The Flower
ICSE Class 9 · Biology · Chapter 4
What this chapter covers
- The flower as the reproductive shoot
- Complete and incomplete flowers
- Essential and non-essential whorls
- The calyx
- The corolla
- The androecium
- The gynoecium
- A bisexual flower studied whole: the Hibiscus
- Inflorescence
- Placentation
Key points
- A flower is the reproductive organ of a flowering plant: a condensed shoot whose leaves have become floral parts.
- The pedicel is the stalk of a flower; the thalamus (receptacle) is its swollen tip, which bears the floral whorls; a bract is a leaf-like structure in whose axil a flower arises.
- The four whorls, outside to inside, are calyx (sepals), corolla (petals), androecium (stamens) and gynoecium (carpels).
- A complete flower has all four whorls; an incomplete flower lacks one or more.
- A bisexual flower has both androecium and gynoecium; a unisexual flower has only one (staminate = male, pistillate = female).
- Every unisexual flower is incomplete, but an incomplete flower can still be bisexual.
- The androecium and gynoecium are the essential whorls; the calyx and corolla are the non-essential (accessory) whorls.
- The calyx protects the bud and may make food or attract insects; the corolla attracts pollinators with colour, scent and nectar.
- A perianth, made of tepals, is formed when calyx and corolla look alike, as in the lily and onion.
- A stamen is a filament plus an anther; a typical anther has two lobes and four pollen sacs that produce pollen grains.
- A carpel is a stigma, a style and an ovary; the ovary contains ovules, each of which holds the egg.
- After fertilisation the ovary becomes the fruit and the ovules become seeds.
- The Hibiscus is a complete, bisexual flower with an epicalyx, a staminal tube and five fused carpels with a five-lobed stigma.
- An inflorescence is a group of flowers arranged in a definite way on a common stalk, the peduncle.
- Placentation is the arrangement of the ovules on the placentae inside the ovary.
Questions answered
Define a flower. Why is it described as a modified shoot?
A flower is the reproductive organ of a flowering plant, a modified and condensed shoot whose leaves are changed into floral parts. It arises from a terminal or axillary bud like a shoot, and its thalamus is a stem axis with nodes and very short internodes bearing the floral parts in place of leaves.
What is the thalamus? Name the four whorls it bears, from the outside inwards.
The thalamus (receptacle) is the swollen tip of the pedicel on which the floral parts are attached. It bears the calyx, corolla, androecium and gynoecium.
Distinguish between a complete flower and an incomplete flower, with one example of each.
A complete flower has all four whorls, for example the Hibiscus or mustard. An incomplete flower lacks one or more whorls, for example the castor flower (no corolla) or the male cucumber flower (no gynoecium).
"All unisexual flowers are incomplete, but all incomplete flowers are not unisexual." Explain.
A unisexual flower lacks either the androecium or the gynoecium, so it always lacks one whorl and is incomplete. An incomplete flower may lack only the calyx or corolla while keeping both stamens and carpels; such a flower is incomplete but bisexual.
Why are the androecium and gynoecium called the essential whorls?
They take a direct part in reproduction: the androecium produces pollen grains that give the male gametes, and the gynoecium produces ovules containing the egg. Without them no seed can form. The calyx and corolla only protect and attract, so they are non-essential.
Give three functions of the calyx.
Any three: it protects the inner whorls in the bud; green sepals make food by photosynthesis; petaloid sepals attract pollinators; it supports the petals; in some plants it persists on the fruit or forms hairs that help disperse it.
What is a perianth? Name a flower that has one.
A perianth is the outer, non-essential part of a flower formed when the calyx and corolla cannot be told apart; its members are tepals. The lily, onion and tuberose have a perianth.
Name the parts of a stamen and state the function of each.
The filament is a stalk that holds the anther in position and supplies it with water and food. The anther, usually with two lobes and four pollen sacs joined by a connective, produces pollen grains.
Past board exam questions
ICSE 2019 sample paper · 1 mark
The term neuter is given to a flower in which both:
- Calyx and corolla are present
- Calyx and corolla are absent
- Androecium and Gynoecium are present
- Androecium and Gynoecium are absent
Show answer
Androecium and Gynoecium are absent