Leaf Anatomy: External Structures


Introduction
Missouri is home to a number of diverse communities, including deciduous forest, prairie, southern pine forest, glades, swamps, and more. Since eastern Missouri is mainly deciduous forest,we are going to focus on this ecosystem. The appendix contains a leaf key with pictures of leaves and other tree identification characteristics. Using this information as an aid, we will learn the basic identification characteristics of approximately 40 species of native Missouri trees, as well as one vine. To do this we are going to go out around campus, the Science Center, and Forest Park to “meet the trees”...

Leaf Characteristics/Terminology
Leaves are often crucial to the identification of a tree. Some leaves have smooth edges while others have rough edges called “teeth”. Some leaves are round while others have odd shapes. Some leaves are subdivided into smaller leaflets, such as palm leaves are. A leaf is the entire green structure that grows from a bud on a stem of a plant, which falls or is shed from the plant when it becomes old. There is a bud at the base of every leaf where it attaches to the stem. Below is a leaf outline with some helpful features noted:

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A. simple untoothed leaf----------B. simple toothed leaf
In each of the above leaf outlines, the leaf surface is one connected and contiguous surface and thus the leaves are termed “simple”. Leaf A is untoothed but does have lobes. The spaces between the lobes are called sinuses. Leaf B is toothed but has no lobes or sinuses. Note that the bottom of leaf B is also lopsided. Many leaves are not symmetrical and the asymmetry of their features is often helpful in their identification. The petiole is the green leaf structure that connects the leaf to the stem. At the base of the petiole is where a bud is found for leaf replacement in the spring.


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C. compound leaf (whorled)------D. compound leaf (pinnate)
Leaves C and D are called compound leaves because the entire green surface of the leaf is broken up into areas that are not contiguous. The divided areas of the leaf are called leaflets. The leaflets of Leaf C are attached to the petiole at one area. This is called a whorled compound leaf. Leaf D has leaflets that are attached to the petiole at different areas. The rachis is the section of the petiole where the leaflets are attached. Note: Leaf C is one leaf. Leaf D is one leaf. At the base of leaflets there are no buds. The rachis/petiole of compound leaves is green and falls from the tree when the leaf is old or the season approaches winter...
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E.------------------F.-----------------G.-----------------H.


The area where a leaf attaches to a stem is called a node. Leaves are attached to stems according to the particular pattern of the species of tree/plant to which they belong. All leaf types tend to attach to stems in one of 3 ways on a particular plant. In Arrangement E, simple leaves are attached to a stem in the most common way - the alternate way. Most trees in Missouri have an alternate leaf arrangement. Arrangement F demonstrates simple leaves attached to a stem in an opposite way with two leaves per node. There are some trees with 3 leaves per node but this is not demonstrated here. Compound leaves also exhibit specific leaf arrangements - most are alternate. The leaves in G are attached to the stem in pairs (2 per node) and are therefore opposite while those in H are attached singly (one per node) and are thus alternate.

Leaf attachment per node is a simple way to distinguish some plants that have very similar leaves. For example, boxelder saplings often look identical to poison ivy saplings because the two have nearly identical leaves but there is a distinct difference in leaf arrangement: boxelder has opposite leaves while poison ivy has alternate ones. Simple facts like this can make plant identification easy.

Definitions:
lobe: large outward extension of the edge of a leaf
sinus: air space between 2 lobes
petiole: the part of a leaf that connects a leaf to a stem
teeth: the rough, often pointed, small protrusions along the edges of some leaves
simple leaf: leaf that is not subdivided into separate areas
compound leaf: leaf that is subdivided into separate unconnected areas (whorled or pinnate)
leaflet: one of the subdivided areas of a compound leaf
rachis: the extension of the petiole of a pinnate leaf which connects the leaflets to one another
node: the area where a leaf attaches to a stem
opposite leaves: when there are two leaves attached per node on a stem
alternate leaves: when there is only one leaf attached per node on a stem