F.
Answer the following questions in detail.
1.
Explain in detail the external structure of leaf.
2.
Describe the functions of leaves.
3.
Explain some modifications in leaves.
4.
With the help of a diagram, explain the arrangement of leave
Picture-based Question
Label the parts of a leaf.
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Answers
Answer:
Ans.1)Structure of a Typical Leaf
Each leaf typically has a leaf blade called the lamina, which is also the widest part of the leaf. Some leaves are attached to the plant stem by a petiole. Leaves that do not have a petiole and are directly attached to the plant stem are called sessile leaves. Leaves also have stipules, small green appendages usually found at the base of the petiole. Most leaves have a midrib, which travels the length of the leaf and branches to each side to produce veins of vascular tissue. The edge of the leaf is called the margin.
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Parts of a leaf: A leaf may seem simple in appearance, but it is a highly-efficient structure. Petioles, stipules, veins, and a midrib are all essential structures of a leaf.
Within each leaf, the vascular tissue forms veins. The arrangement of veins in a leaf is called the venation pattern. Monocots and dicots differ in their patterns of venation. Monocots have parallel venation in which the veins run in straight lines across the length of the leaf without converging. In dicots, however, the veins of the leaf have a net-like appearance, forming a pattern known as reticulate venation. Ginkgo biloba is an example of a plant with dichotomous venation.
Ans.2)The main function of a leaf is to produce food for the plant by photosynthesis. Chlorophyll, the substance that gives plants their characteristic green colour, absorbs light energy. The internal structure of the leaf is protected by the leaf epidermis, which is continuous with the stem epidermis. The central leaf, or mesophyll, consists of soft-walled, unspecialized cells of the type known as parenchyma. As much as one-fifth of the mesophyll is composed of chlorophyll-containing chloroplasts, which absorb sunlight and, in conjunction with certain enzymes, use the radiant energy in decomposing water into its elements, hydrogen and oxygen. The oxygen liberated from green leaves replaces the oxygen removed from the atmosphere by plant and animal respiration and by combustion. The hydrogen obtained from water is combined with carbon dioxide in the enzymatic processes of photosynthesis to form the sugars that are the basis of both plant and animal life. Oxygen is passed into the atmosphere through stomata—pores in the leaf surface.
Ans.3)Leaf modifications
Whole leaves or parts of leaves are often modified for special functions, such as for climbing and substrate attachment, storage, protection against predation or climatic conditions, or trapping and digesting insect prey. In temperate trees, leaves are simply protective bud scales; in the spring when shoot growth is resumed, they often exhibit a complete growth series from bud scales to fully developed leaves.
Weeping willow (Salix babylonica).
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Stipules often develop before the rest of the leaf; they protect the young blade and then are often shed when the leaf matures. Spines are also modified leaves. In cacti, spines are wholly transformed leaves that protect the plant from herbivores, radiate heat from the stem during the day, and collect and drip condensed water vapour during the cooler night. In the many species of the spurge family (Euphorbiaceae), the stipules are modified into paired stipular spines and the blade develops fully. In ocotillo (Fouquieria splendens; Fouquieriaceae), the blade falls off and the petiole remains as a spine.
Ans.4)Leaves are vital to the survival of plants. They help plants in a variety of ways, including producing food and oxygen through photosynthesis, balancing water loss, regulating gas exchange, and transporting the products of photosynthesis.
A leaf has two main parts:
(1) Petiole- the stalk that supports a leaf in a plant and attaches the leaf blade to the stem.
(2) Lamina- the green flat part of a leaf that is specialized for photosynthesis. The lamina contains the following parts:
(i) Veins- the lines on the green flat part in a leaf that provide support for the leaf and transport both water and food.
(ii) Midrib- The central, thick, linear vein that runs along the length of a leaf is called midrib.