Get complete information on Sexual reproduction (Flowers)

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i. The sexual reproductive organs are present in the flowers of angiosperms.

ii. Stamens and carpels represent the male and female sex organs, respectively.

iii. Perfect or bisexual or hermaphrodite: Flower which possesses both the essential organs i.e., stamens and carpels.

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iv. Male or staminate flower: Unisexual flower, carpels absent.

v. Female or pistillate flower: Unisexual flower, stamens absent.

vi. Monoecious: Plant with both male and female flowers e.g., Maize, Castor etc.

vii.Dioecious: Male and female flowers present on the different plants e.g., Papaya, Mulberry etc.

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Development of the male gametophyte:

a. Each stamen consists of a cylindrical, massive thread like filament and a broad terminal bilobed anther.

b. Typical anther is tetrasporangiate with four microsporangia (dithecous) but sometimes with only two microsporangia (monothecus) e.g., Malvaceae.

c. In dithecous anther, the two lobes are separated in the anterior side by a deep groove and attached on back side by the connective.

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d. There is a single vascular bundle in the connective.

e. In the anther wall of a mature anther, the outer most layer is epidermis followed by a single layer of endothecium, then one to three middle layers and innermost is single layered tapetum.

f. Endothecium (radially elongated, fibrous thickening of cellulose) attains its maximum development when the anther is ready to dehisce. The hydroscopic nature of endothecial cells help in the dehiscence of the anthers by the means of slits.

g. Pollen grain (haploid) represents the first stage of male gametophyte and may be referred to as male gametophyte or partially developed male gametophyte or male gamete.

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h. A pollen grain has two layered wall:

(i) An outer exine (tough, cuticularised and with spinous out growth) and

(ii) an inner intine (thin and made up of cellulose). Exine is made up of sporopollenin. Exine possess one or more weak spots known as germ pores (usually 3 in dicots and 1 in monocots ).

i. Development of male gametophy.te starts in pollen grain. Microspore undergoes only two mitotic divisions and produces bigger vegetative cell (tube cell) and a smaller generative cell. At this stage, dehiscence of the anther takes place and the two celled pollen grains are released.

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j. After falling of pollen grains on stigma, pollen grains absorb water and nutrients of the stigmatic secretion through its germ pores.

k. The intine grows out through a germ pore into a slender pollen tube. The generative nucleus divides to form two male nuclei. Usually generative cell comes down into pollen tubes and then divides to form the male gametes. The male gametophyte in angiosperms is highly reduced.

Development of female gametophyte:

a. The ovule is technically a megasporangium having nucellus (parenchymatous tissue), integuments (one or two covering) and a short stalk-like structure (funicle) by which it is attached to the placenta and the integuments enclose-an opening i.e., micropyle.

b. In young ovule, a hypodermal archesporial cell is differentiated which forms the archesporium of the ovule. Archesporial cell divides by a periclinal division and forms:

(i) an outer primary parietal cell (forms nucellus) and

(ii) inner primary sporogenous cell (differentiates as megaspore mother cell).

c. The megaspore mother cell (megasporophyte) divides by meiosis and a linear tetrad of four megaspores is produced. The upper three towards micropyle degenerate and the lowermost towards chalaza is functional.

d. The functional megaspore is the first cell of the female gametophyte. It divides by three mitotic divisions so that eight nuclei are formed: 4 towards the micropyle and 4 towards the chalaza. Out of four nuclei at each pole, one nucleus (polar nucleus) moves towards the centre. The two polar nuclei fuse together just at the time and fertilization of form a diploid secondary or definitive nucleus. Three nuclei towards micropyle form egg apparatus (one egg cell and two synergids): all haploid. The three nuclei towards chalaza form antipodal cells (haploid and vestigeal). So, the mature or organized embryo sac at the time of fertilization is 7-celled and 8 nucleate.

e. 7-celled, 8 nucleate and monosporic embryo sac is known as Polygonum type of embryo sac because it was discovered for the first time in Polygonum by Strasburger.

f. In an embryo sac, all the cells are haploid except the secondary nucleus (2n).

g. P. Maheshwari (1950) classified the female gametophyte into monosporic, bisporic and tetrasporic embryo sac, depending upon the number of megaspore nuclei taking part is the development.

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