What Are the Key Characteristics of a Flower?

A flower is the part of a flowering plant devoted to making seeds, the plant’s way of reproducing. Almost everything about a flower, from its shape to its smell, serves one job: getting pollen from one flower to another. Flowers began as modified leaves, which is why their parts are arranged in tidy rings like leaves around a stem.1PubMed Central. Reflections on the ABC model of flower development

The Four Rings of a Typical Flower

A small set of master genes tells each ring which kind of part to become.6PubMed Central. Reflections on the ABC model of flower development Switching these genes on can turn ordinary leaves into flower parts, so petals, stamens and pistils are best thought of as leaves reshaped for reproduction.7PubMed Central. Reflections on the ABC model of flower development Double flowers in gardens often come from stamens turning into extra petals, which is also why they tend to offer pollinators little pollen.

Some flowers add extra structures, like the trumpet-shaped corona in the center of a daffodil.8PubMed Central. Emergence of Corona Is Independent of the Four Whorls of Floral Organs in Narcissus tazetta The corona is a separate structure, not part of any of the four rings, though it develops closely with the stamens.9PubMed Central. Emergence of Corona Is Independent of the Four Whorls of Floral Organs in Narcissus tazetta And in many flowers such as tulips and lilies, sepals and petals look alike and are together called tepals.

The Stamen Makes Pollen

A stamen has an anther, the pollen sac, usually on top of a thin stalk called the filament.10The Plant Cell. Stamen Structure and Function The filament feeds water and nutrients to the anther and lifts it into position for pollen to spread.11The Plant Cell. Stamen Structure and Function Inside the anther, special cells divide to form tiny pollen grains, each a sealed package carrying the plant’s genes.12The Plant Cell. Stamen Structure and Function

The anther wall has several layers that nourish and protect the developing pollen.13PubMed Central. Evolution and diversity of the angiosperm anther: trends in function and development If those layers form or break down at the wrong time, the plant makes no working pollen and becomes male-sterile.14PubMed Central. Evolution and diversity of the angiosperm anther: trends in function and development

The Pistil Holds the Future Seeds

Despite huge variety, almost every pistil (female part) has three regions: an ovary at the base, a stalk called the style, and a stigma on top.15PubMed Central. Molecular Control of Carpel Development in the Grass Family The stigma, the sticky tip, is where pollen lands.16PubMed Central. Molecular Control of Carpel Development in the Grass Family The ovary at the bottom holds ovules, the tiny structures that become seeds.17PubMed Central. Molecular Control of Carpel Development in the Grass Family

Pistils come in many forms. One can be a single unit, several separate units, or several fused together.18PubMed Central. Molecular Control of Carpel Development in the Grass Family In grasses like rice and wheat, the stigmas are feathery to catch pollen from the air.19PubMed Central. Molecular Control of Carpel Development in the Grass Family In corn, the long silks are stretched-out styles, one per kernel.

Many flowers have both stamens and a pistil in one bloom, but not all do. Squash and cucumber have separate male and female flowers on the same plant. Holly and kiwi have male and female flowers on different plants, so you need both to get berries.

From Pollen to Seed and Fruit

Flowering plants use a two-part process: two sperm cells from one pollen grain join two different female cells.20PubMed Central. The beginning of a seed: regulatory mechanisms of double fertilization One union forms the embryo, the baby plant.21PubMed Central. The beginning of a seed: regulatory mechanisms of double fertilization The other forms the endosperm, the food store inside the seed.22PubMed Central. The beginning of a seed: regulatory mechanisms of double fertilization The starchy part of rice, wheat and corn is endosperm, so much of the world’s diet comes from this second fertilization.

Fertilization usually triggers the ovary to grow into a fruit and the unneeded parts, like petals, to drop off.23PubMed. Fruit development is actively restricted in the absence of fertilization in Arabidopsis In some plants other parts join the fruit, such as the papery husk around a tomatillo or Chinese lantern, which grows from the sepals.24PubMed. Transcriptomic variation of the flower-fruit transition in Physalis and Solanum

When a garden plant flowers but sets no fruit, poor pollination is a common cause. Without it, most flowers simply drop. Few bees, rain or cold during bloom can all cause this, and hand pollinating with a small brush can help squash or tomatoes.

Color, Scent and Nectar Attract Pollinators

Flower colors come mainly from a few pigment families, including the ones behind yellows and oranges and the ones behind reds, purples and blues.25Frontiers in Ecology and Evolution. Major Flower Pigments Originate Different Colour Signals to Pollinators Each pigment group looks different to a bee’s eyes.26Frontiers in Ecology and Evolution. Major Flower Pigments Originate Different Colour Signals to Pollinators Many flowers also carry ultraviolet patterns invisible to us, such as a “bull’s eye” that guides bees to the center.27PubMed Central. Bees, flowers and UV White flowers often stand out most strongly to bees.28Frontiers in Ecology and Evolution. Major Flower Pigments Originate Different Colour Signals to Pollinators Red tubular flowers mostly target birds like hummingbirds.

