- A flower is a modified shoot wherein the shoot apical meristem changes to floral meristem.
- Internodes do not elongate and the axis gets condensed.
- The apex produces different kinds of floral appendages laterally at successive nodes instead of leaves.
- When a shoot tip transforms into a flower, it is solitary.
- The arrangement of cluster of flowers on the floral axis is called inflorescence.
- Based on whether the apex gets converted into a flower or continues to grow, inflorescences are 2 types: Racemose and Cymose.
o Racemose: In this, the main axis continues to grow. Flowers are borne laterally in an acropetal succession.
o Cymose: In this, main axis terminates in a flower, hence is limited in growth. Flowers are borne in a basipetal order.
- It is the reproductive unit in the angiosperms.
- It is meant for sexual reproduction.
- A flower has a stalk (pedicel). Its swollen end is called thalamus (receptacle).
- Reduced leaf found at the base of the pedicel is called bracts. Flowers with bracts are called bracteate and those without bracts, ebracteate.
- A typical flower has 4 kinds of whorls arranged on thalamus- calyx, corolla, androecium & gynoecium.
- Calyx & corolla are accessory organs, while androecium and gynoecium are reproductive organs.
- In flowers like lily, the calyx and corolla are not distinct. It is termed as perianth.
- When a flower has both androecium and gynoecium, it is bisexual. A flower having either only androecium or only gynoecium is unisexual.
Based on symmetry, flowers are 3 types:
o Actinomorphic (radial symmetry): Here, a flower can be divided into 2 equal radial halves in any radial plane passing through the centre. E.g. mustard, datura, chilli.
o Zygomorphic (bilateral symmetry): Here, a flower can be divided into two similar halves only in a particular vertical plane. E.g. pea, gulmohur, bean, Cassia.
o Asymmetric (irregular): Here, a flower cannot be divided into two similar halves by any vertical plane passing through the centre. E.g. canna.
Based on number of floral appendages, flowers are classified as follows:
o Trimerous: Floral appendages are multiple of 3.
o Tetramerous: Floral appendages are multiple of 4.
o Pentamerous: Floral appendages are multiple of 5.
Based on the position of calyx, corolla and androecium in respect of the ovary on thalamus, the flowers are 3 types:
o Hypogynous: Here, gynoecium occupies the highest position while other parts are situated below it. The ovary is superior. E.g. mustard, China rose & brinjal.
o Perigynous: Here, gynoecium is situated in the centre and other parts are located on the rim of the thalamus at the same level. Ovary is half inferior. E.g. plum, rose, peach.
o Epigynous: Here, the margin of thalamus grows upward enclosing the ovary completely and getting fused with it. Other parts arise above the ovary. The ovary is inferior. E.g. Guava, cucumber, ray florets of sunflower.
Parts of a Flower
- It is the outermost whorl of flower. It is made of 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).
- It is the whorl inner to calyx. It is composed of petals.
- Petals have bright colour to attract insects for pollination.
- Corolla may be gamopetalous (petals united) or polypetalous (petals free).
- Shape and colour of corolla vary in plants. Corolla may be tubular, bell-shaped, funnel-shaped or wheel-shaped.
Aestivation: It is the mode of arrangement of sepals and petals in floral bud. It is many types:
o Valvate: Sepals or petals in a whorl just touch one another at the margin, without overlapping. E.g. Calotropis.
o Twisted: One margin of the appendage overlaps that of the next one and so on. E.g. China rose, lady’s finger & cotton.
o Imbricate: Margins of sepals or petals overlap one another but not in any particular direction. E.g. Cassia & gulmohur.
o Vexillary (papilionaceous): In this, there are five petals; the largest (standard) overlaps the two lateral petals (wings) which in turn overlap the two smallest anterior petals (keel). E.g. pea & bean.
- The male reproductive part composed of stamens.
- Each stamen represents the male reproductive organ. It consists of a stalk (filament) and an anther.
- Each anther is usually bilobed.
- Each lobe has 2 chambers called pollen-sacs.
- In pollen-sacs, pollen grains are produced.
- A sterile stamen is called staminode.
- When stamens are attached to petals, they are epipetalous. E.g. brinjal. When stamens are attached to perianth they are epiphyllous. E.g. lily.
- If the stamens are free, it is called polyandrous.
- If they are united, it is called synandrous. It is many types:
o Monadelphous: Stamens are united into one bunch or one bundle. E.g. China rose.
o Diadelphous: Stamens are united into two bundles. E.g. pea.
o Polyadelphous: Stamens are united into more than two bundles. E.g. citrus.
- There may be a variation in the length of filaments within a flower. E.g. Salvia and mustard.
d) Gynoecium (Pistil)
The female reproductive part made up of one or more carpels.
A carpel has 3 parts:
o Stigma: It is the receptive surface for pollen grains. It is usually at the tip of the style.
o Style: An elongated tube that connects ovary to stigma.
o Ovary: It is the enlarged basal part on which the style lies. Each ovary bears one or more ovules attached to a flattened, cushion-like placenta.
Polycarpellary pistils (pistil with many carpels) are 2 types:
o Apocarpous: Carpels are free. E.g. lotus and rose.
o Syncarpous: Carpels are fused. E.g. mustard and tomato.
Placentation: It is the arrangement of ovules on the placenta within the ovary. It is many types:
o Marginal: Here, 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. pea.
o Axile: Here, the placenta is axial and the ovules are attached to it in a multilocular ovary. E.g. China rose, tomato and lemon.
o Parietal: Here, the ovules develop on the inner wall of the ovary or on peripheral part. Ovary is one-chambered but it becomes two-chambered due to the formation of the false septum. E.g. mustard and Argemone.
o Basal: Here, placenta develops at the base of ovary and a single ovule is attached to it. E.g. sunflower, marigold.
o Free central: Here, ovules are borne on central axis and septa are absent. E.g. Dianthus and Primrose.
After fertilisation, the ovules develop into seeds and the ovary matures into a fruit.