The inflorescence

A group of flowers

Walk through a mustard field in winter and look closely at one plant. The yellow flowers do not sit alone. They crowd together on a long stalk. The oldest are at the bottom, and fresh buds wait at the tip.

The way flowers are arranged on the floral axis is called the inflorescence. The floral axis is the part of the shoot that bears the flowers.

Some plants bear a single flower on the axis, as the China rose does. Many others bear a whole cluster of flowers. The cluster is one inflorescence.

The stalk of a single flower is its pedicel. The stalk that carries the whole cluster is the peduncle.

Does the main axis keep growing?

Every inflorescence belongs to one of two groups. To decide which, ask one question: what happens to the tip of the main axis?

• If the tip keeps growing and making new flowers, the inflorescence is racemose.
• If the tip stops growing because it turns into a flower, the inflorescence is cymose.

Racemose inflorescence

In a racemose inflorescence the main axis continues to grow. The flowers are borne on the sides of the axis.

New flowers are added at the growing tip. So the youngest flowers are at the tip, and the oldest are at the base. This order of opening is called acropetal succession.

Cymose inflorescence

In a cymose inflorescence the main axis ends in a flower. So its growth is limited.

The flower at the top opens first. Younger flowers appear below it, on the sides. This order is called basipetal: the newer flowers are towards the base.

Remembering the two types

Look at the tip of the axis.

• Racemose: the tip goes on growing, and the youngest flowers are at the tip.
• Cymose: the tip ends in a flower, and that flower is the oldest.

The word acropetal means "towards the tip". The word basipetal means "towards the base".

Some examples

These examples go a little beyond the textbook, but they help you to picture the two types. The mustard plant has a racemose inflorescence. So has the sunflower, whose big "flower" is really a head of many small flowers, called florets. Jasmine has a cymose inflorescence.

Why the order matters

The oldest flowers are fertilised first, so they are the first to make fruits. On a racemose plant the first fruits appear near the base while the tip is still in flower. On a cymose plant the fruit-making begins at the top.

You should find that on a racemose axis the flower at the base is the oldest. Switch to cymose, and the oldest flower moves to the tip. The two orders are mirror images of each other.

Try it in Python

The program reads the age of each flower, from the base to the tip. It names the kind of inflorescence from the order of the ages.

You should see that plant A is racemose, because its oldest flower, 9 days old, is at the base. Plant B is cymose. Plant C is racemose, because its oldest flower is at the base too.

Practise with new numbers

Common mistakes

• Putting the oldest flower at the tip of a raceme. The tip carries the youngest flowers and the buds.
• Mixing up acropetal and basipetal. Acropetal points to the tip. Basipetal points to the base.
• Thinking that a cymose axis grows for ever. It ends in a flower, so its growth is limited.
• Calling a whole cluster one flower. A sunflower head is an inflorescence of many small florets.

Remember these

• An inflorescence is the arrangement of flowers on the floral axis.
• Racemose: the main axis continues to grow, and flowers open in acropetal order.
• Cymose: the main axis ends in a flower, and flowers open in basipetal order.
• The pedicel is the stalk of one flower. The peduncle is the stalk of the whole cluster.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What decides whether an inflorescence is racemose or cymose?

2. In which order do the flowers open on a racemose inflorescence?

3. Which kind of inflorescence has its oldest flower at the top of the axis?

4. A stalk keeps lengthening, and new buds keep appearing at its top. Which kind of inflorescence is it?

5. Why is the head of a sunflower called an inflorescence and not a single flower?