Flower: Definition, Parts, Function, Symmetry & Diagram

The flower is the reproductive unit in the angiosperms. It is a modified shoot, meant for sexual reproduction. It consists of four whorls which are successively arranged on the thalamus or receptacle. Thalamus is the swollen end of the pedicel or the stalk. The four whorls present in a flower are calyx, corolla, androecium, gynoecium.

The calyx and corolla are non-essential, accessory organs and androecium and gynoecium are the reproductive organs of a flower. In some flowers like lily, the calyx and corolla are not distinct and are termed as perianth.

Terminology used w.r.t. Flower

(i) Bisexual flower: When a flower has both androecium and gynoecium e.g., Pea, Hibiscus

(ii) Unisexual flower: A flower having either only stamens (androecium) or only carpels (gynoecium) e.g., Maize.

(iii) Trimerous flower: When all the floral appendages (whorls) are in the multiples of three.

(iv) Tetramerous flower: When all the floral appendages are in the multiples of four.

(v) Pentamerous flower : When all the floral appendages are in the multiples of five.

(vi) Bracteate flower: Flower with bracts (reduced leaf found at the base of the pedicel).

(vii) Ebracteate flower: Flower without bracts.

Symmetry of Flower

The arrangement of the floral organs around the axis of a flower is known as floral symmetry.

• (I). Actinomorphic flower (radial symmetry) : When a flower can be divided into two equal radial halves in any radial plane passing through the centre e.g., mustard, Datura, chilli.
• (II). Zygomorphic flower (bilateral symmetry): When a flower can be divided into two similar halves only in one particular vertical plane e.g., pea, gulmohur, bean, Cassia.
• (III). Asymmetric flower (irregular): When a fiower cannot be divided into two similar halves by any vertical plane passing through the centre e.g., canna

Position of Floral Parts on Thalamus

Depending upon the position of calyx, corolla, androecium in respect of the ovary on the thalamus, the flowers can be hypogynous, perigynous and epigynous. The flower in which gynoecium i.e., female reproductive part (Ovary) occupies the highest position while the other parts are situated below it are called hypogynous flowers. The ovary in such flowers is said to be superior e.g., mustard, china rose, brinjal, petunia.

If gynoecium is situated in the centre and other parts of the flower are located on the rim or periphery of the thalamus, almost at the same level as the ovary, then flower is called perigynous. The ovary in such flowers are said to be half inferior e.g., plum, rose, peach.

The flowers in which the margin of the thalamus grows upward enclosing the ovary completely and getting fused with it, the other parts of the flower arise above the ovary. Such type of flowers are called epigynous. The ovary is said to be inferior e.g., guava, cucumber, bittergourd, the ray floret of sunflower.

Parts of a Flower

A flower normally has four whorls namely calyx (sepals), corolla (petals), androecium (stamen) and gynoecium (carpel).

Calyx (Sepals)

Calyx is the outermost whorl of the flower and the members are called sepals. The sepals are generally green leaf-like structure that protect the flower in the bud stage. The calyx may be gamosepalous (sepals united) or polysepalous (sepals free).

Corolla (Petals)

Corolla is the second whorl of the flower. The individual leaf segment of the corolla is said to be petals. The corolla or petals are generally brightly coloured, have fragrance, which makes the flower more attractive. The colourful and fragrant petals attract insects for pollination. The shape and structure of corolla varies in different flowers. The different shapes of corolla that exist in nature are – tubular, bell-shaped, funnel-shaped, wheel-shaped etc. Like calyx, corolla may be also free (polypetalous) or united (gamopetalous).

Aestivation

The mode of arrangement of sepals or petals in a floral bud with respect to the other member of the same whorl is called aestivation. It may be of following types

• (i) Valvate: In valvate aestivation, the margin of sepals or petals, present in a whorl just touch each other. There is no overlapping between the sepals or petals. e.g. Calotropis.
• (ii) Twisted : In twisted aestivation, margin of one petal or sepal overlaps the margin of the adjacent successive petal or sepal and so on e.g., China rose, lady’s finger and cotton.
• (iii) Imbricate: In imbricate aestivation, margin of petals or sepals overlaps each other but not in a particular direction. e.g., Cassia, gulmohur.

(iv) Vexillary: In vexillary aestivation, the largest petal (standard) overlaps the two smaller lateral petals (Wings) which in turn overlap the two smallest anterior petals (keel) e.g. Pea and bean flower.

Androecium

Androecium is the third whorl of the flower which arises inner to the corolla. It is the male reproductive system which is composed of stamens. A stamen consists of a filament and anther. Anthers are usually bilobed. Each lobe Contains two microsporangia or pollen sacs. The pollen grains are produced in pollen sacs. A sterile stamen is called staminode. There may be a variation in the length of filaments within a flower, as in Salvia and mustard.

Adhesion of stamens: The stamen may be attached to other floral organs such as petals, sepals etc. When a stamen is attached to the petal, then it is called epipetalous e.g., brinjal and when the stamen is attached to the perianth then it is said to be epiphyllous e.g., lily.

Cohesion of stamens: The stamens may be free or united. when the stamens are free then they are called polyandrous and when stamens are united in a Single bundle, then it is called monoadelphous, as in china rose, when they are united in two bundles, then it is called diadelphous e.g, pea and- when united into more than two bundles it is called polyadelphous e.g., Citrus.

Gynoecium

It is the fourth and the last whorl of the flower. Gynoecium is the female reproductive part of the flower which is composed of one or more carpels. Carpels may be free or fused. When the carpels are free (as in lotus and rose) then they are said to be apocarpous and when the carpels are fused (as in mustard and tomato) then they are said to be syncarpous. A carpel has three parts namely.

(i) Ovary: Ovary is the basal, swollen part of the carpel. It is the lower part of the carpel which bears one or more Ovules. These ovules, after fertilisation mature into seeds. The ovules are attached to a flattened, cushion-like structure called placenta. The ovary. has one or more chambers or loculi. The ovary containing one chamber is unilocular, two chambers is bilocular, three chamber is triocular and so on. The Ovules are borne in these chambers. The wall of the ovary after fertilisation forms the pericarp (fruit wall).

(ii) Style: The tube-like structure which connects the stigma to the ovary is called style. It lies above the ovary in a carpel.

(iii) Stigma: Stigma is generally situated at the tip of the style. Stigma acts as the receptive organ for pollen grains during pollination.

After fertilisation ovules develop into seeds and ovary matures into a fruit.

Placentation

Ovary bears ovules on a cushion-like structure called placenta. An ovary may have one or more placenta. The arrangement of ovules on placenta within the ovary is known as placentation. It is of following types:

• (i) Marginal: The placenta in marginal placentation forms a ridge along the ventral suture of the ovary. The ovules are borne in two alternate rows along the ridge e.g., Pea plant.
• (ii) Axile: In axile placentation, the placenta is present in the axial position and the ovules are attached to it in a multilocular ovary e.g., China rose, tomato and lemon.
• (iii) Parietal: The Ovary is one-chambered (unilocular) but become two-chambered due to the formation of the false septum. The ovules develop on the inner wall of the ovary or on the peripheral part e.g., Mustard, Argemone.
• (iv) Free central: The ovules are borne on central axis and septa are absent in the ovary e.g., Primrose, Dianthus.
• (v) Basal: The placenta develops at the base of the ovary. It has a single ovule attached to the placenta e.g., Sunflower, Marigold.

Fig.: Types of placentation A. Marginal, B. Axile, C. Parietal, D. Free central, E. Basal