Flowers and Inflorescence: How Angiosperms Arrange Their Blooms

A flower is the reproductive structure of a flowering plant. It holds the organs that make pollen and the organs that make seeds, and it usually does both in one small space.

An inflorescence is a group of flowers on one stem. A lilac spray, a grass panicle and a hazel catkin are all inflorescences, and they are built in very different ways.

This article covers the parts of a single flower, and the difference between a solitary flower and an inflorescence. It then sets out the main inflorescence types, and why the arrangement matters to a plant and to anyone trying to name it.

What a Flower Is Made Of

A flower sits at the end of a stalk called the pedicel. Its parts are attached to a small swelling at the top of that stalk, the receptacle.

Botanists describe the flower as four rings, or whorls, of modified leaves. From the outside in they are the sepals, the petals, the stamens and the carpels. A flower is therefore a short shoot that stopped growing and took on a new job.

The Four Floral Organs

The sepals form the outermost ring. They are usually green, and they cover the rest of the flower while it is still a bud.

The petals form the next ring. They are often colored and scented, and their job is to advertise the flower to animals.

The stamens are the male organs. Each one has a thin filament and a sac at the top, the anther, where pollen is made. A grain of pollen carries the male cell that will fertilize an egg.

The carpels are the female organs. One carpel, or several fused together, is called a pistil. It has a stigma at the top to catch pollen, a style below it, and an ovary at the base that holds the ovules.

A labeled diagram of a mature flower: petals, stamens, and the ovary holding its ovules. Image: Mariana Ruiz Villarreal (LadyofHats), public domain, via Wikimedia Commons

Which Parts a Flower Actually Has

Not every flower carries all four rings. A complete flower has sepals, petals, stamens and carpels. An incomplete flower is missing at least one of them.

Grasses and many trees have flowers with no petals at all. Willow and hazel are pollinated by wind, so they do not need to advertise.

A flower with both stamens and carpels is perfect, or bisexual. A flower with only one of the two is imperfect. Maize and cucumber carry separate male and female flowers, sometimes on the same plant.

Ring	What it is made of	What it does
Sepals	The outermost ring of modified leaves	Cover and protect the flower while it is still a bud
Petals	The next ring, usually broader and thinner	Advertise the flower to animals, often by color or scent
Stamens	A thin filament with an anther at the top	Make and release pollen, which carries the male cells
Carpels	One carpel, or several fused into a pistil	Receive pollen and hold the ovules that become seeds

The four floral whorls, what each ring is made of, and what it does for the flower.

One Flower or Many

A plant can put its flowers out one at a time, or it can group them. A single flower on its own stalk is solitary. A group of flowers on one branching stem is an inflorescence.

Clustering is a display trick. One very large flower costs a lot to build. Many small flowers on one stem give a pollinator the same visual target for less material. A bee then sees a bigger patch of color, and visits more flowers in one trip.

Solitary Flowers

Solitary flowers are common in plants with big blooms: tulip, poppy, hibiscus, magnolia. Each flower is large enough to be found on its own.

Most plants cannot afford that. Their flowers are small, so they are grouped. The group is not a flower; it is a system for showing many small flowers at once.

The Main Inflorescence Types

Inflorescence types are named for how the flowers sit on the stem. The names are old, and most of them describe a shape a reader can see at arm's length.

Type	How the flowers are arranged	A familiar example
Spike	Flowers sit directly on a central stem, each with no stalk	Plantain, gladiolus
Raceme	Flowers on short stalks along one central stem	Wisteria, lupin
Panicle	A raceme that branches, so flowers sit on many stalks	Oats, lilac, most grasses
Umbel	All the flower stalks rise from one point, like an umbrella	Carrot, parsley, dill
Corymb	Outer flowers have longer stalks, so the top is level	Hawthorn, candytuft
Cyme	The tip flower opens first, and later flowers come from below	Forget-me-not, phlox
Catkin	A dense, usually hanging cluster of small petal-less flowers	Hazel, willow, birch
Head	Many small flowers packed on one flat base, wrapped in bracts	Daisy, sunflower, Gerbera

The standard inflorescence types, how the flowers are arranged in each, and a plant that shows the shape.

Spike, Raceme and Panicle

A spike has flowers sitting directly on a central stem, with no stalk of their own. Plantain and gladiolus flower this way.

A raceme is the same plan with a stalk for each flower. Wisteria hangs its flowers in long racemes, which is why the whole cluster reads as a single object.

A panicle branches. The main stem divides, and those branches may divide again, so the flowers end up on many stalks. Oats, rice and most grasses carry panicles.

