How to Examine a Plant
When you come across a new plant in the forest, how do you go about studying and identifying it? The following routine provides a systematic way of examining a plant specimen in order to determine its identity to the family, genus, or even species level. Follow “the routine” in conjunction with the Key to Plant Families, The Matrix, or other botanical key to identify your specimen. If you think you know what family a plant belongs to, you can also jump to the appropriate plant family page. All you need for starting material is a branch and some leaves.
Is your specimen a monocot or dicot? Monocots have parallel leaf venation and usually have strap-like leaves. Some, like palms and aroids, have large leaves with pinnate venation, but the secondary veins are parallel. Dicots have veins that fan out in various netted patterns and are usually broad-leaved.
How are the leaves arranged along the stem (phyllotaxy)? Are they alternate? Opposite? Or are there 3 or more leaves at a node arranged in a whorl? Alternate leaves may be distichous, coming off the stem in two ranks in a well-defined left-right-left-right pattern. In other cases alternate leaves may be spirally arranged. Opposite-leaved plants are often decussate, meaning that one pair of opposite leaves is offset by 90° from the next pair of leaves.
- If compound are they pinnate (feathery with multiple leaflets coming off of an elongated continuation of the petiole known as a rachis) or palmate (hand-like with leaflets radiating from a single point)? (note that the “stem” of a leaflet is called the petiolule)
- If pinnate, are they mono- or bi-pinnate? (in bipinnate leaves, the leaflets themselves are pinnate)
- If mono-pinnate, do they have an even number of leaflets occurring in pairs along the rachis (paripinnate)? Or do they have an odd number of leaflets with a terminal leaflet at the end of the rachis (imparipinnate)?
- If palmately compound, how many leaflets do they have?
Are stipules present or absent? For some reason, much of tropical botany seems to hinge on these seemingly insignificant flaps of tissue, or the scars that they leave behind. Use a hand lens! Some are terminal, meaning they are located at the tip of a branch. Most are located where the petiole meets the stem. They are usually tiny and usually fall off by the time the leaf has matured. The only evidence of their presence may be a tiny scar.
Do cut stems or severed petioles yield exudations? Are the exudations clear and watery? Milky white? Yellow, orange, or red? Do exudations flow freely or do they need to be squeezed from the petiole?
Do crushed petioles exude slightly sticky mucilage? Chew up a petiole and then feel between thumb and forefinger while looking for sticky strands as you spread your fingers apart. (Don’t make the mistake of chewing on a leaf that clearly has milky white or colored latex, as it’s likely to be highly toxic! There is a real, but subtle, difference between mucilage and latex exudations.)
Is there an odor? Crush up the leaves and smell them. Or tear off a leaf and immediately smell the broken petiole. What kind of odor do you detect? Primitive “ranalean odor”, anise, turpentine, unripe avocado, guava, eucalyptus, rank, sweet, green bean, or just plain nasty? Or like crushed leaves?
Examine the upper and lower surfaces of the leaf carefully with a hand lens (an essential piece of field equipment). Experiment by holding the leaf up to the light. Are translucent dots, also called pellucid dots, visible? Are the pellucid spots only in the shape of round dots or are some shaped like dashes? Are pellucid dots larger at the margin of the leaf or uniform in size? Leaves with pellucid dots often have an odor as well.
How are the primary, secondary, and tertiary veins arranged?
This routine for studying a plant was developed by Dr. Humberto Jimenez-Saa. If you can answer most of the questions, then you should be able to determine the family of your plant specimen using Susan Letcher’s Key to Plant Families, The Matrix developed by Dr. Humberto Jimenez-Saa, or the key in Alwyn Gentry’s “A Field Guide to the Families and Genera of Woody Plants of Northwest South America”.