1.5 Flower structure and function

Flowers are the reproductive structures of angiosperms, enabling sexual reproduction and genetic diversity. They consist of male and female parts, along with accessory structures that aid in pollination and protection.

Flower diversity is vast, with variations in structure, arrangement, and reproductive strategies. Understanding these differences is key for plant identification and for studying how plants and pollinators interact in ecosystems.

Anatomy of Flowers

Flowers are the reproductive organs of angiosperms (flowering plants). Each flower is built from male and female reproductive organs plus accessory parts that help with pollination and protection. These floral organs are arranged in concentric rings called whorls, all attached to a base structure called the receptacle.

Male Reproductive Structures

The stamen is the male reproductive organ of a flower, made up of two parts:

• Filament: a slender stalk that supports the anther and connects it to the flower
• Anther: the sac-like structure at the tip of the stamen that produces and contains pollen grains (the male gametophytes)

Pollen grains carry the male genetic material, specifically sperm cells, needed for fertilization.

Female Reproductive Structures

The carpel (sometimes called a pistil when carpels are fused) is the female reproductive organ. It has three distinct regions:

• Stigma: the sticky surface at the top that receives pollen grains during pollination
• Style: the elongated tube connecting the stigma to the ovary, through which the pollen tube grows after pollination
• Ovary: the enlarged base containing ovules, which house the egg cells and develop into seeds after fertilization

Accessory Floral Parts

These structures don't directly produce gametes but play supporting roles:

• Sepals: the outermost whorl, typically green and leaf-like, that protect the flower bud before it opens
• Petals: the often colorful parts of the flower that attract pollinators
• Nectaries: glands that secrete sugary nectar, rewarding pollinators and encouraging repeat visits
• Bracts: modified leaves that can be colorful and help attract pollinators. In poinsettias, for example, the bright red "petals" are actually bracts.

Flower Diversity and Types

Flowers vary widely in their structure, arrangement, and reproductive strategies. These variations form the basis for classifying flowers into different types, and they're also central to understanding how plants interact with their pollinators.

Complete vs. Incomplete Flowers

• Complete flowers have all four main floral parts: sepals, petals, stamens, and carpels.
• Incomplete flowers lack one or more of those parts. For instance, corn is a monoecious plant where male flowers (tassels) lack carpels and female flowers (ears) lack stamens.

Perfect vs. Imperfect Flowers

This classification focuses specifically on reproductive parts:

• Perfect (bisexual) flowers contain both stamens and carpels in the same flower.
• Imperfect (unisexual) flowers have either stamens or carpels, but not both.

Two related terms come up often here:

• Monoecious plants bear separate male and female flowers on the same individual (e.g., cucumber, corn).
• Dioecious plants have male and female flowers on separate individuals (e.g., holly, willow).

Symmetry in Flowers

• Radial symmetry (actinomorphic): the flower can be divided into equal halves along any plane through the center (e.g., lily, buttercup).
• Bilateral symmetry (zygomorphic): the flower can only be divided into equal halves along a single plane (e.g., orchid, snapdragon).
• Asymmetrical: no plane of symmetry exists; the flower is irregular in shape (e.g., canna).

Floral Formulas and Diagrams

• Floral formulas are shorthand representations of flower structure using symbols and numbers to indicate the number and arrangement of each floral part.
• Floral diagrams are schematic cross-sections showing how floral parts are arranged when viewed from above.

Both tools are used to describe and compare flower morphology across species, and you'll likely encounter them in lab.

Inflorescence Arrangements

An inflorescence is a cluster of flowers arranged on a stem in a specific pattern. The type of inflorescence can affect how efficiently a plant gets pollinated and how it disperses seeds.

Racemose Inflorescences

Racemose (indeterminate) inflorescences have a main axis that keeps growing and producing flowers along its sides. Flowers typically open sequentially from the base upward.

Examples include:

• Racemes (snapdragon)
• Spikes (gladiolus)
• Corymbs (yarrow)
• Umbels (dill)

Cymose Inflorescences

Cymose (determinate) inflorescences have a main axis that ends in a flower, with lateral branches developing below it. Flowers open from the center (or apex) outward.

