“U Guesam”, or, Advice on Keying Plants

by Chris Laning

Using a good key is the most reliable way to identify a plant—if you know how. Keying takes practice, and it’s certainly an art that comes easier to some people that others. But it’s not some deep, dark mystery. Basic intelligence, common sense, and determination are the prerequisites.

  1. The plant always comes first. Books can be wrong, you can be wrong, but the plant is never wrong. Look it over carefully. Describe it to yourself before you read the key. Sometimes, after reading some interesting statement, you may see things that aren’t there, or miss things that are.
  2. Now to the book. Here’s how a key works: keys have successive pairs of statements, called leads. The two leads of a pair may be right together, or many lines apart, but they are numbered or indented alike. Start with the first pair. Find both, read all of both, and choose the one which fits your plant best. Don’t just choose the first that sounds good; the other may be better. When you’ve chosen one lead, then go where it tells you to. It may give you the number of the next pair, or the next may be just under it on the page. Read both of this new pair, choose one, and continue.
  3. If the leads have several phrases, compare them one phrase at a time. The first phrase listed is usually (though not always) the most reliable. But don’t just go by that one; check the others too.
  4. If you are the least bit unsure what a word means, look it up. Even professors look up words in the glossary. A slight misunderstanding can send you down entirely the wrong path.
  5. If your specimen seems not to fit either lead very well, stop and think. You may be in the wrong place. Go back and check your other choices. If you’re sure you’re right so far, note the place, and try whichever lead looks the least unlikely.
  6. It’s helpful, at least at first, to note choices on paper as you make them, marking dubious ones. Then possible mistakes are easier to find.
  7. Never skip steps, even if you see some picture or lead farther on that looks like it fits. If it does, you will get there anyway by following the key methodically. But it may actually be on quite a different path, and mislead you badly. Similarly, if you think you know the identity of your plant, try to forget that fact momentarily while you key it. Keying backwards from the result makes it much too easy to miss some important point—like flower color—which might tell you you’re wrong.
  8. Sometimes a key difference is whether your plant has, or doesn’t have, some feature (hairs, basal leaves, bracts). For some reason, it’s much harder to convince oneself that the plant doesn’t have something. If you look carefully and can’t find the feature, believe your own eyes, and go on.
  9. Sometimes you really can’t tell about some crucial choice because your specimen is incomplete, lacking fruits or whatever. You may not be completely stuck, though. If you try going each way in turn, one may lead to something reasonable, and the other might not.
  10. Eventually you will come to a family (or genus or species) name. It is vital to stop here and read the description of that family (or whatever) very carefully. Keys are simply the means by which you arrive at a tentative decision, which must be checked out. Discrepancies between description and plant are often your only warning of an error. Try not to convince yourself your plant has something (or doesn’t) just because the description says it should (or shouldn’t).
  11. Never identify something from only one plant, if you can help it. Things like leaf length vary between plants, or even on the same plant, so measure several if you must choose on that basis. And if you’ve collected several of something, check them all.

You may or may not find it cheering to hear that some troubles with keying are not your fault. Some plants (think of Asteraceae for instance) are just plain tricky. They have sepals that look like petals, or inflorescences that look like flowers, and if you’re calling these things by the wrong names, you get nowhere.

In the same way, not all books are well written, and bad ones are harder to use. Botanists are human, fallible, and occasionally forget they’re writing for people who don’t already know the plants. Now and then you will find terms unexplained, exceptions ignored, and choices fuzzy, even in Jepson, which on the whole is fairly good.

Fortunately, there’s plenty of help available:

  1. The Professor. If you’re stuck on something, bring it to lab. He probably won’t tell you what it is (he might not even know) but he can help you key it. He also has access to the herbarium and may be able to find you a better key in the literature than what’s in Jepson.
  2. Other students in the class. If several of you independently get the same name for the same thing, you can have more confidence in it. However it’s not wise just to take someone’s word without checking it yourself—they too may be wrong!
  3. Yourself. When you’re really stuck on something and no help is immediately available, the best thing you can do is to put the specimen away and don’t look at it for at least 24 hours (several days is better). Surprisingly often, when you go back to it, you will find that you subconsciously have made some decision, and it will key more easily. This is especially likely when you have been trying to squeeze the poor plant into some species where it doesn’t really go. When you come back to it, you’ll often realize this, and can then see where it does go.

Copyright © 1997 by Chris Laning. Used with permission.
Last revision February 24, 2002.

HTML/CSS-coded by Curtis Clark, jcclark@csupomona.edu.