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| Parts of a Flower - Continued |
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| Androecium |
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| Androecium is the male reproductive part and constitutes the third whorl in the flower. It is formed of one to many stamens. Each stamen consists of a filament and an anther at its tip. |
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| The anther is described as dithecous when it consists of two anther lobes. The two anther lobes are connected to each other by the connective e.g., Crotalaria, Solanum etc. If the anther consists of only one anther lobe it is called monothecous e.g., Hibiscus. |
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| The anthers in Nerium are said to be sagitate as they are arrow shaped. In Hibiscus the anthers are kidney shaped and are called reniform. |
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| The attachment of the anther to the filament is specific. When the filament is attached to the back of the anther throughout its length, the anther is said to be dorsifixed. e.g., Michelia. But in some the extreme tip of the filament is attached to the base of the filament, when the anther is described as basifixed e.g., Solanum. Sometimes, the filament is attached to the middle of the anther so that the anther swings freely on the filament. This type of anther is called versatile. e.g., Grasses, lilies |
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| fig. 27.55: Androecium: structure and types of stamen |
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| In dithecous anthers, the two lobes are clearly visible externally. In young anther, each anther lobe contains chambers, called microsporangia. But as the anther matures the partition between the two cavities in each anther lobe is disrupted and thus each anther lobe comes to have one large cavity. These cavities are filled with pollen grains produced by the anther. When the wall of the anther lobe ruptures (dehiscence) , the pollen grains are released (dispersed). |
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| fig. 27.57 |
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| In polypetalous flowers, the stamens are usually free and arise from the torus. But in gamopetalous flowers, the stamens are often fused with the petals. Such stamens are called epipetalous e.g. Solanum. All the stamens in the flower may be of the same length or of different lengths. In Brassica, there are six stamens, of which two are short and four are long. Such an androecium is said to be tetradynamous. But in Leucas, there are four stamens in pairs, of which two are long and two are short. Such an androecium is said to be didynamous. |
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| The stamens may be free or united in various ways. If all the stamens unite in one bundle by connation of their filaments, the androecium in said to be monadelphous e.g., Hibiscus, Crotalaria. If the stamens unite into two bundle, the androecium is described as diadelphous. e.g. Lathyrus, pea. In some there are numerous stamens and their filaments unite into a number of bundles. Such an androecium is called polyadelphous. e.g. Bombax Citrus. But in Sunflower all the filaments are free but the anthers are united. Such a condition is described as syngenesious. |
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| fig. 27.58 |
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| In Cucurbita, both the anthers and the filaments are fused. Such a condition is known as synandrous. |
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| Structure of the pollen grain |
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| The mature pollen grain has a wall made up of two layers, the outer thick exine and the inner thin intine. The Exine has thin circular areas called germ pores or apertures. Through one of the germ pores the pollen tube grows out. The wall encloses two unequal cells. The large one is called the vegetative cell and the smaller the generative cell. The generative cell produces two male gametes (sperms). |
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| fig. 27.59 Section through an anther |
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| Gynoecium or Pistil |
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| Gynoecium is the innermost whorl and is the female reproductive part of the flower. The gynoecium has three parts, the basal ovary, the elongated style and the terminal stigma. |
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| fig. 27.60 |
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| The ovary is made up of one to many units called carpels. Depending on the number of carpels present, ovary is described as follows: |
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| a. Monocarpellary (having one carpel) e.g. Beans. |
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| b. Bicarpellary (having two carpels) e.g. Brassica |
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| c. Tricarpellary (having three carpels) e.g. Ricinus (Castor) |
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| d. Tetracarpellary (having four carpels) e.g. Bergia |
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| e. Pentacarpellary (having five carpels) e.g. Hibiscus, apple |
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| The carpels in the ovary may be free from one another or fused. The ovary is described as apocarpus, when the carpels are free e.g. Michelia. The ovary is described as syncarpous, when the carpels are united e.g. Ricinus, Citrus. |
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| Depending upon its position the ovary may be |
