The flower  

The flower is a modified shoot, meant for sexual reproduction, so it is a reproductive unit in the angiosperms. A typical or complete flower has four main parts i.e. Calyx and Corolla (as accessory organs) and Androecium and Gynoecium (reproductive organs). All these parts are arranged in a cyclic manner on the thalamus. (Thalamus or receptacle: A swollen end of the stalk or pedicle of the flower.)

  • In some flowers like lily, the calyx and corolla are not distinct and are termed as perianth
  • A Flower with only one reproductive organ either stamen or carpel is Unisexual flowers are imperfect
  • Flowers with both of these organs (stamen and carpel) are present is called bisexual or hermaphrodites.
  • A plant bearing bisexual, unisexual, and even neutral flowers is called polygamous. e.g. Mangifera indica, Polygonum SPS.
Symmetry of Flower

The number, shape, size, and arrangement of floral leaves in a flower determine the symmetry. Following types of symmetry found in flowers:

  • Actinomorphic (radial symmetry): When a flower can be divided into two equal radial halves in any radial plane passing through the center, it is said to be actinomorphic, e.g., mustard (Brassica), datura, chili
  • Zygomorphic (bilateral symmetry): In this type the flower can be divided into two similar halves only in one particular vertical plane, it is zygomorphic, e.g., pea, Gulmohur, Bean, Cassia.
  • Asymmetric (irregular): If the flower cannot be divided into two similar halves by any vertical plane passing through the center it is called an Asymmetric flower. e.g. Canna
Other types of flowers on a different basis
  • Monochlamydeous flower: in this type of flower the accessory whorl is not distinguished as calyx and corolla. e.g. monocots
  • Dichlamydeous flower: Accessory whorl of the flower is well distinguished into calyx and corolla. e.g. dicots.
  • Achlamydeous flower: In these flowers the accessory whorl is absent.
  • Bracteate Flowers: Flowers with bracts-reduced leaf found at the base of the pedicel- are called bracteates
  • Ebracteate Flower: those flowers are without bracts called ebracteate.

Types of flower on basis of position of floral parts on thalamus

On the basis of the position of floral parts on the thalamus
  1. Hypogynous: In this type of flower, the gynoecium occupies the highest position while the other parts are situated below it. The ovary in such flowers is said to be superior, e.g., mustard, china rose, and brinjal.
  2. Perigynous: In these flowers, the gynoecium is situated in the center and other parts of the flower are located on the rim of the thalamus almost at the same level, it is called perigynous. The ovary here is said to be half inferior (or it can be said that ovary is semi superior or semi inferior), e.g., plum, rose, peach.
  3. Epigynous flower: In these flowers, the margin of the thalamus grows upto the top of the ovary completely, and getting fused with it (ovary completely embedded in the thalamus), the other parts of the flower arise above the ovary. i.e. the ovary is said to be inferior. e.g. flowers of guava and cucumber, and the ray florets of sunflower
Parts of flower

Typical flower parts divided into two groups as accessory whorl- Calyx, corolla, and necessary or essential whorls– androecium and gynoecium.

Parts of a flower

Accessory parts: these are formed by the transformation of the leaves into the non-fertile parts of the flower, such as sepals, petals, tepals, and lodicules. The parts of accessory whorl are:

1. Calyx (K)
    • it is the outermost whorl of the flower and the 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 calyx with fused sepal) or Polysepalous (sepals free). The function of the calyx is to protect floral parts during anthesis.
    • Epicalyx (EK)- it is the group of bracteoles in a flower. Bracteole is a membranous structure present below calyx. these are found in members of the family Malvaceae.
    • Perianth (Pe or P): Neither distinguished as calyx nor corolla. its unit is called
    • Lodicules: These are rudiment of tepals, found in a member of the family
2. Corolla (C)

It is the second whorl of flowers and made up of Petals that are usually brightly colored to attract insects for pollination. Corolla with free petals is called polypetalous, and corolla with fused petals is called gamopetalous.