American wisteria (Wisteria frutescens) hangs its flowers in racemes, each flower on its own short stalk along one stem. Image: Dcrjsr, CC BY 3.0, via Wikimedia Commons

A grass panicle: the stem branches again and again, so the flowers sit on many stalks. Image: W.carter, CC BY-SA 4.0, via Wikimedia Commons

Umbel and Corymb

An umbel puts all its flower stalks at one point, like the ribs of an umbrella. The stalks are nearly equal in length, so the flowers form a flat or rounded top.

Carrot, parsley, dill and hogweed carry compound umbels: the first umbel's stalks end in a second, smaller umbel. The whole family, Apiaceae, is named for the shape.

A corymb looks flat-topped too, but it is built differently. The outer flowers have longer stalks than the inner ones, so a cluster that starts rounded ends up level. Hawthorn and candytuft are corymbs.

A compound umbel: the stalks rise from one point, then branch again into smaller umbels. Image: Paul van de Velde, CC BY 2.0, via Wikimedia Commons

A corymb diagram. The lower flowers have longer stalks, so every flower reaches roughly the same height. Image: Amada44, public domain, via Wikimedia Commons

Cyme

A cyme is built from the top down. The flower at the tip opens first, and later flowers come from buds below it, so the oldest flower sits in the middle.

In a monochasium, one side branch grows at a time, which gives a curled or zigzag shape. Forget-me-not and phlox are familiar cymes.

A monochasium, the simplest cyme: one side branch at a time, with the tip flower opening first. Image: Kagami, CC BY-SA 3.0, via Wikimedia Commons

Catkin and Head

A catkin is a dense, usually hanging spike or raceme of small flowers with no petals. Willow, hazel, birch and oak all use them, and all four are pollinated by wind.

A head, or capitulum, is the trick of the daisy family. Hundreds of small flowers, called florets, sit on one flattened base, and a ring of bracts wraps the outside.

What looks like one petal on a daisy is a whole flower, the ray floret. The center is a crowd of disc florets. A sunflower and a Gerbera are built exactly this way.

Hazel catkins: dense, hanging clusters of tiny flowers with no petals, built for wind pollination. Image: W.carter, CC BY-SA 4.0, via Wikimedia Commons

A Gerbera head. What looks like one flower is hundreds of small florets on a flat base. Image: Jeevan Jose, CC BY-SA 4.0, via Wikimedia Commons

Indeterminate and Determinate

Two plans sit behind all of these shapes: racemose and cymose. The first keeps growing at the tip, and the second stops at each tip.

A racemose inflorescence has its newest flower at the top and its oldest at the base, so flowering moves upward. Spike, raceme, panicle, umbel, corymb and catkin are all racemose.

A cymose inflorescence opens its tip flower first, and any further flowers come from side buds. Flowering moves downward or outward instead.

The difference sounds academic until you watch a plant. On a raceme, the bottom flowers are open while the tip is still in bud. On a cyme, the middle is already open and the outer buds are the last to go.

The pattern also shapes the fruit. An inflorescence that sets seed becomes an infructescence, and in a few plants the fruits fuse into one. A pineapple is the fused fruit of a whole inflorescence, not of a single flower.

Why the Arrangement Matters

For the plant, the arrangement is a way to be seen. A flat umbel gives a small insect a landing platform. A hanging catkin puts pollen where the wind can take it. A head gives a tiny flower the display area of a large one.

For a reader, the arrangement is a shortcut to a family. A head points to Asteraceae. An umbel points to Apiaceae. The tight whorl of mint flowers, a verticillaster, points to Lamiaceae.

It is not a name on its own. A corymb and an umbel can look alike from above, and a young panicle passes for a raceme until it branches. Used together with the flower's own parts, though, the arrangement narrows a plant faster than almost any other feature.

Key Terms in Plain English

These are the words in this article that would send you to a dictionary. Each one is given here the way it is actually used above.

Term	What it means
Flower	The reproductive structure of a flowering plant, where pollen and seeds are made.
Inflorescence	A group of flowers arranged on one stem.
Pedicel	The stalk of a single flower.
Peduncle	The stalk that carries a whole inflorescence.
Receptacle	The swollen tip of the stalk where the flower parts are attached.
Sepal	One of the outer, usually green, leaf-like parts that cover a bud.
Petal	One of the often colored parts that attract pollinators.
Stamen	The male organ of a flower: a filament with an anther at the top.
Anther	The sac at the top of a stamen where pollen is made.
Filament	The thin stalk of a stamen.
Pollen	The grains that carry the male cells of a flowering plant.
Carpel	The female unit of a flower, containing the ovules.
Pistil	One carpel, or several fused carpels, seen as a single female organ.
Stigma	The sticky tip of the pistil that catches pollen.
Style	The stalk between the stigma and the ovary.
Ovary	The base of the pistil, which holds the ovules and becomes the fruit.
Ovule	The structure inside the ovary that becomes a seed after fertilization.
Complete flower	A flower with sepals, petals, stamens and carpels.
Perfect flower	A flower with both stamens and carpels.
Bract	A modified leaf at the base of a flower or inflorescence.
Floret	One of the small individual flowers in a dense head.
Capitulum (head)	An inflorescence of many small flowers packed on one flat base.
Catkin	A dense, usually hanging cluster of small flowers without petals.
Racemose	Growing at the tip, so the newest flower is the highest one.
Cymose	Stopping at each tip, so the tip flower opens first.
Infructescence	The fruiting stage of an inflorescence.