Examples include:

• Dichasial cymes (baby's breath)
• Monochasial cymes (forget-me-not)
• Scorpioid cymes (heliotrope)

Specialized Inflorescence Types

Some plants have unique inflorescence structures adapted for specific pollination or dispersal strategies:

• Spadix and spathe in aroids (calla lily): a fleshy spike (spadix) surrounded by a large modified bract (spathe)
• Capitulum in Asteraceae (sunflower): what looks like a single flower is actually hundreds of tiny flowers packed onto a flat receptacle
• Hypanthodium in figs: flowers are enclosed inside a fleshy, hollow receptacle, with tiny wasps entering to pollinate them

Pollination Mechanisms

Pollination is the transfer of pollen grains from an anther to a stigma. Plants have evolved a wide range of mechanisms to make this happen, using both living (biotic) and non-living (abiotic) agents. Flower structure is closely tied to these mechanisms, with specific adaptations that facilitate pollen transfer.

Self-Pollination vs. Cross-Pollination

• Self-pollination occurs when pollen from the same flower (or another flower on the same plant) fertilizes the ovules. It's reliable but limits genetic diversity.
• Cross-pollination transfers pollen between different plants, promoting genetic diversity.

Some plants actively prevent self-pollination through mechanisms like self-incompatibility (biochemical rejection of self-pollen) or temporal separation of male and female maturity. In protandry, pollen is released before the stigma is receptive; in protogyny, the stigma matures first.

Abiotic Pollination Agents

• Wind pollination (anemophily) is common in grasses, sedges, and many trees (oak, pine). These flowers tend to be small and inconspicuous, producing large quantities of lightweight pollen.
• Water pollination (hydrophily) is rare but occurs in some aquatic plants like eelgrass and hornwort.

Biotic Pollination Syndromes

A pollination syndrome is a set of floral traits associated with attracting a particular type of pollinator:

• Bee pollination (melittophily): bright blue or yellow flowers with landing platforms and nectar guides (snapdragon, lavender)
• Butterfly pollination (psychophily): flowers with long, narrow tubes and ample nectar (phlox, milkweed)
• Moth pollination (phalaenophily): white or pale flowers that open at night with strong fragrance (jasmine, yucca)
• Bird pollination (ornithophily): sturdy, often red flowers with abundant nectar (columbine, hibiscus)
• Bat pollination (chiropterophily): large, strong-scented flowers that open at night (agave, saguaro cactus)

Pollinator Adaptations in Flowers

Flowers have evolved specific features to improve pollination success:

• Nectar guides: patterns or UV-reflective lines on petals that direct pollinators toward the nectar source (visible in pansies)
• Landing platforms: fused petals or modified structures that give pollinators a place to land (snapdragon, orchid)
• Floral scents: volatile compounds that attract pollinators from a distance (rose, jasmine)
• Trapping structures: some orchids, like Catasetum, have bucket-shaped flowers that temporarily trap pollinators, ensuring pollen gets attached before the animal escapes

Fertilization Process

Fertilization is the fusion of sperm and egg to produce a zygote, which then develops into a seed. In angiosperms, this process takes place within the ovule and involves a special mechanism called double fertilization.

Pollen Grain Structure and Germination

Pollen grains are protected by two layers: a tough outer wall called the exine and a thinner inner wall called the intine. When a pollen grain lands on a compatible stigma, it absorbs moisture and nutrients, triggering germination. During germination, a pollen tube begins growing out of the grain and penetrates into the stigma and style.

Pollen Tube Growth and Guidance

The pollen tube extends through the style toward the ovary. Chemical attractants secreted by synergid cells in the embryo sac guide the tube to the ovule. As the tube grows, the pollen tube nucleus divides to produce two sperm cells, which travel down the tube toward the ovule.

Double Fertilization

Double fertilization is unique to angiosperms and involves both sperm cells:

1. One sperm cell fuses with the egg cell, forming a diploid zygote that develops into the embryo.
2. The other sperm cell fuses with the two polar nuclei in the central cell, forming a triploid endosperm that nourishes the developing embryo.

This process ensures that the embryo and its food supply (endosperm) develop at the same time, so resources aren't wasted on unfertilized ovules.

Flower Development and Regulation

Flower development is tightly controlled by genetic and hormonal factors. The switch from vegetative growth (leaves, stems) to reproductive growth (flowers) is triggered by environmental cues like day length (photoperiod) and temperature. Once that switch is flipped, specific genes determine which organs form and where.

