The Language of Leaves
By Dr. Leonard Perry, Extension Professor
University of Vermont
Do the words obovate, acicular, or sinuate mean anything to you? If not, perhaps it's time to learn a little about the "language of leaves."
These are some of the many terms you may run across when reading about plants or trying to identify them. Many of the terms on plant structure, used for identification, apply to the leaves. The terms can be grouped into those applying to the leaf blade, the leaf shape, leaf margins, leaf divisions, and leaf surfaces.
Depending on the arrangement of the leaf blade on the stem, leaves may be across from each other (opposite), alternating up the stem (alternate), or with several leaves arising around the stem (whorled) from the same point (node) as in the Crown Imperial (Fritillaria imperialis). Leaves may arise from the stem, from the base of the plant (basal) as in many ornamental grasses, or with several leaves at the base (rosette). The latter is often what is seen with perennials in late fall--the leaves that overwinter.
The part of the leaf that attaches the leaf blade to the stem is called the "petiole." If there is no petiole, rather the leaf blade attaches directly to the stem, the leaf is "sessile" as in the upper leaves of the Black-eyed Susan (Rudbeckia). If the leaf stalk (petiole) clasps the stem, going part or all the way around it, the terms "sheathing" or "clasping" are used. Sometimes the petiole is curled upward, forming a "V" or "U" shape in cross section. Then the petiole is called "channeled."
There are many shapes of leaves. Linear means narrow, being the same width throughout, and the leaf blade being four times or more as long as wide. If similar, but less long, the leaf is called oblong. If similar to linear, only more narrow, it is acicular. Egg-shaped is ovate, as in many violets, while the reverse (leaf is broadest above the middle) is obovate. Lance or sword-shaped is "lanceolate" as in Lilium lancifolium. Circular, like an orb, is "orbicular." And so the list goes on. Good plant references usually have these terms defined, often with drawings as well.
Margins of leaves are often seen in descriptions. They can be smooth or "entire," wavy or "sinuate," or with indentations called "teeth." Toothed leaves can be further subdivided with such terms as "serrate" (sharply cut teeth pointing forward), "crenate" (broad, rounded teeth) as in speedwells, or "dentate" (teeth pointed outwards, like steps), among others.
If the leaf is not divided, it is said to be "entire." A compound leaf has several distinct divisions--once, twice, or more. The Astilbe has three divisions, so is "ternately compound." If these divisions arise from stem divisions they are "pinnate," if from the same point as in a palm leaf they are "palmate," as in the Meadowsweet Filipendula palmata.
Finally there are the leaf surfaces, smooth or "glabrous" perhaps being the most common. There are several terms for a waxy coating, giving a "glaucous" effect as in many sedums and iris. More terms exist for leaf hairs, such as "tomentose" or "rugose." Note the hairs the next time you see a "rugosa" rose of the goldenrod Solidago rugosa.
Knowing terms such as these for leaves, and similar terms for flowers, will help you identify the plants you see. And it will help make the world of plant names less threatening and confusing.
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