Morphology of Flowers: The flower is a modified shoot meant for the production of seeds. It consists of four different circles (whorls) arranged in a definite manner. A flower is built upon a stem or pedicel with an enlarged end known as the thalamus or receptacle. The four whorls of the flowers can be described as under:
1. Calyx: It is the outermost whorl of flower and is generally green in color, the individual member of which is called sepal.
2. Corolla: It is the second whorl of flower and is either white or bright-colored, each member of which is known as a petal.
3. Androecium: It is the third circle of flowers and constitutes the male part. The individual component is called the stamen and each stamen consists of filament, anther, and connective.
4. Gynoecium: This is the fourth circle of the flower and constitutes the female part. Each component is known as carpel or pistil and is made of stigma, style, and ovary.
When all the four whorls, are present in a single flower, it is described to be a complete flower, the absence of any one of them describes it as an incomplete flower. A flower is described to be hermaphrodite or bisexual when it contains stamens and carpels. The absence of any one of them describes it as a unisexual flower. When calyx and corolla are in a flower are similar in color and shape, then both of them (calyx and corolla) together are called Perianth, i.e. garlic, onion, asparagus.
When a flower is divided into two equal parts by any vertical section passing through the center, then it is described as a regular or symmetrical or actinomorphic flower as in ipomoea, rose, datura, and shoe flower. But when it cannot be divided equally into two parts by one vertical section, then it is described as an irregular or asymmetrical or zygomorphic flower.
When the stamens arise from petals instead of the thalamus, the petals are called epipetalous. When the stamens get united with gynaecium the structure is known as gynostemium. The union of stamens among themselves is known as cohesion. When the filaments of the stamen get united to form a single bundle, it is known as monadelphous. When it forms two bundles, it is known as diadelphous. When anthers get united to form a column (but filaments are free), the stamens are known as syngenesious. When the ovary consists of only one carpel, it is said to be monocarpellary and when it contains more than one carpel, it is said to be polycarpellary. When the carpels in the ovary are free, the ovary is described as apocarpous and when they are united it is known as syncarpous.
Arrangement of Floral Parts on Thalamus
Depending upon the arrangement of floral parts on the thalamus, the flowers may be of three types.
(i) Hypogynous flower (Superior ovary): Herein the thalamus is conical, flat, convex and stamens, sepals, and petals are arranged at the base and ovary at the apex, e.g.: brinjal, china rose, mustard, etc.
(ii) Perigynous flowers (Half-superior Ovary): Thalamus is flat, sepals and stamens grow around the ovary. The flowers are said to be perigynous as in Rose, Strawberry peach.
(iii) Epigynous flower (Inferior ovary): The thalamus is fused with ovary wall, calyx, corolla, stamen appear at the top and the gynaecium at the bottom as in Sunflower, cucumber, apple, etc.
Placentation
The type of distribution of placentae in the ovary is called placentation. They are of the following types.
1. Marginal: It is characteristic to monocarpellary ovary and placentae arise on the ventral suture, e.g. bean and pea.
2. Axil: It is characteristic of the polycarpellary syncarpous, bilocular, or multilocular ovary. Ventral sutures of each carpel meet at the center and each of them has marginal placentation, e.g. onion, china rose, ipomoea.
3. Parietal: It is characteristic of the polycarpellary syncarpous ovary and the placentae develop on the ventral suture but the ovary is unilocular as in Papaya and Cucurbita.
4. Free Central: It is characteristic of the polycarpellary syncarpous ovary, which is unilocular. Ovules arise on the central axis, but it is not connected with the peripheral wall, e.g. Dianthus, Saponaria, Portulaca.
5. Basal: It is characteristic of the polycarpellary and unilocular ovary. Only one ovule is present and it arises from its base as in sunflower.
Pollination
Pollination is the process of transference of pollen grains from the anther of a flower to the stigma of the same flower or another flower of the same or allied species. Pollen grains are produced by bursting the anther and are carried by various agencies to the stigma.
The agencies may be insects of various types, wind or even water. There are two types of pollination, i.e. (a) Self-pollination or autogamy and (b) cross-pollination or allogamy
(a) Self-pollination: There are two types of self-pollination, i.e.
- Homogaxny: In this case the anthers and stigmas of a flower mature at the same time.
- Clestogamy: It is found in flowers that never open or the underground flowers of Commelina bengalensis.
(b) Cross-pollination: Pollination through the agency of insects, animals (such as snails, bats, squirrels, birds, and even human beings) wind and water is cross-pollination.
Pollination by insects is known as entomophily. To attract various insects, plants adopt different means such as color, nectar, and scent. Entomophilous pollination is very common in plants.
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