Floral Meristem Identity Genes

Genes like LEAFY and APETALA1 control the transition from a vegetative meristem (which produces leaves) to a floral meristem (which produces flower parts). These genes are activated by environmental and internal signals. Mutations in these genes can cause the plant to produce abnormal or leafy structures where flowers should be.

ABC Model of Floral Organ Identity

The ABC model explains how combinations of three gene classes determine which organ forms in each whorl of the flower:

• A-class genes alone (e.g., APETALA1, APETALA2) → sepals (outermost whorl)
• A + B-class genes (e.g., APETALA3, PISTILLATA) → petals (second whorl)
• B + C-class genes (e.g., AGAMOUS) → stamens (third whorl)
• C-class genes alone → carpels (innermost whorl)

A and C genes are mutually antagonistic, meaning where A is active, C is not, and vice versa. The model has since been expanded to include D-class genes (ovule development) and E-class genes (required for all organ identity), but the core ABC framework is what you need to know for this course.

Hormonal Control of Flowering

Several plant hormones regulate flower development:

• Gibberellins promote flowering in many species, especially in response to long days or cold exposure (vernalization).
• Auxins help initiate floral meristems and guide the growth of floral organs.
• Cytokinins regulate cell division and differentiation in developing flowers; their levels typically rise during flower development.
• Ethylene triggers flower senescence (aging) and abscission (petal drop), causing flowers to wilt and fall off.

Ecological Significance of Flowers

Flowers are central to the ecology of terrestrial ecosystems. Their evolution has been tightly linked to the diversification of animal pollinators, creating intricate coevolutionary relationships. Beyond pollination, flowers provide resources for herbivores, seed dispersers, and decomposers.

Flowers as Reproductive Units

As the sites of pollination and fertilization, flowers enable sexual reproduction and genetic recombination in angiosperms. Successful fertilization leads to seed and fruit production, ensuring species propagation. The sheer diversity of flower forms and reproductive strategies is a major reason angiosperms are the most species-rich group of land plants.

Coevolution with Pollinators

Many flowers and their pollinators have evolved together, developing specialized adaptations that benefit both parties. Classic examples include the long proboscis of hawk moths matched to the deep nectar spurs of certain orchids, and the tubular flowers of trumpet vine shaped to fit hummingbird bills. These coevolutionary relationships have driven diversification in both plant and animal lineages.

Flowers in Plant-Animal Interactions

• Flowers provide nectar and pollen as food for insects, birds, and mammals, supporting pollinator populations and broader biodiversity.
• Some plants have evolved structures that manipulate animal behavior. Carrion flowers mimic the smell of rotting flesh to attract fly pollinators, while pitcher plants use modified floral structures to trap insects.
• Flowers can also serve as shelter or breeding sites for animals. Certain bats roost in large floral bracts, and some insects induce gall formation on floral tissues.

Economic Importance of Flowers

Flowers have major economic value as ornamental plants and as the basis for agricultural crop production. The global floriculture industry, including cut flowers, potted plants, and bedding plants, is a multi-billion dollar market.

Ornamental and Cut Flower Industry

Roses, chrysanthemums, and tulips are among the most widely cultivated ornamental flowers. The cut flower industry spans production, transportation, and retail, and it's an important economic sector in countries like the Netherlands, Colombia, and Kenya, providing employment across the supply chain.

Flowers as Food and Medicine Sources

• Food: Saffron (harvested from Crocus sativus stigmas) is one of the world's most expensive spices. Squash blossoms are eaten in many cuisines, and edible flowers like nasturtium and violets are used as garnishes.
• Medicine: Chamomile flowers are brewed as a calming tea. Pyrethrin, a natural insecticide, is extracted from chrysanthemum flowers. Essential oils from lavender and jasmine are used in both aromatherapy and pharmaceutical products.

Role of Flowers in Agriculture

Flowers are where pollination and seed development happen, making them essential for crop production. Many high-value crops depend on insect pollination for fruit set, including almonds, apples, and strawberries. Forage crops like clover and alfalfa also require pollination for seed production. Beyond pollination, hybridization and selective breeding of flower varieties have produced improved crop cultivars with traits like disease resistance and higher yields.