Flowers of Plants

During their reproductive phase, angiosperms or flowering plants first produce flowers which develop into fruit that contain one or more seeds.  The various shapes, sizes, and forms of flowers that appear on angiosperms are useful to taxonomists when identifying and classifying such plants.

The main stalk that supports a simple or solitary flower is called a peduncle.  A peduncle can also support an inflorescence or cluster of flowers.  In the case of an inflorescence, each flower in the cluster is typically connected to the peduncle by its own smaller stalk called a pedicel.   The enlarged tip of the pedicel where the flower develops is called the receptacle.  The receptacle can form up to four types of modified leaves including sepals, petals, stamens, and carpels.

The Structure of Flowers

The stamens and carpels are the reproductive parts of flowers in angiosperms.  While the sepals and petals do not participate in the reproductive process, they do use their color to attract pollinators to the flowers.

Sepals are part of flowers of angiosperms.  They are green leaflike structures around the base of the flower.  They typically serve to product the flower in bud and to support the petals in bloom.  The sepals make up the outmost whorl of flower parts that collectively are called the calyx.

Petals are modified leaves that surround the reproductive portions of flowers.   Collectively, they are referred to as the corolla.  They are frequently brightly colored and fragrant to attract pollinators. 

Together, sepals and petals form the perianth, the outer whorls of the flower.  The calyx and corolla surround the sexual organs of the flower.

The stamen is the male reproductive organ of a flower.  It consists of two parts: an anther and a filament.  The anther is a pollen sac that typically has two lobes. The filament attaches to the anther either at is base or in between the two lobes and holds the anther in place so that the wind or pollinators can carry the pollen to the female reproductive portion of the flower.

The carpel is a bowling pin shaped structure surrounded by the petals and consists of a stigma, a style, and an ovary.  The top of the bowling pin is the stigma, the opening through which pollen enters the carpel.  The style is the skinny neck of the bowling pin that connects the stigma to the ovary, the larger base of the bowling pin.  The ovary contains ovules, inside of which eggs form.  When those eggs are fertilized, they become seeds.  Flowers can have one or more carpel, which collectively are referred to as the pistil.  This pistil is the female reproductive organ of a flower.

A perfect flower is one which has sepals, petals, stamens, and carpels.  These flowers have everything needed to reproduce. 

Flowers that are missing any one of the four floral parts are called imperfect flowers.  Some examples are pistillate or carpellate flowers that have one or more pistils but no stamen.  These flowers are essentially female.  Staminate flowers have stamens but no pistils.  In some cases, flowers have neither stamen nor pistils, making them sterile and preventing them from participating in the reproductive process.

When a single plant has either perfect flowers or both pistillate and staminate flowers, it is called monoecious, meaning “one house” in Greek.  Most flowering plants are monoecious.  Some plants may start out with only staminate flowers and later develop both staminate and pistillate flowers.

When a single plant develops either pistillate or staminate flowers but never both, they are referred to as dioecious, which means “two houses” in Greek.  For the pistillate plant to produce seeds, pollen must be carried by the wind or a pollinator from a nearby staminate plant to the pistillate plant.

Those plants that depend on pollinators for fertilization often have colorful and fragrant flowers to attract.  Those that depend on the wind are often not so colorful or fragrant, but instead have flowers high in the plant so that can be carried by the wind longer distances.

Inflorescence Types

Solitary flowers bear only one flower per stem or peduncle.  However, flowers often occur in clusters called inflorescences.

Flowers in an inflorescence that do not have a pedicel and attach directly to the peduncle are called sessile flowers or florets.  An inflorescence that consists of sessile flowers is called a spike. The gladiolus is an example of flowers arranged in a spike inflorescence.

A raceme is like a spike except each of the flowers or florets are attached to the peduncle with a short pedicel.  Snapdragons are examples of such an inflorescence.

A panicle is an inflorescence that has many branches that are often racemes. They can have determinate or indeterminate growth.

A corymb is a term for an inflorescence that is made of flowers or florets whose pedicels develop at random along the peduncle to create a flat or slightly convex top.  The outermost florets have longer pedicels while the innermost have shorter florets.  The flowers can either be parallel or alternate.

An umbel is an inflorescence where all of the pedicels spread from a common point at the tip of the peduncle like the ribs of an upside-down umbrella or parasol.  The resulting flower arrangement can be flat, convex, or almost spherical.

 A pseudanthium or composite flower is made of a cluster of hundreds, sometimes thousands, of stemless florets.  Composite flowers often have their florets arranged with hundreds of disc flowers that make up the central region with a ring of sterile ray flowers around the outside as in a sunflower.

A spadix is a spike of flowers densely arranged.  It is usually surrounded by a spathe which is a leaflike curved bract.  Calla lilies and peace lilies are examples of this type of inflorescence.

Floral Induction

Angiosperms must produce flowers to reproduce.  Before doing so, the plant must first undergo two important transitions. 

During their juvenile period, plants first must transition from the immature juvenile vegetative state of a seedling where they are not capable of producing flowers to the mature adult vegetative state where they can produce flowers.  This period of juvenility can last weeks, months, or even years before a plant is even capable of producing flowers, depending on the type of plant.

The second important transition occurs when the mature plant switches from its mature vegetative growth phase to its reproductive phase, resulting in the production of flowers.  This second transition occurs when the apical meristems in the shoot tips or buds are induced to form reproductive organs and is known as floral induction.  The timing of induction is typically affected by many physiological processes as well as external environment cues like the quality and duration of light, temperatures, the amount of moisture in the soil, and more. 

Reproductive Development

Once an angiosperm has been induced to produce flowers and those flowers have had a chance to fully develop and open, then reproduction can occur.  For reproduction to occur, pollen grains produced by the anther of a compatible plant must find their way to and make contact with a flower’s stigma.  This process is known as pollination.

If a plant’s flower is pollinated by pollen from the same flower or that of another flower on the same plant, then it is called self-pollination, because no other plant participated in the pollination process.  If the plant’s flower is pollinated by pollen from the flower of another plant, then it is called cross-pollination.

Once the pollen grain or gametophyte has landed on stigma, it germinates producing a pollen tube that grows down into the style until it reaches an ovule.   The two sperm cells, or male gametes, from the pollen grain travel down the pollen tube.  One sperm nucleus combines with an egg or female gamete within the carpel to form a zygote, which will develop into an embryo.  The other sperm nucleus fuses with the two polar bodies in the embryo sac to produce the endosperm, which develops into food tissue for the embryo.

With conifer gymnosperms, trees produce separate male cones that bear pollen and female cones that contain exposed ovules not protected behind a wall.  Once the cones open, pollination occurs when pollen from the male cones are transferred by wind to the female cones.  Fertilization occurs and “naked seeds” develop on the scales of the female cones.