Norton Brown Herbarium
Part 7: Leaf Pubescence and Other Characteristics
Introduction
Leaves are usually the most recognizable parts of a plant and can be useful for plant identification.  Unique combinations of shape , color, tips, bases, or hairs make leaves great indicators of plant identities.        Simply recognizing if the position of leaves on the stem is opposite or alternate to each other can greatly narrow down the number of possibilities for the identity of a particular plant.  Other useful features are whether the leaf is simple or compound (made up of several parts), or if it is deciduous (falls off in autumn) vs. evergreen.  Because there are so many features to consider, the terminology can become a bit overwhelming.  In this guide, we will present discriptions and visuals of many of the terms you will see in the interactive key .
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Part 1: A Leaf Defined
A leaf is basically the power generator of a plant.  Leaves have specialized cells that carry out photosynthesis when exposed to sunlight.  In photosynthesis sunlight, water supplied by the roots, and carbon dioxide from the air produce oxygen and sugars (stored plant food).
Although there are several types of leaves , a typical leaf has a blade (broad, flattened portion) and a stalk called the petiole.  A leaf without a petiole, where the blade is directly attached to the stem or the branch, is called sessile.  Any part of the leaf can be pubescent (hairy), glabrous (hairless), or can have glands, which are structures that excrete oil or nectar (e.g., on leaf petioles of cherry trees).  Each leaf is associated with a bud that sits at the base of the petiole where it is attached to the stem ( see Picture 11 ).
At the base of the petiole, there may also be a stipule or what looks like a small leaf or two.  A stipule will frequently shed early in the season or it may persist.  Some stipules extend around the entire stem, leaving a scar that encircles the axis when the stipule drops off, such as in Magnolias.
The visible lines running through the leaf blade are veins that carry the water and nutrients involved in photosynthesis to and from the rest of the plant.  The network of veins includes a midvein (aka midrib) that runs through the center of the leaf.  Secondary veins branch off from the midrib and tertiary veins branch off the secondaries.  The patterns formed by the veins are called the leaf venation.  If the veins branch from along the midvein, as in Picture 1, then the leaf has pinnate venation.  If the veins originate from one point at the base of the blade, as seen in Picture 4, the leaf has palmate venation.
The edge of the blade is called the leaf margin.  Here, you may find teeth, or serrations, that can be useful in plant identification. Terms for different types of leaf margins can also be found here .
leaf blade
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Part 2: Leaf Type
After defining the plant habit (is it a tree? a shrub? a vine?), classifying the leaf type is among the easiest and most important steps in identifying a plant.
Broad-Leaved plants are the most common plants in the Mid-Atlantic region.  These are plants with wide leaf blades exposing a large surface area to the sun for photosynthesis and examples include the oaks, maples, and dogwoods.  Alternatively, firs, pines and spruces, have needle-like photosynthetic organs. Scale-like leaves of junipers or cedars are comparatively short, overlapping, and resemble the scales of a fish.
Picture 2:  Types of leaves
Most Broad-Leaved trees and shrubs in the Mid-Atlantic region are deciduous, that is, they shed their leaves in the fall every year and remain dormant throughout the winter.   However, there are a few Broad-Leaved trees that are evergreen, which means that they keep at least some leaves throughout the entire year.
Picture 3: Leathery Magnolia leaves
Southern magnolia trees and rhododendron shrubs have broad leaves that are thick and leathery and can survive the cold and dry winters in this region.
Most needle-like and scale-like leaves are evergreen.  Exceptions are the needle-like but deciduous leaves of the American and Eurasian larches that turn a bright yellow before shedding in the fall.
Part 3: Leaf Type: Simple and Compound
Picture 4: A simple leaf.
Broad-leaved plants can be simple or compound.  Oaks and most maples are examples of trees with a simple leaf where a single, undivided blade is attached to the stem by a petiole (leaf stalk). There is a bud (next year's growth) in the leaf axil at base of the petiole where it connects to the stem. The axil is the petiole/stem juncture that forms an angle, while the entire juncture is the node.
If a leaf is divided into distinct smaller parts (called leaflets), it is a compound leaf.  The stalk of a leaflet is called petiolule, not to be confused with the petiole, which is the stalk of the entire leaf.  You will never find a bud where the petiolule (leaflet) connects, but only at the node where the petiole meets the stem ( see Picture 11 ).  There are several types of compound leaves. Palmately compound leaves are made up of leaflets that are all attached at the same point where the petiole meets the blade.
A blade composed of 5 leaflets
Picture 5: A palmately compound leaf.
Pinnately compound leaves are also composed of leaflets, but these are connected to a main axis called a rachis (looks like the continuation of the petiole).  If there is a single terminal leaflet on the rachis, the leaf is odd-pinnately compound. When there is a pair of leaflets at the tip of the rachis the leaf is even-pinnately compound (i.e., the leaf is composed of an even number of leaflets).  If the leaf is twice compound because the rachis branches, it is bipinnately compound.  If the leaf is three times compound, then it is tripinnately compound.
A blade divided into 3 leaflets is trifoliate.  A good trifoliate leaf to know in the Mid-Atlantic region is poison ivy (“leaves of three, leave it be”).
Picture 6: A once odd
pinnately compound leaf.
Picture 9: Tripinnately compound leaf.
