page thirty-three

The Herbarium

Thirty-two rooms have measured, dated, built and found things. None of them has asked the oldest question in the field: what is this? Botanists answer it with a key, a ladder of two-way choices that ends at a name. Today's number invents a flora, writes the key to it, and leaves six unnamed sheets of pressed plants on the bench. One of them is not in the book.

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The bench

Each couplet gives you two leads. Pick the one that is true of the sheet and follow it to the next couplet, until the key gives you a name. Then read that species' account and check it against the plant before you write the slip. Move over the sheet to look through the hand lens.

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Flora
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The arrow keys walk to other days. Every day is a different country with different plants and a different key.

The key

The whole key, as it would be printed in the flora. The number on the right says where each lead goes next: another couplet, or a name. Your path through it for the sheet on the bench is shaded green, and the couplet you are on is yellow. The page wrote this key itself, choosing at every couplet the character that splits the remaining plants most evenly.

Terms

What the words in the leads mean.

The flora

The species accounts, in alphabetical order. A key gets you to a name, and the account is how you check it.

What is real here and what is not

The method is the one botanists use. The plants are not: every genus, species, society and collector here is the day's number, so do not go looking for them.

  1. 01

    Keys are real

    Dichotomous keys go back to Lamarck's Flore françoise of 1778, and nearly every field flora still uses them. Each couplet is a pair of contradictory statements, numbered so you can jump around a printed page. A good key leans on characters you can see on most specimens most of the year, and a balanced one gets you to a name in as few steps as possible.

  2. 02

    So are the sheets

    Herbarium specimens are pressed, dried and strapped to stiff paper, with a label giving the place, the date, the collector and their running number. Flower colours fade, so collectors write the colour in life on the label. Determination slips are added over the years, and the latest one is the name the plant currently goes by.

  3. 03

    So is the trap

    A key only uses the characters it needs to separate the species it covers, so a plant from outside the flora will often key out smoothly to the wrong name. Checking the result against a full description is standard practice for that reason, and new species do still turn up in old herbarium cupboards, sometimes decades after they were collected.

  4. 04

    The rest is simplified

    Real plants have dozens of useful characters, from stipules and fruit shape to leaf-base hairs seen only at ×20, and they vary. Real keys often have to cope with specimens that have no flowers. Here every plant has eight clean characters and no variation, and genera are simply the plants that share a petal number.

Thirty-three rooms, and one number doing all the work. Today it invents a country's flowers, presses six of them and writes the book you need to name them, leaving out one plant on purpose.