The Flower

Structure of a Bisexual Flower:

• First of all, there is the stalk (pedicel) which supports the flower.
  • Some flowers may be without a stalk (sessile).
• The tip of the flower stalk may be expanded to form a cup-shaped receptacle thalamus.
  • The floral parts are borne on the thalamus in four whorls.
  • First whorl — green sepals (collectively called calyx).
  • Second whorl — large brightly colored petals (collectively called corolla).
  • Third whorl (male parts) (o = symbol for male) — long thread-like processes somewhat projecting out and each usually ending in a bilobed tip.
    • These are stamens (collectively named androecium).
    • Each stamen consists of a threadlike filament and a bilobed anther.
  • Fourth whorl (female parts) (centrally located pistil that may be formed of a single female unit (carpel) or of several fused carpels (collectively called gynoecium).
    • Each carpel consists of a basal ovary, a middle style, and an uppermost stigma.

Complete and Incomplete Flower:

• A complete (or perfect) flower is one that contains all four floral structures.
• If one or more sets of floral structures are missing, the flower is called incomplete (or imperfect).
  • Essential (reproductive) and non-essential (non-reproductive) parts of a flower.
  • The essential parts of a flower are those that are directly concerned with reproduction.
    • These parts consist of the stamens (male parts) and the carpels (female parts).
  • The non-essential (accessory) parts are simply the helping pans that either protect the reproductive parts of the flower or make the flower attractive for pollination.
  • These parts include the sepals and petals.
  • In some cases, the petals and sepals are undifferentiated together called perianth.
  • When the perianth is non-green it is described as petaloid.
    • When the perianth is green like the sepals, it is described as a sepaloid perianth.
• Bracts:
  • When a flower arises in the axil of a leaf-like structure, this structure is known as a bract.
  • Bracts may be green like ordinary leaves or at times they are colored.
  • The large and colorful bract of Bougainvillea is easily mistaken for petal.
    • The actual flower in this case is small, somewhat cylindrical, and attached on the inner side of the bract.
• Nectaries:
  • Most flowers produce a sweet fragrant liquid called nectar.
  • Groups of nectar-secreting cells, nectaries are situated usually at the base of the pistil or on the bases of the petals.
    • The nectar attracts insects like honeybees, for cross-pollination.
  • In some cases, nectaries are very prominent as in Nasturtium.

Sexuality in Flowers:

Male, female and bisexual flowers:

• The anthers of the stamens produce pollen which forms the male cells (male gametes).
• The ovary of the carpel bears the ovule which encloses the egg cell (female gamete).
  • The stamens and the carpels are the male and female parts of the flower respectively.
• A flower that contains both stamens and carpels is called bisexual or hermaphrodite or just a perfect flower.
  • A flower that has only one of these parts, that is, having only the stamens or only the carpets is called a unisexual or incomplete (imperfect) flower.
  • Example: papaya and palm.
  • A unisexual flower that contains only the stamens is called the male or staminate flower.
  • A flower that contains only the carpets is called the female or the pistillate flower.

Neutral Flower:

• A flower in which both male and female reproductive organs are lacking.
  • Example.- Ray florets of sunflower.

General Description of Flower Parts:

Calyx:

• Usually, there are five sepals.
  • Sometimes they may be less or in a few cases even more.
• The sepals may be free polysepalous or fused gamosepalous.
  • Sometimes, as in Hibiscus (shoe-flower), there may be a second series of sepals called episepalous, collectively called epicalyx.
  • When the flower opens the sepals may fall off or persist.
  • Sepals are usually green but in some cases, they are brightly colored (called petaloid) as in Gul Mohur (flame of the forest) where they are red.
  • The sepals protect the young flower bud and when green they also perform photosynthesis.

Corolla:

• The petals are generally arranged in a single whorl but sometimes there may be a double whorl (Example: Poppy) or a spiral (Example: Water lily).
• They may be free polypetalous or united gamopetalous forming a tube.
  • The petals attract insects for pollination.
  • They also protect stamens and pistils especially when the petals form a tube.

Androecium:

• The number of stamens in different flowers may vary from a few to a large number.
  • Each stamen consists of a long filament and an anther attached to its extremity.
  • The anther is usually two-lobed.
    • Each lobe has two pollen sacs [four sacs in all].
    • Within the pollen are contained pollen grains.
    • When fully matured the pollen sacs rupture to liberate pollen grains.
    • The pollen grains are generally powdery particles of different shapes and sizes.
    • In androecium, stamens may be free (polyandrous) as in Petunia or joined in different ways in single, double, or several groups.
  • Monadelphous: Stamens are united in one group by their filaments. Only antlers are free.
  • Example: china rose (staminal tube), cotton.
  • Diadelphous: The filaments are united in two bundles.
  • Example: pea (out of ten, nine stamens form a staminal tube while one is free).
  • Polyadelphous: The filaments are united in several groups. Example: Bombax.

Gynoecium:

• The gynoecium is also known as the pistil.
  • It is composed of one or more carpels.
  • Each carpel consists of three parts—stigma, style, and ovary.
  • Stigma is the terminal knob-like part, it may be divided into two or more lobes and may assume a feathery appearance.
  • The stigma is covered with hair or with glandular papillae. It serves as the landing place for pollen during pollination.
  • Style is the tubular slender stalk that connects the stigma to the ovary.
  • The ovary is the swollen basal portion composed of one or many carpels.
  • The inner cavity of the ovary may be a single chamber or divided into several chambers (locales) each containing a number of rounded bodies, the ovules.
    • The cushion or swollen region in the ovary attaching the ovules to the wall of the ovary is called the placenta.

Inflorescence and Placentation:

• The inflorescence is the mode of arrangement of flowers on the axis of the plum.
  • The flowers may be arranged in several different ways in different plants, this manner of arrangement is called an inflorescence.
• Placenta: Tissue that attaches the ovule to the wall of the ovary.
  • Ovules turn into seeds in the mature fruit (transformed ovary).
  • Then different arrangements can be seen in fruits such as those peas, tomatoes, mango, lotus, etc.