A Word About Leaves Throughout this website, we have attempted to maintain a uniform format in the description of the positioning and appearance of each tree’s leaves. You will see first the arrangement of the leaves. That is, the position of a leaf in reference to another leaf along the stem. There are basically four categories: opposite, sub-opposite, alternate, and whorled. Each is illustrated and described below. Opposite Opposite leaves are directly accross from each other on the stem. Sub-opposite A condition where the leaves are not directly opposite, nor spaced far enough apart to be considered alternate. Needle or Scale Needles are narrow, frequently pointed and also vary in length. They can be round or flat in cross section and grow as a single needle or in bundles. Scales are short and flat and grow close to the stem. Alternate Alternate leaves are spaced in alternating fashion along the axis of the stem and are seldom, if ever, directly across from each other. Whorled A condition where three or more leaves are present at one spot on the stem. Leaf Morphology Leaf morphology is described as a way to establish a concise mental picture of the leaf as it appears on its stem. There are two main categories: simple and compound and three sub-categories within compound—palmately compound, pinnately compound, and bipinnately compound. These are illustrated and described below. Note: The position of the bud determines whether the leaf is simple or compound. Simple Leaves The bud is located in the axil of a single leaf and the stem. The papery part of the leaf is not divided into sub-parts. Compound Leaves These can be composed of anywhere from 3 to 1500 leaf-like structures. Palmately Compound Each leaflet is attached at a common point. Pinnately Compound The bud is located in the axil of a structure with more than one leaf (or leaflet) attached. Bipinnately Compound These are leaves that are divided twice. The leaf (or leaflet) of a pinnately compound leaf, now has another leaf-bearing axis to which additional leaflets are attached. Leaf Margins This key uses leaf margins as a way to identify one tree from another. A leaf margin is simply the edge of the leaf. Edges can be smooth, have different types of teeth, be wavy or incised, or have lobes. Here are the classifications used in this key.