The flower is the most attractive, brightly, coloured and conspicuous part of the plant. It helps in sexual reproduction and later forms fruit and seeds.
PARTS OF A FLOWER
The flower is attached with the stem or its branches with the help of a stalk called pedicel. If the flower is without pedicel, it is called sessile.
The pedicel may arise from the axil of a green leaf- like structure called bract. Sometimes, the pedicel may bear two small, leafy, green bract-like structures called bracteoles.
These four whorls from outer to inner side are :
The first two whorls are non-essential floral parts whereas, the inner two whorls are essential parts which are involved in the process of reproduction. These floral structures are explained as follow :
They constitute the outermost and lowermost whorl. They are mostly small, green, leaf-like structures which cover an unopened floral bud.
(a) Gamosepalous – When all the sepals are fused together, fully or partially, they are termed as gamosepalous e.g. Sunflower, China rose, Marigold, Petunia etc.
(b) Polysepalous – When all the sepals are free from each other they are called polysepalous. e.g. Ranunculus, Brassica.
Petals, that constitute the second whorl are collectively called corolla. They arise inner to and above the sepals on thalamus.
They are usually brightly coloured and showy leaf-like structures in the day blooming flowers and strongly scented and whitish in night blooming flowers.
They are of 2 type :
(a) Gamopetalous :- When all the petals are fused fully or partially e.g. Petunia.
(b) Polypetalous :– When the petals are free from each other e.g. Hibiscus.
In some flowers, such as Asparagus, non-essential parts of the flower are not differentiated into sepals and petals. They are, collectively known as perianth. The individual perianth lobes are called tepals. The tepals may be greenish (seploid) or beautifully coloured (petaloid).
The corolla or perianth may sometimes bear outgrowths, such as spurs, nectaries and corona.
Inner to and above the petals are present male reproductive organs called stamens. They collectively constitute the androecium. The stamens are present either in a single or more whorls.
Each stamen may be regarded as a highly modified leaf (called microsporophyll). A typical stamen is differentiated into two parts i.e.
The filament is a long stalk. It supports a fertile head called anther. The two anther lobes are attached at the back by a small connective. Each anther consists of two lobes.
Each lobe is differentiated into two chambers called pollen sacs, filled with pollen grains. The wall of pollen grain is made-up of two layers, i.e.
(i) Exine – It is the outer, ornamented, protective, cutinized and rough layer. It often bears spiny outgrowths.
(ii) Intine – It is the thin, delicate, cellulose layer that grows into the pollen tube.
The stamens exhibit great diversity with respect to the fusion, their attachment to different whorls and size.
Accordingly, they are categorized into the following five types :
- Polyandrous – All the stamens are free from each other. Their filaments may be of same or different sizes. e.g., Brassica.
- Monoadelphous – The filaments of all the stamens fuse into a staminal tube but, their anthers are free e.g. China rose, cotton etc.
- Diadelphous – The filaments of stamens fuse into two bundles but anthers are free e.g. pea, sesbania, etc.
- Polyadelphous – The filaments of stamens unite to form more than two bundles e.g. silk cotton, Citrus.
- Syngenesious – The filaments are free but anthers of all the stamens unite together forming a cylindrical tube around the style e.g. sunflower
Located in the centre of the flower is the female reproductive organ called gynoecium or carpels. A flower is composed of one or more carpels.
It consists of three parts i.e.
Stigma – It is the tip of the carpel which is slightly swollen and usually sticky. It is meant for receiving pollen grains and their germination.
Style – It isa long tubular stalk that connects stigma with ovary.
Ovary – It is the swollen, basal part of the carpel. The ovary may be divided into either a single chamber (unilocular) or many chambers (multilocular). Each chamber contains one or more rounded, egg – like structures called ovules which on maturity produce seeds. Each ovule encloses a large ovul structure called embryo sac.
CHARACTERISTICS OF THE FLOWERS
The flowers possess following characteristics :
- The flower is a highly modified and specialized shoot, meant for sexual reproduction.
- The nodes and internodes are highly condensed to form a flat thalamus or receptacle.
- The thalamus can be flat, conical, concave or dome-shaped in different species.
- The thalamus is very short. It is usually borne at the end of a stalk called pedicel.
- The flower arises in the axil of a leaf or leaf- like structure called bract.
- The floral parts are borne on the thalamus in the form of four whorls.
- The four rings or whorls of floral structures from outer to inner side are, calyx (sepals), corolla (petal), androecium (stamens) and gynoecium (carpels).
- Any flower having all the four whorls is called complete. Whereas, those flowers in which any whorl is missing are called incomplete.
- Androecium and gynoecium are called essential parts as they directly help in sexual reproduction. Whereas, calyx and corolla are non-essential parts as they are not directly involved in the process of reproduction.