Terms used in this article, defined in plain words.

Key Takeaways

• A flower is the reproductive structure of a flowering plant, and it holds both the pollen-making and the seed-making organs.
• Its parts are four rings of modified leaves: sepals, petals, stamens and carpels, attached to a receptacle at the top of a stalk.
• The stamen has a filament and an anther, and the anther makes pollen; the pistil has a stigma, a style and an ovary holding the ovules.
• A complete flower has all four rings, and a perfect flower has both stamens and carpels; grasses and many trees have flowers with no petals.
• A solitary flower stands alone on its stalk, while an inflorescence groups many flowers on one stem, which is cheaper than building one huge bloom.
• A spike has stalkless flowers, a raceme has stalked ones, and a panicle is a raceme that branches.
• An umbel spreads all its stalks from one point, and a corymb levels its top by giving the outer flowers longer stalks.
• In a cyme the tip flower opens first; the oldest bloom is in the middle and the newest are on the outside.
• Catkins are dense, petal-less clusters built for wind, while a head packs hundreds of florets onto one base as the daisies do.
• The arrangement is a shortcut to a plant family and, with the flower's own parts, one of the fastest ways to narrow an identification.

Frequently Asked Questions

What is the difference between a flower and an inflorescence?

A flower is one reproductive unit. An inflorescence is a group of flowers arranged on one stem. A single daisy head looks like one flower but is actually an inflorescence of many small florets.

What are the four parts of a flower?

The four rings are the sepals, which cover the bud; the petals, which attract pollinators; the stamens, which make pollen; and the carpels, which contain the ovules. All four are modified leaves.

What is the difference between a raceme and a spike?

Both have flowers along one central stem. In a raceme each flower has its own short stalk, and in a spike the flowers sit directly on the stem with no stalk.

What is the difference between an umbel and a corymb?

In an umbel all the flower stalks rise from one point and are nearly equal in length. In a corymb the stalks attach at different points, and the outer ones are longer, so the cluster still tops out level.

What is a catkin?

A catkin is a dense, usually hanging cluster of small flowers that have no petals. Willow, hazel, birch and oak use catkins and are pollinated by wind.

What is a head in botany?

A head, or capitulum, is an inflorescence in which many small flowers called florets sit on one flattened base, ringed by bracts. The daisy and the sunflower are heads.

What does determinate mean in an inflorescence?

A determinate, or cymose, inflorescence ends in a flower, so the tip stops growing. The tip flower opens first and later flowers come from side buds below it.

Why do plants grow flowers in clusters?

A cluster gives pollinators a larger visual target for less material than one huge flower. It also lets a plant offer many flowers in one visit, which raises the chance that pollen reaches another plant.

Key references
• Flower — the four whorls and their parts (Wikipedia)
• Inflorescence — types and arrangement (Wikipedia)
• Raceme — stalked flowers on one axis (Wikipedia)
• Catkin — the wind-pollinated cluster (Wikipedia)
• Asteraceae — the head and its florets (Wikipedia)
• Pollination — how pollen reaches a stigma (Wikipedia)
• Multiple fruit — fruit from a whole inflorescence (Wikipedia)

Sources: the Wikipedia article on flowers, for the definition of a flower as the reproductive structure of an angiosperm, the four whorls as modified leaves, the parts of the stamen and the pistil, and the complete, incomplete, perfect and imperfect distinctions; the Wikipedia article on inflorescences, for the definition of an inflorescence, the racemose and cymose plans, and the spike, raceme, panicle, umbel, corymb, cyme, catkin and head types; the Wikipedia article on the raceme, for the stalked flowers on one unbranched axis; the Wikipedia article on catkins, for the dense petal-less clusters of wind-pollinated trees; the Wikipedia article on Asteraceae, for the capitulum, its ray and disc florets, and the surrounding bracts; the Wikipedia article on pollination, for the advantage a display gives an animal-pollinated plant; and the Wikipedia article on multiple fruit, for the pineapple as the fused fruit of an inflorescence. Each source was opened and read in full. Figures are from Wikimedia Commons under the credit named in each caption. This article is written for education.