Flowers release a mix of airborne scent chemicals to draw pollinators.29PubMed Central. Volatile Organic Compounds Emitted by Flowers: Ecological Roles, Production by Plants, Extraction, and Identification The exact blend can attract particular kinds of insects.30PubMed Central. Volatile Organic Compounds Emitted by Flowers: Ecological Roles, Production by Plants, Extraction, and Identification

Nectar is a sugary liquid made by glands called nectaries, offered as payment for carrying pollen.31PubMed Central. The developmental basis of floral nectary diversity and evolution Besides sugar, nectar contains amino acids, the building blocks of protein.32PubMed Central. Nectar in Plant–Insect Mutualistic Relationships: From Food Reward to Partner Manipulation It sits deep in the flower near the stamens and pistil, so visitors brush against pollen while drinking.33PubMed Central. Nectar in Plant–Insect Mutualistic Relationships: From Food Reward to Partner Manipulation Some flowers, like certain magnolias, even warm themselves up, which spreads their scent and gives insects a warm shelter on cold spring days.34PubMed Central. Floral thermogenesis: An adaptive strategy of pollination biology in Magnoliaceae

In your own garden, plant a mix of colors and shapes to draw different pollinators. Scented flowers are often busiest at dusk with moths. Avoid spraying insecticides on open blooms. Double-petaled varieties often have little nectar or pollen, so they feed visitors poorly.

How Flowers Avoid Pollinating Themselves

Self-pollination often produces far fewer fruits than cross-pollination: in one spurge, only about 1 in 10 self-pollinated flowers set fruit.35PubMed Central. Linking Self-Incompatibility, Dichogamy, and Flowering Synchrony in Two Euphorbia Species: Alternative Mechanisms for Avoiding Self-Fertilization? Mixing genes with another plant generally gives healthier offspring.

Flowers have several ways around the problem. Some keep stamens and stigma physically apart.36PubMed. The differential contributions of herkogamy and dichogamy as mechanisms of avoiding self-interference in four self-incompatible Epimedium species Others ripen their male and female parts at different times.37PubMed. The differential contributions of herkogamy and dichogamy as mechanisms of avoiding self-interference in four self-incompatible Epimedium species In some plants, physical separation works better than timing at preventing self-pollination.38PubMed. The differential contributions of herkogamy and dichogamy as mechanisms of avoiding self-interference in four self-incompatible Epimedium species Many plants also chemically reject their own pollen. Fruit trees like many apples need a second variety nearby for this reason.

Some flowers self-pollinate happily. Peas and tomatoes often pollinate themselves, which guarantees seed when pollinators are scarce. Gardeners can also save true-to-type seed from such plants more easily, as long as they aren’t hybrid varieties.

Shapes Built for Specific Pollinators

Round flowers like daisies can be approached from any side, while flowers like snapdragons and orchids have a single left-right symmetry. These mirror-image flowers are often visited by fewer kinds of pollinators.39PubMed Central. Floral symmetry: pollinator-mediated stabilizing selection on flower size in bilateral species Their size stays more uniform, because it has to fit the same few visitors.40PubMed Central. Floral symmetry: pollinator-mediated stabilizing selection on flower size in bilateral species

Grasses and many trees rely on wind, so they skip showy petals, scent and nectar. Their height, flower position and exposed stamens and stigmas are what count for spreading and catching pollen.41PubMed Central. Wind of change: new insights on the ecology and evolution of pollination and mating in wind-pollinated plants These flowers release huge amounts of light pollen, which is why grass and tree pollen cause most hay fever. Showy insect-pollinated flowers are less often to blame for hay fever, since their heavier pollen isn’t carried far by wind.

Some flowers, like tomatoes, only release pollen when a bee vibrates them.42PubMed Central. Biomechanical properties of a buzz-pollinated flower The anthers amplify the bee’s buzz, shaking about 1.7 to 5 times harder than the rest of the flower.43PubMed Central. Biomechanical properties of a buzz-pollinated flower

That’s why tomatoes in a greenhouse may need help. Honeybees can’t buzz-pollinate, but bumblebees can. Gently shaking or tapping plants mimics the buzz, and an electric toothbrush held to the flower stem is a common gardener’s trick.

Flowers That Trick Their Pollinators

Many flowers attract flies by smelling like dead animals or dung.44South African Journal of Botany. Convergent evolution of carrion and faecal scent mimicry in fly-pollinated angiosperm flowers and a stinkhorn fungus Their scent contains the same sulfur compounds found in rotting meat.45South African Journal of Botany. Convergent evolution of carrion and faecal scent mimicry in fly-pollinated angiosperm flowers and a stinkhorn fungus One pipevine imitates the smell of dead insects to lure tiny flies.46Frontiers in Ecology and Evolution. Flowers of Deceptive Aristolochia microstoma Are Pollinated by Phorid Flies and Emit Volatiles Known From Invertebrate Carrion

The giant corpse flower draws crowds at botanic gardens for its rotten smell. Carrion flowers are often dark red or brown like flesh.

Some orchids release scents that copy a female insect’s mating signal, so males try to mate with the flower and carry pollen away.47PubMed Central. Floral visual signal increases reproductive success in a sexually deceptive orchid Their petals matter too: the orchid succeeds more often when its visible flower parts are present.48PubMed Central. Floral visual signal increases reproductive success in a sexually deceptive orchid

Hidden Features of Petals and Nectar

Rose petals have tiny bumps that make water bead into round drops.49PubMed. Understanding the petal effect: Wetting properties and surface structure of natural rose petals and rose petal-derived surfaces Unlike on a lotus leaf, those drops cling firmly instead of rolling off.50PubMed. Understanding the petal effect: Wetting properties and surface structure of natural rose petals and rose petal-derived surfaces The curl of some petals comes from cells widening on just one side.51PubMed. The mechanism underlying asymmetric bending of lateral petals in Delphinium (Ranunculaceae)

Bacteria and yeasts living in nectar can change its sugar, acidity and smell.52PubMed Central. Tiny but mighty? Overview of a decade of research on nectar bacteria Pollinators can taste and smell these changes when choosing flowers.53PubMed Central. Potential effects of nectar microbes on pollinator health Still, most plants don’t depend on microbes to attract pollinators.54Annual Review of Ecology, Evolution, and Systematics. The Floral Microbiome: Plant, Pollinator, and Microbial Perspectives