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| Flower |
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| The angiosperm plants produce characteristic structures called flowers for the purpose of reproduction. A flower is defined as a modified vegetative shoot meant for sexual reproduction. The flower consists of a very short axis on which whorls of different parts arise. |
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| Usually the flower arises in the axil of a modified leaf called bract. When the flower consists of a stalk called pedicel, it is described as Pedicellate. |
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| When the pedicel is absent the flower is described as sessile. When bracts are present at the base of the pedicel, the flower is said to be bracteate. When bracts are absent, the flower is ebracteate. Similar small green bract-like structures are sometimes present on the pedicel and are called as bracteoles. A flower having bracteoles is described as bracteolate. When bracteoles are absent it is described as ebracteolate. |
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| fig. 27.48 : A flower in L.S. showing position of different whorls on the thalamus |
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| Bracts are of various types: |
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| 1. Foliaceous: Leaflike as in Adhathoda |
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| 2. Petaloid: Petal like, where the bracts are large and coloured as in |
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| Bougainvillea |
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| 3. Involucre of bracts: Here many bracts form one or more whorled structures called involucre. e.g. Head inflorescence of Helianthus. |
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| 4. Spathe: Special type of bract covering the flowers in spadix inflorescence. e.g., Aroids, Cocos nucifera. |
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| Reference of flowers in relation to axis and bract: |
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| The flower arise in the axil of a branch (a reduced leaf) and the inflorescence axis or if solitary, in the axil of a leaf and the stem. The following terms are used to refer the two halves of the flower in relation to the axis and the bract. |
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| The axis side half of the flowers is called adaxial, posterior or ventral and the branch side half is abaxial, anterior, or dorsal. |
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| The flowers arise in the axial of a bract (a reduced leaf) and the inflorescence axis, if solitary, in the axil of a leaf and the stem. |
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| The tip of the pedicel enlarges to form the thalamus or torus. On the torus four whorls of floral leaves are present usually, viz., calyx, corolla, androecium and gynoecium. Calyx, and corolla are commonly referred to as non-essential parts of the flower as they do not take part in reproduction directly. But still they help in attracting insects for pollination. Androecium and Gynoecium are called the essential parts of the flower as they take part in reproduction directly. When all the four whorls are present in a flower, it is described as complete. Sometimes one or more whorls are absent. Such a type of flower is called incomplete. |
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| Generally, the calyx is green and the corolla is brightly coloured. A flower is described as heterochlamydeous when there is clear differentiation into calyx and corolla as in Hibiscus. A flower is described as homochlamydeous when calyx and corolla are undifferentiated as in Michelia. The flower becomes achlamydeous when both the calyx and corolla are absent as in Euphorbia pulcherrima. |
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| When the flower can be divided into two equal halves radially in more than one plane, the section passing through the axis of the flower, the flower is described as actinomorphic and is called regular or symmetrical. e.g. Tribulus. The flower is described as zygomorphic when it can be divided into two equal halves in only one plane. e.g. Crotolaria, Leucas. |
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| If the flower cannot be cut into two equal halves in any plane it is asymmetric, e.g. Canna. |
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| The flower is described as bisexual or hermaphrodite when both androecium and gynoecium are present e.g., Crotolaria. The flower is unisexual when either only androecium or only gynoecium is present, e.g. Castor. In species with unisexual flowers, flowers may occur on the same individual plant (monoecious the plant itself is bisexual) as in Cucurbita and Ricinus or they may occur on difference specimen (dioecious) as in Papaya. If unisexual and bisexual flowers occur on the same individual plant then the species is polygamous (e.g., Toddalia). Depending upon the distribution of the flowers, the species is polygamo-monoecious or polygamo-dioecious. |
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| Fusion of Floral parts |
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| If members of the same whorl are united partly or completely the condition is connate (e.g. sepals, petals, stamens or carpels, among themselves) and when members of different whorl unite partly or completely the condition is adnate e.g. petals and stamens and carpels. |
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| In some flowers, the gynoecium is placed at the summit of the torus and all other parts of the flower arise from below the gynoecium. Such flowers are described as hypogynous and the ovary is superior. e.g., Pea. |
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| fig. 27.49 Hypogyny, Epigyny and Perigyny |
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| In some plants like Roses, Peach etc., the thalamus becomes cup shaped and the gynoecium is situated inside the cup. The other floral parts arise from the edge of the cup shaped thalmus. Such flowers are called perigynous and the ovary is said to be semisuperior. |
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| In some plants like Psidium guajava, Coriandrum, etc., the thalamus completely encloses the ovary and fuses with it. The other parts of the flower arise from the top of the ovary. Such flowers are called Epigynous and the ovary is described as inferior. In all other cases the ovary is superior. |
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