The forms of corolla may be as follows:

  • Cruciform: It consists of 4 petals arranged crosswise, each petal is clawed. e.g. mustard (Brassica compestris), so the name of the family is Cruciferae.
  • Caryophyllaceous: it consists of 5 petals, each with a comparatively long claw and the limbs of petals are placed at the right angles to the claw, e.g. Dianthus.
  • Rosaceous: it consists of 5 petals, with short claws or none. the limbs spread regularly outwards e.g. Rosa indica.
  • Campanulate or Bell-shaped: the shape of the corolla is like a bell. e.g. Physalis.
  • Tubular: Corolla is tubular or cylindrical and gamopetalous. e.g. Disc floret of Helianthus sp.
  • Infundibuliform or Funnel-shaped: Corolla is in shape like a funnel, e.g. Datura
  • Rotate or wheel-shaped: the tube of corolla is found narrow and short and the limbs are right angles to the tube e.g. Solanum melongena (Brinjal).

forms of corolla

Aestivation

The mode of arrangement of sepals or petals in the floral bud with respect to the other members of the same whorl is known as aestivation. there are the following types of aestivation: valvate, twisted, imbricated, Quincuncial, and vexillary.

  1. Valvate: When sepals or petals in a whorl just touch one another at the margin, without overlapping, as in Calotropis, it is said to be valvate.
  2. Twisted: If one margin of the appendage overlaps that of the next one and so on, it is called twisted. e.g. in china rose, lady’s finger, and cotton.
  3. Imbricate: If the margins of sepals or petals overlap one another but not in any particular direction, the aestivation is called imbricate. e.g. Cassia and Gulmohar,
    • Quincuncial: it is a special type of imbricate aestivation in which 2 petals are external, 2 are internal and one petal in one margin is overlapped. e.g. Cucurbita maxima, Ranunculus.
  4. Vexillary: the posterior one is the largest and almost covers the two lateral petals and these overlap two anterior petals. As in pea and bean flowers, there are five petals, the largest (standard) overlaps the two lateral petals (wings) which in turn overlap the two smallest anterior petals (keel); this type of aestivation is known as vexillary or papilionaceous.

Aestivation

 

Essential parts: it includes the fertile part of flowers, such as stamen and carpel.

3. Androecium (A)

It represents the male part of the flower, composed of a group of stamens. Each stamen consists of three parts:

  1. Filament: Lower thread-like part of the stamen is called the filament.
  2. Anther: upper swollen part of stamen usually having two lobes or theca. In the Malvaceae family, it shows only one lobe. Each lobe contains each lobe has two chambers, the pollen sacs. the pollen grains are produced in pollen sacs. The two lobbed anther is called dithecous anther and with one lobe it is called monothecous.
  3. Connective: the filament of stamen is extended in between the two anther lobes and is called connective.

Note: the stamen without anther is called Staminode i.e. sterile stamen.

stamen

Different Ways of attachments of filament to the anther: 

  1. Adnate: The filament runs along the back of anther e.g. MIchelia (Champa).
  2. Basifixed: Anther is fixed to the filament by its base e.g. Datura, Brassica, Raphanus.
  3. Dorsifixed: Anther is fixed to the filament by its back and the anther is immobile. e.g passionflower
  4. Versatile: The anther is attached on tip of the filament, at a point so it is able to swing freey. e.g. Triticum (grasses) and wheat.
Cohesion of stamens

(Different conditions of stamen)

  • Polyandrous condition: In a flower the stamen of an androecium may be free from one another, the condition is called Polyandrous.
  • Adelphous condition: when stamens are united by their filaments but the anther remains free then it is called adelphous condition.
  • Syngenasious condition: In a flower, if stamen is united by their anther only and the filaments remain free, then the condition is called synganasious condition. Example- Helianthus species. Asteraceae family.
  • Synandrous condition: If both filaments, as well as anthers of different stamen, are united then it is called synandrous condition. e.g. Colocasia, Cucurbita.

