Flower Structure and Function

• Flowers are vital for plant identification
• Flowers are compressed, modified fertile shoots with modified leaves, allowing fruit and seed production

Functions of Flowers

• Biological function of a flower is the production of male and female gametes for seed production after fertilization

Structure of a Typical Flower

• Flowers sit atop stems called pedicels
• The top of the pedicel has a swollen tip called the receptacle
• The receptacle bears the flower's parts including sepals, petals, stamens, and carpels

The Floral Parts (Components)

• Non-essential parts include the calyx (sepals) and corolla (petals)
• Essential parts include the androecium (stamens) and gynoecium (carpels)

Sepals

• Sepals form the first whorl on the receptacle
• Sepals are frequently green
• The calyx is the term for all sepals together
• Sepals protect the flower bud before it opens

Petals

• Petals are often brightly colored modified leaves
• The corolla is the term for all petals together
• When the calyx and corolla are alike, it is called the perianth
  • Sepaloid refers to a perianth that's green
  • Petaloid refers to a perianth that's colored
• Petals attract pollinators and protect reproductive organs

Stamens

• Stamens are male reproductive organs
• The androecium refers to all stamens together

Structure of Stamen

• Anthers are divided into two anther lobes by connective tissue
• Each anther lobe possesses two pollen sacs containing numerous pollen grains

Carpels

• Carpels (or pistils) are female reproductive organs
• All carpels together are known as the gynoecium

Structure of Carpel

• Each carpel consists of the stigma, style, and ovary

Bract

• Bract is an additional component consisting of a leafy structure from the axil of which the flower arises
  • Bracteate is used to describe a flower with bracts

Involucre

• Involucre is an additional component existing as a group of bracts arranged in one or more whorls surrounding a group of flowers
• Flowers can either be bracteate (with a bract) or ebracteate (without a bract)

Flower Classifications

• Flowers can be solitary or part of an inflorescence
• Inflorescence is a group of flowers arranged in a particular way on a stem
• The stem holding the inflorescence is the peduncle
• The main axis of inflorescence is the rachis
• Flowers can either be pedicellate (possessing a pedicel) or sessile (lacking a pedicel)

Classifications Based on Parts

• Complete flowers contain all said types of modified leaves: sepals, petals, stamens, and carpels
• Incomplete flowers lack at least one of these types of modified leaves

Classifications Based on Symmetry

• Actinomorphic (regular) flowers are symmetrical in all directions
• Zygomorphic flowers are symmetrical in one direction only
• Asymmetric flowers lack symmetry in any direction

Classifications Based on Segments Per Whorl

• Trimerous flowers have segments in multiples of three
• Tetramerous flowers have segments in multiples of four
• Pentamerous flowers have segments in multiples of five

Classifications Based on Number of Whorls

• Pentacyclic flowers show five whorls
• Tetracyclic flowers show four whorls

Classifications Based on Arrangement

• Cyclic flowers have floral leaves arranged in definite circles or whorls
• Acyclic flowers have floral leaves arranged in a spiral manner

According to presence or absence of androecium &/or gynoecium

• Hermaphrodite (perfect) flowers have both male and female organs present
• Unisexual flowers have only one of the reproductive organs present
  • Staminate have only stamen
  • Pistillate have only pistils
• Sterile flowers lack both male and female organs

Classifications Based on Number of Stamens

• Monoandrous flowers have one stamen
• Diandrous flowers have two stamens
• Triandrous flowers have 3 stamens
• Tetrandrous flowers have four stamens
• Pentandrous flowers have five stamens
• Polyandrous flowers have many stamens

Classifications Based on Sepal and Petal Fusion

• Apetalous flowers don't have petals
• Gamopetalous (sympetalous) flowers have united petals
• Polypetalous flowers don't have united petals
• Gamosepalous flowers have united sepals
• Polysepalous flowers don't have united sepals

Calyx Structures

• Foliaceous (leafy) calyxes, e.g. Stramonium
• Very small calyxes
• Membranous calyxes, e.g. Pyrethrum
• Bristle calyxes, e.g. Arnica

Corolla Insertion

• Hypogynous: Corolla attaches to the receptacle below the ovary
  • The ovary is superior
• Perigynous: Corolla is inserted on the edge of the hollow receptacle, which doesn't fuse with the ovary
  • Called semi-inferior or half inferior
• Epigynous: Corolla attaches to the receptacle above the ovary
  • The receptacle fuses with the ovary
  • The ovary is inferior

Androecium Details

• No pollen grains mean it is sterile
  • This state is called Staminode
• Stamens are = Androecium
• Lacking a filament meaning the anther is sessile

Androecium length variations

• Didynamous: Formed of 2 long and 2 short stamens
• Tetradynamous: Formed of 4 long and 2 short stamens

Stamens: Cohesion Through

• Free stamens
• United stamens involving
  • Fusion through anthers = Syngenesious, e.g. Compositae
  • Fusion through filaments
    • Monadelphous (leguminosae)
    • Diadelphous
    • Polyadelphous (clove...tetradelphous)

Fusion Through Filaments

• Monadelphous stamens are united in a tube through the filaments
• Diadelphous stamens are arranged in two groups united by filaments
• Polyadelphous stamens are filaments united in three or more groups

Pollen Grains

• Pollen grains consist of a wall with two membranes
  • Exine: outer thick cutinized wall
  • Intine: inner delicate cellusic membrane
• The exine may be smooth, warty, pitted, or spiny
• Pollen grains contain a germ pore and furrow for germination

Gynaecium Characteristics

• Consists of one or more carpels

Gynaecium Classification Based on Number of Carpels

• Monocarpellary: One carpel
• Bicarpellary: Two carpels
• Multicarpellary: Many carpels (>2)

Gynaecium Classification Based on Fusion

• If gynoecium is formed of more than one carpel

  • Free i.e. Apocarpous
  • United i.e. Syncarpous

Ovary

• Fused margins occur when when carpels are more common

• Carpels fuse at their margins - Ovules attach to the placenta

    -Ovary is at the base of the carpel and is divided into these groups
  

• Simple, when gynoecium is monocarpellary

• Compound, when gynoecium is bi- or multicarpellary

Style variations:

• Very long or very short
• Could also potentially be absent
• May be flattened

Stigma variations:

• Style present means that stigma is stalked
• In the absence of style stigma becomes sessile

General Stigma Considerations

• Usually stigma is colored, sticky and covered with papillae for optimal pollen grain absorbtion

Stigma Forms

• Capitate
• Globose
• Tubular
• Lobed
• Stellate

Receptacle Features

• Usually flattened, convex, or conical
• Can elongate below the calyx creating a hypanthium