Q.What is a flower? Describe the parts of a typical angiosperm flower.

A flower is a modified shoot meant for the sexual reproduction of angiosperms. The shoot apical meristem changes into a floral meristem, the internodes do not elongate, and the axis becomes condensed into the thalamus (receptacle) on which the floral parts are borne as modified leaves arranged in whorls.

A typical angiosperm flower has four whorls arranged successively on the thalamus:

• Calyx: The outermost whorl, made up of sepals, which are usually green, leaf-like and protect the flower in the bud stage.
• Corolla: The whorl of petals, usually brightly coloured to attract insects for pollination.
• Androecium: The male reproductive whorl, composed of stamens; each stamen has a filament and an anther, the anther bearing pollen grains. …

What is a flower?

A flower is a modified shoot in which the shoot apical meristem changes to a floral meristem. As the axis becomes condensed, the internodes do not elongate and the axis gets modified into the thalamus (also called the receptacle). Successive nodes then bear, in place of leaves, the different floral appendages, which are essentially modified leaves arranged in whorls. Because it produces the male and female gametes and brings about fertilisation, the flower is regarded as the reproductive unit in angiosperms, meant for sexual reproduction.

Parts of a typical flower

A typical flower has four different kinds of whorls arranged successively on the swollen end of the stalk (pedicel), i.e. on the thalamus. These four whorls are calyx, corolla, androecium and gynoecium.

• Calyx: The outermost whorl of the flower, and its members are called sepals. Sepals are generally green, leaf-like and function to protect the flower in the bud stage. When the sepals are free they are said to be polysepalous; when united they are gamosepalous.
• Corolla: The whorl inner to the calyx, composed of petals. Petals are usually brightly coloured and often scented to attract insects and other agents for pollination. Like the calyx, the corolla may be free (polypetalous) or united (gamopetalous); its shape and colour vary greatly.
• Androecium: The male reproductive whorl, composed of stamens. Each stamen consists of a stalk called the filament and a terminal anther. The anther is usually bilobed (dithecous), and each lobe carries pollen sacs in which the pollen grains are produced. A sterile stamen is a staminode. …

Method: Reasoning From "A Flower Is a Modified Shoot"

Instead of memorizing the four whorls as an unconnected list, derive them by reasoning through what happens when an ordinary vegetative shoot is converted into a flower. This developmental angle explains why the parts exist and are arranged the way they are, rather than asking you to simply recall four names.

Start from a normal shoot: it has an apical meristem, internodes that stretch out the stem, and leaves produced at successive nodes.

Now apply the single change that defines a flower: the apical meristem switches from producing vegetative growth to producing a floral bud, and, critically, the internodes stop elongating. Because the internodes freeze in place, all the appendages that would normally have been spread out along a long stem instead get compressed onto one short, swollen tip, the thalamus. …