FLOWER
Flower is defined as a highly modified and condensed shoot with limited growth and specialized for sexual reproduction in higher plants.
A flower generally develops on the lateral side of the peduncle, at a node, in the axil of a leaf like structure called bract. The bract protects the flower when it is in bud condition. A flower with a bract is termed as a bracteate flower as in Caesalpinia and sunflower. A flower without a bract is described as ebracteate flower as in Datura and Neem.
Parts of a typical flower:
A typical flower has a pedicel, a thalamus and four floral whorls.
A) Pedicel:
1) The stalk ot the flower is called pedicel which projects the flower for pollination.
2) A flower with pedicel is called pedicellate flower or a stalked flower and flower without pedicel is known as non-pedicellate flower or sessile flower.
3) It is generally an elongated, cylindrical and green structure.
4) In some flowers a small leaf-like structure called bracteole may also be found on the pedicel besides the tract.
5) A flower having a bracteole is known as a bracteolate flower (Clitoria, Hibiscus) and the one without bracteole is called an ebracteolate flower (Lily).
B) Thalamus:
1. The upper, swollen, condensed, knob-like part of the pedicel is called thalamus. It is also called receptacle or torus.
2. The thalamus acquires different shapes such as conical, elongated, biconvex, cup-shaped etc.
3. It is differentiated into nodes which the floral whorls are inserted and internodes that are highly condensed.
C) Floral whorls:
1. A circle of modified floral leaves is called a floral whorl.
2. A typical Dicotyledonous flower has four floral whorls namely calyx, Corolla, Androecium and Gynoecium.
3. They are arranged in concentric manner with calyx as outer and Gynoecium being the innermost whorl.
4. A flower with all the four whorls is described as a complete flower and the flower having absence of one or more floral whorls is called an incomplete flower.
5. Among these, calyx and corolla are called accessory whorls or helping whorls of the flower. The Androecium and Gynoecium are called essential whorls or reproductive whorls.
a) Calyx:
1) Calyx is the outermost, accessory whorl of the flower. Its individual member or unit is called sepal. The sepals are usually small and green.
2) When the sepals are free from each other, the condition is termed as polysepalous calyx. When the sepals are United, it is called as Gamosepalous calyx.
3) Sepals generally wither off after pollination. However, if the calyx persists even after fruit formation it is called persistent calyx.
Example- Brinjal.
Functions of Calyx:
a) The calyx protects the flower in bud condition.
b) Being green in colour, it helps in photosynthesis.
c) It many become modified for dispersal of fruits and seeds by wind as in Tridax.
d) Sometimes it becomes brightly coloured- called petaloid calyx to attract the insects for pollination.
e) It encloses and protects corolla, androecium and Gynoecium.
b) Corolla:
1. Corolla is the second accessory whorl of the flower present on the inner side of the calyx. Its individual member or unit is called petal.
2. The petals are usually brightly coloured and fragrant.
3. When the petals of the Corolla are free from ecah other, the Corolla is called polypetalous Corolla. When the petals are United, it is known as gamopetalous Corolla.
Functions of Corolla:
1) Corolla protects essential whorls of the flower.
2) Being brightly coloured and fragrant, it helps in attracting insects for pollination.
3) Sometimes it produces extra outgrowths called coralline Corolla which facilitates pollination.
Perianth:
1. When the members of accessory whorls of a flower are not differentiated into sepals and petals, it is called perianth.
2. In such flowers the floral members are identical in shape, size and colour.
3. Each member of perianth is called a tepal.
4. Tepals of the perianth may be free (polyphyllous perianth) or united (gamophyllous perianth).
c) Androecium:
1. Androecium is the third, essential and male reproductive whorl of the flower.
2. It consists of individuals members or units called stamens or microsporophylls.
3. Each stamen consists of a filament, connective and anther.
4. The filament projects the anther to facilitate dispersal of pollen grains.
5. The anther is fertile part of the stamen which is usually bilobed. Each anther lobe contains two microspsorangia filled with microspores or pollen grains at maturity. Hence the anther is bilobed and tetrasporangiate. An anther with two microspsorangia is called two celled or dithecous (Datura) while anther with a single microsporangium is called one-celled or monothecous (Hibiscus).
6. Connective is the cementing tissue which connects the anther lobes and the filament together.
d) Gynoecium:
1. Gynoecium is the fourth, innermost and female reproductive whorl of the flower.
2. Its individual member or unit is called a carpel or megasporophyll.
3. Each carpel consists ot the three parts-ovary, style and stigma.
4. The ovary is basal, hollow, swollen, fertile part of the carpel. It's cavity is called ovarian cavity or locule. The cavity bears one or more ovules attached to placenta. The mode of arrangement of ovules or placenta in the ovary is called placentation.
5. The style is middle, narrow, elongated, sterile part of the carpel which connects ovary with stigma. It is filled or lined with transmitting tissue.
6. The stigma is the terminal part of the carpel which receives pollen grains during pollination. Therefore, it becomes sticky at maturity. It also facilitates germination of pollen grains.
7. Gynoecium is also called pistil. It may be simple (monocarpellary) and compound (polycarpellary).
A flower having both Androecium and Gynoecium is said to be bisexual or hermaphrodite flower and the flower which has only one of them is called Unisexual flower. A flower having only stamens is called a staminate or male flower and the flower with only carpels is termed as Unisexual pistillate or female flower.
A plant with both male and female flowers is said to be a monoecious plant e.g. Maize. A plant bearing either male or female flowers is termed as a dioecious plant e.g. Cucurbita. A plant with bisexual and Unisexual and even neuter flowers is said to be a polygamous plant.
e.g. Mango.
Insertion of floral whorls on thalamus:
Normally the calyx, corolla, Androecium and Gynoecium of a flower are present on the thalamus in a proper sequence. Mostly Gynoecium is present at the tip of the thalamus. In many flowers, tthe relative position of calyx, corolla and androecium with respect to the ovary becomes disturbed due to position of the thalamus. Accordingly there are three kinds of flowers.
1) Hypogynous Flower:
The thalamus of the flower is conical, dome-shaped, elongated or disc-shaped. The members of calyx, corolla and androecium are inserted at the base of the ovary. Such a condition is called hypogyny and such a flower is known as hypogynous Flower.
In such a flower ovary is superior and other floral leaves are interior e.g. Hibiscus.
2) Perigynous Flower:
The thalamus of the flower is cup-shaped and the ovary is present in the concavity of the thalamus. The members of calyx, corolla, and androecium are inserted at the rim, around the ovary. This condition is termed as perigyny and such a flower is described as perigynous flower.
In such flowers ovary is half-superior and half interior e.g., Rose, Caesalpinia.
3) Epigynous Flower:
The thalamus of the flower is cup-shaped and the ovary is sunken in the thalamus. The wall of the ovary and thalamus are often fused together.
The members of calyx, corolla and androecium are inserted above the level of the ovary. This condition is termed as epigyny and such a flower is called epigynous Flower as in Guava. In such flowers ovary is inferior.
Flower | Definition, Anatomy, & facts - Chemosmart
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September 10, 2021