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| i) Superior: When all the floral parts arise from the base of the ovary as in hypogynous flowers. |
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| ii) Inferior: When all the floral parts arise from above the ovary as in epigynous flowers. |
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| iii) Semi-inferior: Where the floral parts are seen developing from about the middle of ovary as in perigynous flowers. |
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| fig. 27.61 |
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| When the ovary is syncarpous, it may show one to many chambers called locules. Depending on the number of locules, the ovary is described as follows: |
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| a. Monolocular or unilocular (having one locule) e.g. Beans |
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| b. Bilocular (having two locules) e.g. Ixora |
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| c. Trilocular (having three locules) e.g. Ricinus |
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| d. Tetralocular (having four locules) e.g. Ocimum |
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| e. Pentalocular (having five locules) e.g. Apple. |
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| fig. 27.62 |
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| The ovary encloses the ovules. The ovules are attached to the inner wall of the ovary with the help of a special type of tissue called placenta. |
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| Placentation: The arrangement of ovules on the placenta inside the ovary is called placentation. There are several types of palcentation. |
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| a. Marginal placentation: In this type, the ovary is monocarpellary, monolocular and the placenta is on the ventral suture. The ovules are attached to the placenta which is on the margin. e.g. Beans, Pea, etc. |
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| b. Axile placentation : In this type, the ovary has two or more carpels, syncarpous, and has two or more locules. The ovules are borne at or near the centre on the axis formed by the union and fusion of the septa (partitions) and usually in vertical rows. e.g., Apple, Hibiscus etc. |
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| fig. 27.63: Types of Placentation |
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| c. Free Central placentation: In this type the ovary is monolocular, wherein the ovules are borne on a central axis that reaches the top of the ovary. e.g., Primula, Sandal Wood. |
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| d. Free basal placentation: Similar to free central but the placental column does not reach the top of the ovary. e.g., Portulaca, Celosia. |
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| Pariental Placentation: In this type, the ovary has two or more carpels, and is syncarpous, and monolocular. Here there are as many placentae as the number of carpels and the ovules are attached to those placentae at the periphery. e.g., Cucurbita, Argemone. |
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| e. Basal Placentation: In this type, the ovary is monolocular. The ovules are few or reduced to one and borne at the base of the ovary. The ovule when solitary often fills the ovary cavity. e.g., Sunflower. |
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| f. Apical placentation: The ovary is monolocular. The one or two ovules are borne at the roof of the ovary. e.g., Morus |
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| Structure of the Ovule: Each ovule is attached to the placenta by a stalk called funicle. The body of the ovule fuses with the funicle to form the hilum in the seed. The ovule consists of one or two integuments, accordingly the ovules are called unitegmic or bitegmic respectively. The integuments leave a small opening on one side called the micropyle. The end of the ovule having the micropyle is called the micropylar end. The opposite end is called the chalazal end. The integuments enclose a tissue called the nucellus. Nucellus in turn encloses the embryosac, the female gametophyte. |
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| The embryo sac is usually oval in shape. At the centre it has two nuclei called secondary nuclei. The embryosac has three cells called antipodals towards the chalzal end and three cells forming the egg apparatus towards the micropylar end. In the egg apparatus, the middle cell is called the egg cell and the other two cells are called synergids. |
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| Mature ovules are classified into several types based principally on the position of the micropyle with reference to the funicle, as follows: |
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| a. Orthotropous: Here, the ovule is erect which the micropyle and the funicle lie on the same axis (vertical line). |
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| b. Anatropous: The ovule takes a 180o turn so that the micropyle and the funicle come to lie close to each other. |
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| c. Hemianatropous: In this type, the ovule takes a 90o turn so that the micropyle is at right angles to the funicle. |
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| d. Campylotropous: The ovule is like a horse shoe. |
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| e. Circinotropous: Here the long funicle is circinate around the ovule. |
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