Picture 10: A trifoliate leaf.
A note of caution: It may be difficult at first to tell a leaf from a leaflet, that is, if a leaf is simple or compound.  If there is a petiole leading to an undivided leaf blade, then you have a simple leaf, but if there are leaflets it is a compound leaf.  As mentioned before, the simplest way is to look for a bud in the axil of the leaf (where the petiole joins the axis).  A leaf will have a bud (called a lateral or axillary bud) in this location; a leaflet will not (see picture 11 and Appendix III: Stems and Twigs ).  The leaflet-stalk (petiolule) is attached to the rachis that bears all of the leaflets and is connected to the petiole.
Picture 11: A single ash leaf that is pinnately compound.  To know if a leaf is compound or simple, look at the location of the bud.  A bud only occurs at the base of the leaf and there are no buds at the bases of leaflets.  The rachis is the axis that connects the leaflets.
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Leaf Arrangement
There are different terms for the ways that leaves are arranged along the axis of the stem (not to be confused with the rachis of a compound leaf).  Most woody plants have an alternate leaf arrangement where there is only one leaf at each node.  Here the alternately arranged leaves either spiral down the stem with minimal overlap, allowing for the most surface area to be exposed to sunlight, or they do not spiral and are arranged in one plane (distichous).
Where leaves have an opposite arrangement, pairs of leaves are borne opposite each other at the same node.  There are fewer woody plants with opposite rather than alternate leaf arrangement.  There is a popular mnemonic device for remembering trees and shrubs of some families with oppositely arranged leaves, which is MADCAP Horse.  While it does not cover all of the woody plants with opposite leaves, it is a helpful general rule for some groups.  MADCAP Horse stands for:
M – Maple
Cap – members of the family Caprifoliaceae, such as honeysuckle and
       former members of this family, the viburnums (now in Adoxaceae)
Horse – Horsechestnut
When the pairs of opposite leaves are arranged along the stem perpendicular with respect to the next leaf pair, again for maximum exposure to sunlight, this is called decussate.  Leaves that are almost, but not quite, opposite are termed subopposite.  If there are three or more leaves at the same node, then the leaf arrangement is whorled.
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Part 5: Leaf Shapes
Some common leaf shapes are round, egg-shaped (ovate), triangular (deltoid), or heart-shaped (cordate).  It is important to note that within species and even individual plants you may see more than one typical leaf shape, and most plants will have several possible leaf shapes.  This depends on environmental factors (light, soil conditions, water availability) or genetic variability.  We present the most common leaf shapes below and in the interactive key .
The shapes are generally determined by the location of the widest part of the leaf, as indicated by the lines in the pictures below.  The prefix "ob-" signals the reverse of the indicated shape.  For example, an ovate leaf has the widest part of the leaf at the base and an obovate leaf differs in that the widest part of the leaf is toward the tip.
Picture 15: Ovate (egg-like) leaf shape. The base is the widest part of the leaf and the tip is narrower but rounded.
Picture 16: Lanceolate leaf. The widest part of this leaf shape is at the middle while both ends are tapered (the tip more acutely so).
Picture 17: Obovate leaf. The widest part of the leaf is towards the tip
Picture 18: A cordate leaf. Heart-shaped, where the base is notched and with a tapering tip.
Picture 19: Elliptical leaf shape. An oval leaf where the middle is the widest part and both ends taper evenly.
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Part 6: Leaf Tips, Bases and Margins
Besides basic shape, leaves have a number of other physical characteristics that are useful for identification.  For example, the tips of leaves may be rounded, notched, or long and tapering.  Picture 20 shows the leaf tips you will encounter in the key.  The most common are acute (sharp angle), acuminate (coming to a long point), and obtuse (rounded).  Leaf bases are shown in Picture 21.
Picture 20: Leaf tips.
Picture 20: Leaf bases.
The margins of leaves (edges) may be toothed or without teeth (entire).  There are many technical terms describing types of teeth.  A leaf margin is serrate if the teeth point forward towards the tip or dentate if the teeth point outwards.  A margin with small serrations is serrulate and one with a tooth within a tooth is doubly serrate.  If a leaf margin has rounded teeth, then it is crenate. Here are some common leaf margins.
Picture 23: Features of an oak leaf.
Several species of oaks and maples have unique and easily identifiable leaf shapes, because of the placement of projections called lobes and indentations that are called sinuses (Picture 23).  A leaf can sometimes be identified by the number of lobes and unique structures found on the lobe.  For example, some species of oaks have sharp-pointed lobes that are tipped by a bristle (the red oak group), such as the leaf in Picture 23. Other species of oaks have rounded lobes and lack a bristle at the tip (the white oak group).
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Part 7: Leaf Pubescence
Any part of a leaf can be pubescent (hairy), but more often the underside of the blade is pubescent rather than the upper surface.  The texture, cell-type, color and location of hairs can be important characteristics for plant identification.  There are as many technical terms for hairs as there are botanists in the world, and it is often best to describe the hair type in detail rather than resort to a term that may have different meanings from author to author.
Leaves that are not pubescent are glabrous, or hairless.  Occasionally, leaves (as well as stems and fruit) may have a whitish bloom (glaucous) that easily rubs off.  This is a waxy layer and in some species of bayberries the bluish bloom can be wiped to green (see Picture 24).