Sexual Reproduction

Category : 12th Class

Sexual reproduction in flowering plants involves transformation of diploid sporophytic cells into haploid gametophytic cells by meiosis and subsequent fusion of haploid gametes of opposite sex to form diploid zygote. The zygote then develops into an embryo which ultimately forms a diploid plant body. In flowering plants, all these steps of sexual reproduction occur within specialized reproductive organs, called the flowers.

(1) Structure of the flower : Morphologically flower is a modified shoot meant for sexual reproduction of the plant. Typically, it is a condensed branch in which internodes have become condensed, bringing nodes very close to one another, and the leaves are modified to form floral whorl that directly or indirectly participate in the process of reproduction.

The flower is commonly borne on short or long stalk called the pedicel. It has an upper swollen region known as receptacle (thalamus or torus).

(2) Parts of a flower : A typical angiospermic flower consists of four whorls of floral appendages attached on the receptacle : calyx, corolla, androecium and gynoecium. Of these, the two lower whorls (i.e., calyx and corolla) are sterile and considered as nonessential, accessory or helping whorls. The two upper whorls (i.e., androecium and gynoecium) are fertile and considered as essential or reproductive whorls.

(3) Functions of a flower

(i) Flowers are modifications of shoot to perform the function of sexual reproduction. The fertile leaves become microsporophylls (stamen) and megasporophylls (carpels) which bear anthers and ovules respectively. The anthers produce pollen grains and the ovules possess eggs.

(ii) Flowers of most of the angiosperms are shaped variously to help diverse modes of pollination.

(iii) Flowers provide seat for germination of pollen, development of pollen tube, formation of gametes and fertilization.

(iv) The ovary part of the carpel gets transformed into fruit and the ovules are transformed into seeds after fertilization.

(v) Some floral parts like calyx and various modifications in ovaries help in the dispersal of fruits and seeds.

(4) Relative position of floral organs on thalamus : Depending upon the form of thalamus and the position of floral whorls with respect to the ovary, the flowers are of the following three types :

(i) Hypogyny : In this case the thalamus is convex and ovary occupies the highest position on it. The outer three whorls, viz. sepals, petals and stamens inserted one above the other but below the ovary. Since the ovary lies above the other parts, it is described as superior and the rest of the floral whorls as inferior. A flower having hypogyny is called hypogynous. e.g., China rose, Brinjal, Mustard, etc.

(ii) Perigyny : In some cases, the receptacle or the thalamus forms a swallow or deep cup-shaped structure around the ovary. The pistil is attached at the centre of the concave thalamus. The sepals, petals and stamens are attached at the margins of the thalamus, the flowers are said to be perigynous and ovary is half inferior or half superior. Different type of flowers show different degrees of perigyny. e.g., Rose, Pea, Bean, Prunus, etc.

(iii) Epigyny : In this condition the margin of thalamus grows further upward completely enclosing the ovary and getting fused with it and bear the sepals, petals and stamens above the ovary. The ovary in such cases is said to be inferior and the rest of the floral members superior. e.g., Apple, Sunflower, Cucumber, Guava, etc.



(5) Placentation : The ovary contains one or more ovules, which later become seeds. The ovule bearing regions of the carpel is called placenta. The arrangement of placentae and ovules within the ovary is called placentation. The placenta is the cushion-like structure to which the ovules are attached inside the cavity of the placenta, placentation is of the following types :

(i) Marginal : In this type of placentation, the ovary is simple, unilocular and the ovules are arranged along the margin of the unilocular ovary. The placenta develops along the ventral suture of the ovary. e.g., Pea, Gram, Goldmohur, etc.

(ii) Axile : It is found in a compound ovary which is two or more chambered, usually as many as the number of carpels e.g., Petunia and Asphodelus. The placentae bearing the ovules develop from the central column or axis which is formed by the fusion of margins of carpels. In certain cases the number of chambers (loculi) increases due to the false septum formation. e.g., Datura, Tomato, etc.

(iii) Free central : In this free central placentation, the gynoecium is polycarpellary and syncarpous. The ovary in early stages is multilocular, but soon the septa break down leaving it as a unilocular structure. e.g., Dianthus, Slience, Primula, etc.

(iv) Parietal : In parietal placentation, the ovary is usually one-chambered but in some cases it becomes bilocular due to the formation of false septum called replum, e.g., Brassica compestris (Sarson). The placentae bearing the ovules develop on the inner wall of the ovary at places where the margins of two adjoining carpels meet. The number of placentae corresponds to the number of fused carpels. e.g., Poppy, Mustard, Cactus, etc.

(v) Basal : In this type of placentation, ovary is bicarpellary, syncarpous and unilocular and a single ovule is borne at the base of ovary. e.g., Marigold, Sunflower, etc.

(vi) Superficial : The ovary is multicarpellary, syncarpous, and large number of loculi without specific order e.g., Waterlily (Nymphea).


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