Flower

FLOWER 

 

  • The flower serves as the reproductive unit in angiosperms, designed for sexual reproduction.
  • A typical flower consists of four whorls: calyx, corolla, androecium, and gynoecium.
  • Calyx and corolla are accessory organs, while androecium and gynoecium are reproductive.
  • In some flowers like lilies, calyx and corolla together are known as perianth.



  • A bisexual flower has both androecium and gynoecium; unisexual flowers have either stamens or carpels.
  • Flower symmetry can be either actinomorphic (radial symmetry) or zygomorphic (bilateral symmetry).

-Actinomorphic flowers can be divided into two equal radial halves in any plane through the centre, seen in plants like mustard, datura, and chilli. 

-Zygomorphic flowers can only be divided into two similar halves in a specific vertical plane, seen in plants like pea, gulmohur, bean, and Cassia.

 

  • Asymmetric (irregular) flowers cannot be divided into two similar halves by any vertical plane, as seen in canna.
  • Floral appendages in multiples of 3, 4, or 5 define trimerous, tetramerous, or pentamerous flowers.
  • Flowers with reduced leaf-like structures (bracts) at the pedicel base are bracteate, while those without are ebracteate.
  • Flower positions relative to the ovary on the thalamus result in three types: hypogynous, perigynous, and epigynous.

-In hypogynous flowers, the gynoecium is highest, while other parts are beneath. Ovary is superior. Examples: mustard, china rose, brinjal. 

 

-Perigynous flowers have gynoecium at the centre, other parts on the thalamus rim at similar levels. Ovary is half-inferior. Examples: plum, rose, peach.

 

-Epigynous flowers show thalamus margin growing over and fusing with the ovary, with other parts above. Ovary is inferior. Examples: guava, cucumber, sunflower's ray florets.

 

 

Parts of flowers

Normally flower has four floral whorls, viz., calyx, corolla, androecium and gynoecium. 

Calyx:

 

  • The calyx is an essential part of a flower, forming the outermost whorl of floral structures.
  • It is composed of individual units known as sepals. The calyx serves several crucial functions within the flower's life cycle.
  • Sepals are the individual components that make up the calyx. They are often leaf-like and can vary in size, shape, and colour across different plant species. 
  • Green is a common colour for sepals, resembling leaves, and they are typically located directly beneath the petals.


  • Function of calyx:

Protection in Bud Stage: One of the primary roles of sepals is to provide protection to the delicate inner parts of the flower while it is still in its bud stage. They encase the developing floral structures, shielding them from potential damage, such as mechanical stress, weather conditions, or herbivores.

 

  • Calyx Variation:

The calyx can exhibit different arrangements based on how the sepals are united or separated.

 

Gamosepalous:

In some flowers, the sepals are fused or united, forming a single structure. This type of calyx is referred to as gamosepalous. 

The fused sepals can have various shapes and sizes, contributing to the overall appearance of the flower.

 

Polysepalous:

Other flowers have sepals that remain separate from one another, forming distinct and individual units. This type of calyx is termed polysepalous. 

The sepals in polysepalous calyces are often more pronounced and can stand out distinctly from one another. 

Corolla:

 

  • The corolla constitutes the second whorl of a flower, positioned inside the calyx. It is composed of individual segments called petals.
  • Petals play a significant role in attracting pollinators, primarily insects, through their vibrant and eye-catching colours. This attraction is vital for successful pollination and the subsequent reproduction of the plant.
  • Function of Petals:

One of the primary functions of petals is to assist in pollination. Their striking colours and  patterns serve as visual cues that help guide pollinators toward the flower's reproductive structures, such as the stamen and stigma.

Additionally, petals can emit fragrances that further entice pollinators, enhancing the flower's chances of successful cross-pollination.

 

  • The corolla exhibits remarkable diversity across different plant species. Petals can vary significantly in terms of shape, size, texture, and arrangement.
  • Corolla shape can range from tubular, where petals form a cylindrical tube, to bell-shaped, funnel-shaped, or even wheel-shaped, each with distinct characteristics that contribute to the flower's overall appearance.
  • Variation in Corolla:

Similar to the calyx, the corolla can also be categorised into two main types based on the arrangement of its petals:

 

Gamopetalous:

In certain flowers, the petals are fused or united, forming a single corolla structure. This type is referred to as gamopetalous corolla.

The fused petals can create various shapes that enhance the flower's visual appeal.

 

Polypetalous: 

Other flowers have separate petals that do not fuse, resulting in distinct and individual petal units. This type is termed polypetalous corolla. 

The separate petals provide unique textures and often contribute to the flower's intricate design.

 

 

Aestivation 

 

  • Aestivation refers to the specific arrangement of sepals or petals within a floral bud concerning other members of the same whorl.


  • It plays a critical role in the protection and development of the inner floral structures.


  • Types of Aestivation:

Valvate Aestivation: In valvate aestivation, sepals or petals within a whorl merely touch each other at their margins without overlapping. 

