What is a flower? Describe the parts of a typical angiosperm flower.
A flower is the reproductive unit in the angiosperms, meant for sexual reproduction. It is a modified shoot. A typical flower has four different kinds of whorls arranged successively on the swollen end of the stalk or pedicel, called thalamus or receptacle.
These four whorls are calyx, corolla, androecium and gynoecium. The calyx and corolla are accessory organs, while androecium and gynoecium are reproductive organs. In some flowers like lily, the calyx and corolla are not distinct and are termed as perianth.
The calyx is the outermost whorl and its members are called sepals. Generally, sepals are green, leaf-like and protect the flower in the bud stage. The calyx may be gamosepalous (sepals united) or polysepalous (sepals free).
The corolla is composed of petals which are usually brightly coloured to attract insects for pollination. Corolla may be gamopetalous (petals united) or polypetalous (petals free). The shape and colour of corolla vary greatly in plants. The mode of arrangement of sepals or petals in floral bud is known as aestivation.
When a flower has both androecium and gynoecium, it is bisexual. A flower having either only stamens or only carpels is unisexual.
Explanation
This answer comprehensively covers the definition of a flower as stated in NCERT textbook (reproductive unit in angiosperms, modified shoot meant for sexual reproduction) and systematically describes all four floral whorls. The calyx (outermost whorl with sepals) and corolla (petals) are accessory organs, while androecium and gynoecium are reproductive organs. Key terms like thalamus, perianth, gamosepalous/polysepalous, gamopetalous/polypetalous, aestivation, and bisexual/unisexual flowers are included as per the textbook context.