Normally, plants have a monopodial branching habit, where the end bud continually takes the lead. In a few plants, such as the flowering dogwood, pagoda dogwood, and sassafras, the branching habit is termed sympodial. Sympodial branching is where the side bud takes the lead and the branches seemingly form in horizontal tiers.
The pith character is useful for identification purposes. Normally if a stem is cut in longitudinal sections, the pith appears to be continuous and of a uniform structure throughout. In some plants, such as Nyssa sylvatica or blackgum, the pith is interrupted at frequent intervals by partitions of firmer and darker tissue. Pith of this type is referred to as diaphragmed. In other plants, such as the black walnut, the parts of the pith between these firmer partitions have disappeared and have become hollow except for the partitions. This type of pith is referred to as chambered. Finally, in some shrubs the pith is entirely absent and is referred to as hollow.
Thorns are considered to be modified branches. Spines are considered to be
modified stipules, or outgrowths of the cortical tissues of the stem. Lenticels,
or breathing pores, appear as warty dots or patches on the surface of the stem
and frequently aid in identification. Bark characteristics, twig color, and
the presence or absence of hairs are other characteristics used to distinguish
between twigs of different trees.
The more bud characters are used, the more useful the key becomes. Its use extends into the winter months when the leaves are no longer present on deciduous plants. However, if the key was based on bud and twig characteristics alone, it would eliminate the possibility of dividing the plants into the several groups mentioned at the beginning of this section. In most cases, the bud characters used in this key can be determined with the naked eye. However, a hand lens will always help in identification. Bud characteristics are difficult to use from late spring to early fall when rapid growth occurs and foliage is shading twigs and retarding color development.
Buds occupy a lateral or terminal position on the stem. The lateral or axillary buds are those that develop in the leaf axis, or directly above the leaf scar. The terminal bud is formed on the end of the twig and denotes the end of the present year's growth. Often a number of buds develop at each node. These accessory, or collateral, buds are often a plump flower bud on either side of the leaf bud. Sometimes there are one or more buds above the normal lateral bud. This type of bud is known as a superposed bud.
Occasionally, plants have what appear to be terminal buds when really they are not. This condition is brought about when the end of the twig dies back and sloughs off to the nearest lateral bud, leaving a small stub that may only be visible using a hand lens. When a bud occurs in this position, it is termed a pseudo-terminal bud. This pseudo-terminal bud, or lateral bud, can be determined by the leaf scar directly below it. A true terminal bud has no leaf scar.
Buds are immature branches, miniature leaves, or flowers, usually with a protective covering. With a few plants, no covering is provided, and the miniature leaves are visible. This type of bud is known as a foliate (naked) bud. Buds are protected by scales of varying numbers. If the scales meet like a clam shell without any overlapping visible edges, they are termed valvate buds. When many overlapping scales are present, they are known as imbricated buds. The number of overlapping scales can vary from a few to many and can be used to distinguish between plants.
A complete leaf is composed of a blade (a leaf in common parlance), a petiole (leaf stalk), and stipules (appendages at the base of the petiole). Stipules are usually borne one on each side of the petiole. They may be persistent and stay as long as the leaf or be shed as the leaf expands and appear to be absent on the mature leaf.
The blade of the leaf varies in size, shape, and marginal features. These characteristics, together with color and extent of hairiness, are often used as identification points. Several degrees of hairiness exist. If no hairs are present, the leaf is glabrous. Other terms describing hairiness are: ciliate (margin fringed with hairs), pubescent (covered with short, soft hairs), silky (covered with adpressed, fine, and straight hairs), villous (covered with long, soft, usually curved hairs), tomentose (densely woolly), tomentum (dense covering of matted hairs), and glaucous (covered with bluish-white or bluish-gray, waxy bloom).
Leaf tips are: acute (sharp-pointed), acuminate (tapered or long-pointed), obtuse (rounded), truncate (flat), emarginate (shallow notch at tip), and cuspidate (coming to a sharp bristle-like tip). Leaf bases are: obtuse (rounded), acute (coming to a point), cuneate (coming to a sharp point), oblique (the leaf blade intersects the midrib at different points), and cordate (heart-shaped). Leaf shapes are: elliptic (outline of an ellipse–about two times as long as wide), lanceolate (lance-shaped–about four times as long as wide), linear (long and narrow with parallel sides), oblong (about three times as long as wide with nearly parallel sides), reniform (kidney-shaped), ovate (outline like a hen's egg), obovate (inversely ovate), and acicular (needlelike). A leaflet is a part of a compound leaf. Rachis is an axis bearing leaflets or flowers. Stomata are small openings in the epidermis or surface of leaf, and leaves bearing them are said to be stomatic or stomatiferous. The petiole varies in length and shape and sometimes helps in the identification of species.
Stipules vary a great deal in size, from mere threadlike appendages to those the shape and nearly the size of a small leaf. Stipules may or may not be persistent. When they fall, they leave a stipular scar on the twigs that is closely connected with the leaf scar. Stipular scars vary greatly in size and shape from a small dot to a line that encircles the twig. Stipules or their scars may be present or absent.
When leaves fall, they leave a scar varying greatly in size and shape. These are usually consistent within the genus. On the surface of the leaf scar are small dots or lines that denote the bundle scars (part of the original sap-conducting tissues). These bundle scars vary in number and pattern and are often used as characteristics for winter identification.
A few terms used to describe flowers are: catkin (scaly-bracted), spike (usually of unisexual flowers), bract (modified and reduced leaf subtending a flower cluster), and peduncle (stalk of a flower cluster). Several terms used to describe fruit are: drupe (a fleshy fruit with a single seed covered with a bony, inner part), samara (a winged seed), and pod (a bean-like fruit opening on one side).
Measurements in standard botanical works are usually given in the metric system. In this key, the figures have been roughly converted to the English system to aid the reader. Equivalents are: 1 millimeter (mm) equals approximately 1/25 of an inch, 10 millimeters equals 1 centimeter (cm); 2 1/2 centimeters equals approximately 1 inch; 1 meter equals approximately 40 inches.