For example, Calotropis exhibits valvate aestivation. This arrangement ensures minimal exposure of inner floral parts.

 

Twisted Aestivation: Twisted aestivation involves the overlapping of one margin of an appendage over the next, creating a spiral-like arrangement. 

Flowers like china rose, lady's finger, and cotton display twisted aestivation. This arrangement offers effective protection during the bud stage.

 

Imbricate Aestivation:Imbricate aestivation is characterised by overlapping margins of sepals or petals, but the direction of overlap is not specific. 

Cassia and gulmohar flowers showcase imbricate aestivation. This arrangement enhances protection and adds structural stability.

 

Vexillary Aestivation:Vexillary or papilionaceous aestivation is distinctive and observed in pea and bean flowers. 

Here, five petals are present: the largest (standard) overlaps the two lateral petals (wings), which, in turn, overlap the two smallest anterior petals (keel). 

This unique arrangement ensures efficient protection and facilitates pollination.

 

 

 

 

Importance of Aestivation:

Aestivation is crucial for safeguarding the delicate inner floral parts during the bud stage, shielding them from mechanical damage, external elements, and potential harm.

Additionally, the aestivation pattern can influence pollination mechanisms. Certain aestivation types may facilitate specific interactions with pollinators, contributing to the reproductive success of the plant. 

Androecium:

 

  • The androecium is the male reproductive part of a flower, composed of stamens.
  • Each stamen consists of a filament (stalk) and an anther.



  • The anther is usually bilobed and contains two chambers called pollen-sacs, where pollen grains are produced.
  • A sterile stamen is referred to as a staminode.
  • Attachment and Union of Stamens:

Stamens can be attached to other flower parts such as petals or to each other. 

When stamens are attached to petals, they are called epipetalous (e.g., brinjal). 

If stamens are attached to the perianth (the collective term for sepals and petals), they are termed epiphyllous (e.g., lily).

 



  • Types of Staminal Union:

Monadelphous:

Stamens unite into a single bundle.

Example: China rose.

 

Diadelphous:

Stamens unite into two bundles.

Example: Pea.

 

Polyadelphous:

Stamens unite into more than two bundles.

Example: Citrus.

 




  • Variation in Filament Length:

Some flowers exhibit variations in filament length within a single flower.

 

Types based on their length

Isostemonous: All the stamens in a flower are of the same length.

 

Obdiplostemonous: Stamen length occurs in two distinct whorls.

 

Diplostemonous: Stamen length occurs in two whorls, with the inner whorl being shorter than the outer whorl.

 

Polystemonous: There are multiple stamen lengths within a flower.

 

Haplostemonous: All stamens in the flower are of varying lengths.

 

Trimorphic: Stamen lengths occur in three distinct categories.For example, Salvia and mustard can have stamens with varying filament lengths. 

Gynoecium:

 

  • The gynoecium is the female reproductive part of a flower, composed of carpels.
  • A carpel consists of three main parts: stigma, style, and ovary.
  • The ovary is the basal part, the style connects the ovary to the stigma, and the stigma is the receptive surface for pollen grains.



  • Components of a Carpel:

Ovary: The enlarged basal part of the carpel contains ovules.

 

Style: An elongated tube that connects the ovary to the stigma.

 

Stigma: Usually located at the tip of the style, this is where pollen grains land for fertilisation.

 

  • Ovule Attachment and Placenta:

Each ovary bears one or more ovules.

 

Ovules are attached to a flattened, cushion-like structure called the placenta within the ovary.

 

  • Types of Carpels:

Apocarpous: When multiple carpels are present, they may be free and not fused.

Examples: Lotus, rose.

 

Syncarpous: Carpels are fused together.

Examples: Mustard, tomato.

 

 

  • Fertilisation and Fruit Development:

After fertilisation, ovules develop into seeds, containing the embryo and stored nutrients.

 

The ovary matures into a fruit, which protects and aids in the dispersal of seeds.

 

  • Placentation:

Placentation refers to the arrangement of ovules within the ovary of a flower.

 

Different types of placentation include marginal, axile, parietal, basal, central, and free central.

 

Types of Placentation:

-Marginal Placentation:

Placenta forms a ridge along the ventral suture of the ovary.

Ovules are borne on this ridge, arranged in two rows.

Example: Pea.

 

-Axile Placentation:

Placenta is located in the centre (axis) of the ovary.

Ovules are attached to the placenta in a multilocular (multi chambered) ovary.

Examples: China rose, tomato, lemon.

 

-Parietal Placentation:

Ovules develop on the inner wall of the ovary or the peripheral part.

Ovary appears one-chambered, but a false septum may form, making it two-chambered.

Examples: Mustard, Argemone.

 

-Free Central Placentation:

Ovules are borne on a central axis, and septa (dividing walls) are absent.

Examples: Dianthus, Primrose.

 

-Basal Placentation:

Placenta develops at the base of the ovary.

A single ovule is attached to the placenta.

Examples: Sunflower, marigold.