Cohesion may be of the following types:

  1. Monadelphous stamen– all filaments are united into a single bundle but anthers are free, e.g. Hibiscus rosa-sinesis, Hibiscus esculentus.
  2. Diadelphous stamens– the filaments are united in two bundles, the anter remaining free e.g. Pisum sativum.
  3. Polyadelphous stamen: filaments are united into more than two bundles but anthers are free. e.g. castor (Ricinus), Lemon (Citrus).
Adhesion of stamens:

in adhesion, the stamens are attached to the tepals, petals, or gynoecium. These are maybe of following:

  1. Epiphyllous Condition: when stamens are attached to perianth e.g. members of
  2. Epipetalous Condition: when stamens are attached to petal. e.g. Datura, solanum.
  3. Gynandrous Condition: when stamens are attached to the carpel (the unit of gynoecium), e.g. Calotropis.

Sometimes stamen forms a hairy scale-like structure, this type of outgrowth is called the staminal corona, e.g. Nerium.

4. Gynoecium or Pistil (G):

The gynoecium is the female reproductive part of the flower and is made up of one or more carpels (megasporophylls). A carpel consists of three parts namely ovary, style, and stigma.

  1. Ovary- Ovary is the enlarged basal part containing the ovule. Each ovary bears one or more ovules attached to a flattened, cushion-like placenta
  2. Style- elongated thread-like structure attached to the apex of the ovary is called style.
  3. Stigma- The style connects the ovary to the stigma, the stigma is usually at the tip of the style and is the receptive surface for pollen grains.

reproduction in flower

Gynoecium may be monocarpellary (one carpel) e.g. pea, or multicarpellary e.g. papaver.

  • if the carpels are free it is called apocarpous gynoecium, e.g. Rose, Lotus.
  • If the carpels are fused it is called syncarpous gynoecium. e.g. Lycopersicon, Brassica. Syncarpous gynoecium may be bicarpellary, tricapellary, tetracarpellary, pentacarpellary or multicarpellary.
  • if the ovary has one chamber i.e. locule, called the unilocular ovary. It also may be divided into several chambers (locules) and may be bilocular, trilocular, tetralocular, Penta, or multilocular depending upon the number of locules in the ovary.
    • In mustard the carpel is unilocular but a false septum called replum is present due to this, the carpel seems to be bicarpellary.

ovary of flowers on the basis of chamber (locules)

Placentation:

the ovules attached on the inner surface of the ovary wall on one or more cushions called the placenta. The arrangement of ovules within the ovary is known as placentation. This may be of the following types:

  • Marginal: In the marginal placentation, the placenta forms a ridge along the ventral suture of the ovary and the ovules are borne on this ridge forming two rows. e.g. in pea.
  • Parietal: In this type of placentation, the ovules develop on the inner wall of the ovary or on the peripheral part. The ovary is one-chambered but it becomes two-chambered due to the formation of the false septum, e.g., mustard (Brassicaceae), Argemone (Papaveraceae), Cucurbita.
  • Axile: In a flower when the placenta is axial and the ovules are attached to it in a multilocular ovary, the placentation is said to be axile. e.g. china rose (Malvaceae) , tomato and lemon (Citrus), Solanum nigrum (Solanaceae).
  • Free Central: the ovary is unilocular, the ovules are borne on the central axis and septa are absent, the placentation is called free central. e.g. Dianthus (Caryophyllaceae), and Primrose.
  • Besal: In basal placentation, the placenta develops at the base of the ovary and a single ovule is attached to it. e.g. sunflower (Compositae) Asteraceae, marigold, Wheat, Maize (Poaceae).

placentation in the flower

 

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Reproduction in Organisms - PCSSTUDIES - Biology · May 12, 2021 at 7:15 pm

[…] Sexual reproduction in flowering plants (Angiosperms) […]

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