Floral diagrams and formulae · Biology Class 11–12 | StemPy Lab
Course 7.3·Lesson 2 of 3
Floral diagrams and formulae
A floral diagram shows the number of parts of a flower, how they are arranged and how they are related, with the mother axis as a dot at the top. A floral formula writes the same flower in symbols: Br, the symmetry sign, K, C or P, A and G, with brackets for fusion and a line under G for a superior ovary.
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What a floral diagram shows
A floral diagram is a map of a flower, seen from above as if the flower were cut across. It shows three things.
How many parts each whorl has.
How the parts are arranged round the centre.
How the parts are related to one another, for example which are joined.
The diagram follows a few simple rules.
A dot at the top stands for the mother axis, the branch on which the flower sits. It tells you which side of the flower is the back.
The calyx is drawn outermost. The corolla comes next, then the androecium. The gynoecium is at the centre.
A bract, if there is one, is drawn as a small arc on the side away from the mother axis.
Parts that are joined are linked by a line.
The next lesson shows the diagrams of three families. Here, remember only the order of the whorls: calyx, corolla, androecium, gynoecium, from the outside to the middle.
What a floral formula says
A floral formula says the same things in symbols. It is short enough to write under a diagram, and a botanist can read a whole flower from it.
A list of fifteen symbols of a floral formula, each in a blue box with its meaning beside it. Br: bracteate, the flower has a bract. A circle with a plus: actinomorphic, radial symmetry. A percent sign: zygomorphic, one plane of symmetry. A male and female sign joined: a bisexual flower. The male sign: a male flower. The female sign: a female flower. K: the calyx, the sepals. C: the corolla, the petals. P: the perianth, tepals, when sepals and petals look alike. A: the androecium, the stamens. G with a line under it: a gynoecium with a superior ovary. G with a line over it: a gynoecium with an inferior ovary. K with 5 in brackets: brackets show the parts are fused, here five sepals united. A with 3 plus 3: a plus shows two whorls or two groups. C and A under one line: a line over two symbols shows that one part is joined to the other.
The key to the symbols is in the picture. Here is what to remember.
Br means the flower has a bract. It is bracteate.
A circled plus means a radial flower, which is actinomorphic. A percent sign means a flower with one plane of symmetry, which is zygomorphic.
One more sign tells the sex of the flower. The sign for a bisexual flower is a male and a female sign joined. A male flower and a female flower have their own signs.
K is the calyx, C the corolla and A the androecium. P is the perianth, used when the sepals and petals look alike and are called tepals.
G is the gynoecium. A line under G means a superior ovary. A line over G means an inferior ovary.
Rules for writing a formula
Write the symbols in this order: Br, the symmetry sign, the sex sign, K, C or P, A and G. Then follow these rules.
A number after a letter says how many parts there are. So K5 means five sepals, and A10 means ten stamens.
Brackets mean the parts are fused. So K(5) is five sepals joined into one cup, and G(2) is two carpels joined into one ovary.
A plus joins two groups. A3+3 means two whorls of three stamens. A(9)+1 means nine stamens joined, and one free.
A line over two symbols shows that one part is joined to the other. A line over C and A would mean that the stamens are joined to the petals.
A line under G shows that the ovary is superior. A line over G shows that it is inferior.
A number without brackets means the parts are free. So C5 means five separate petals, and C(5) means five petals joined into a tube.
A formula written out
Take the mustard flower from the last lesson. It has no bract, and it is radial. It has four free sepals in two pairs, four free petals and six free stamens, which are two short ones and four long ones. Its two carpels are joined, and its ovary is superior.
Radial: the circled plus.
Four sepals in two pairs, free: K2+2.
Four petals, free: C4.
Six stamens in two groups, free: A2+4.
Two carpels, joined, ovary superior: G(2), with a line under the G.
The formula is ⊕K2+2C4A2+4G(2). Every symbol answers a question about the flower.
Write the formula
Set the parts of a flower. The program under the picture writes its formula.
Try it in Python
The program writes a formula from a table of parts. A function writes one group of parts as a symbol with or without brackets.
Practise with new numbers
Common mistakes
Writing K(5) for five free sepals. Brackets mean fused. Five free sepals are written K5.
Reading the percent sign as a percentage. It is the sign of a flower with one plane of symmetry.
Mixing up superior and inferior. A line under G is a superior ovary, and a line over G is an inferior ovary.
Writing the gynoecium first. The order is K, C, A, G.
Counting the mother axis as a floral part. It is only a dot on the diagram, to show the back of the flower.
Remember these
A floral diagram shows the number of parts, their arrangement and how they are related.
The dot at the top is the mother axis. The calyx is outermost and the gynoecium is at the centre.
A formula is written in the order Br, symmetry, sex, K, C or P, A, G.
Brackets show fusion. A plus shows groups. A number alone shows free parts.
A line under G is a superior ovary. A line over G is an inferior ovary.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
What does the dot at the top of a floral diagram show?
Read the formula K(5) C5 A(9)+1 G1. What do the brackets and the plus tell you?
Which whorl is drawn outermost, and which is at the centre?
How is an inferior ovary shown in a formula?
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