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| Leaf |
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| The leaf is a green, flattened structure borne on the stem at the nodes and is a lateral appendage of the stem. A typical leaf is also known as the foliage leaf in order to distinguish it from other forms of leaves. |
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| 1. The leaves enclose cells containing green plastids called chloroplasts. The green colour of the chloroplasts is due to the presence of a pigment called chlorophyll. Due to this pigment, plants are capable of synthesising their food by a process called photosynthesis. |
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| 2. The stomatal openings of the leaves help in the entry of atmospheric carbon dioxide during photosynthesis and in the removal of excess of water in the form of vapour during transpiration. |
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| 3. Exchange of respiratory gases takes place through leaves. |
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| 4. In some plants like Amaranthus, the leaves serve to store food materials. |
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| 5. In plants like Bryophyllum, the leaves take part in vegetative propagation. |
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| A typical dicot leaf (e.g., Hibiscus) consists of the following parts, namely, Axillary bud, petiole and lamina. |
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| fig. 27.32 Parts of a Typical Leaf |
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| Leaf Base |
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| Leaf base is the portion by which the leaf is attached to the stem. In monocots the leaf base is of sheathing type. In dicots the leaf base could bear two lateral outgrowths called stipules. In some plants like Pea, bean, etc. the leaf base is swollen and is called pulvinus. |
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| Petiole |
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| It is the stalk of the leaf. A leaf with a petiole is described as petiolate, e.g., Hibiscus. A leaf without a petiole is called sesile, e.g., Calotropis. |
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| In Lemon, Orange and such plants the petiole develops two wing-like extensions on two sides. Such a petiole is said to be a winged petiole. |
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| The angle between the petiole and the stem is called axil, and a bud present in the axil is the axillary bud. |
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| Leaf Blade or Lamina |
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| It forms the green expanded portion of the leaf. In the dicot leaf, the lamina has a central midrib running from the base to the apex. The lamina on either side produces veins which in turn produce veinlets, to form a network of veins. Hence the venation is described as reticulate type. The edge of the lamina is called the leaf margin, which may be entire or smooth, or variously cut and lobed. In most dicots, the leaf grows horizontally and only the upper surface is exposed to sunlight (dorsiventral). Exception : Eucalyptus |
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| Monocot Leaf (e.g., Zea mays) |
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| In general, the monocot leaf resembles the dicot leaf. The part of leaf attached to the stem is the leaf base which expands into a sheath (sheathing base). The leaves are upright and both the sides are equally exposed to sunlight (isobilateral). The leaf shows parallel venation. |
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