Flowers are the reproductive structures of a flowering plant. Flowers are the primary structures used in grouping plant families.

Function

• Reproduction, beginning with pollination and fertilization.
• Advertisement and rewards to lure a pollinator.
• Horticultural uses
• Aesthetic qualities
• Cut flowers and potted blooming plants
• Edible flowers and herbs
• Plant identification

Structure

Pistil – central female organ of the flower.  It is generally bowling-pin shaped and located in the center of the flower. 

Stigma – receives pollen, typically flattened and sticky
Style – connective tissues between stigma and ovary
Ovary – contains ovules or embryo sacs
Ovules – Unfertilized, immature seeds

Stamen – male flower organ

Anthers – pollen-producing organs
Filament – stalk supporting anthers

Petals – usually colorful petal-like structures making up the “flower”, collectively called the corolla.  They may contain perfume and nectar glands. 

Sepals – protective leaf-like enclosures for the flower buds, usually green, collectively called calyx.  Sometimes highly colored like the petal as in iris.

Receptacle – base of the flower

Pedicel – flower stalk of an individual flower in an inflorescence

Monocot or Dicot

The number of sepals and petals is used in plant identification.  Dicots typically have sepals and petals in fours, fives, or multiples thereof.  Monocots typically have flower parts in threes or multiples of three.    

Terms Defining Flower Parts

Flowers
Complete – flower containing sepals, petals, stamens, and pistil
Incomplete – flower lacking sepals, petals, stamens, and/or pistils

Perfect – flowers containing male and female parts
Imperfect – flowers that lack either male or female parts

Pistillate  – flowers containing only female parts
Staminate – flowers containing only male parts

Plants
Hermaphroditic – plants with perfect flowers  (apples, tulips)
Monoecious  (mə-nē'shəs) – plants with male flowers and female flowers on the same plant  (corn, squash, and pine)
Dioecious (dī-ē'shəs) – plants with male flowers and female flowers on separate plants  (maple, holly)

Gynoecious – plants with only female flowers
Andromonoecious – plants with only male flowers

Inflorescence (flower arrangement on a stem)

Catkin (ament) – a spike with only pistillate or staminate flowers (alder, poplar, walnut, and willows)
Composite – a complex flower, composed of ray flowers (usually sterile with attractive, colored petals) and disc flowers in center that develop into seed (sunflower and aster)
Corymb – stemlets (pedicels) arrangement along main stem, gives florets display with a flat rounded top.  Outer florets open first.  (yarrow, crabapple)
Cyme – a determinate, flat or convex flower, with inner floret opening first. 
Panicle – an indeterminate flower with repeated branching.  It can be made up of racemes, spikes, corymbs, or umbels.
Raceme – flowers attached to a main stem (peduncle) by stemlets (pedicel). (snapdragon, bleeding heart, Canterbury bells)
Single (or solitary) – one flower per stem  (tulip, crocus)
Spadix – showy part is a bract or spathe, partially surrounding the fertile flower.  (calla, caladium)
Spike – flowers attached to main stem, without stemlets, bottom florets open first.  (gladiolus, ajuga and gayfeather)
Umbel – florets with stalks attached to main stem from one point, forming a flat or rounded top.  Outer flowers open first.  (dill, onion)

Symmetrical – symmetrical flowers  (lily)
Asymmetrical – asymmetrical flowers  (snapdragon)

Additional Information – CMG GardenNotes on How Plants Grow (Botany):

#121 Horticulture Classification
#122 Taxonomy
#131 Plant Structures: Cells, Tissues, and Structures
#132 Plant Structures: Roots
#133 Plant Structures: Stems
#134 Plant Structures: Leaves
#135 Plant Structures: Flowers
#136 Plant Structures: Fruit
#137 Plant Structures: Seeds
#141 Plant Growth: Photosynthesis, Respiration and Transpiration
#142 Plant Growth: Light
#143 Plant Growth: Temperature
#144 Plant Growth: Water
#145 Plant Growth: Hormones

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