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Let us learn about Diversity in Modification of Leaves. After reading this article you will learn about: 1. Structures of the Leaves 2. Functions of the Leaves.
Structures of the Leaves:
They are as follows:
1. Bladder:
In bladderwort (Utricularia) the leaves are very much segmented and they simulate roots excepting that they are green in colour. Some of these segments become modified into bladders. Each bladder is about 3 mm in diameter and is provided with a trapdoor entrance. The trap door acts as a short of valve which can be pushed open inwards from outside, but never from inside to outside.
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This trap-door entrance allows aquatic animalcules to pass in, but never to come out. The inner surface of the bladder is dotted all over with numerous digestive glands. These glands secrete the digestive agent and absorb the digestive products.
2. Pitcher:
In the pitcher plant (Nepenthes) the leaf becomes modified into a pitcher. The morphology of the leaf of pitcher plant is that the pitcher itself is the modification of leaf blade, the tendrillar stalk supporting the pitcher is the modification of the petiole, and the laminated structure that of the leaf base.
The inner surface of the pitcher corresponds to the upper surface of the leaf and the lid of the pitcher arises as an outgrowth of leaf apex. The function of the pitcher is to capture and digest insect. When young the mouth of the pitcher remains closed by its lid which later on opens and stands erect.
The inner side of the pitcher remains covered with numerous, smooth and sharp hairs, all pointing downwards. Lower down the inner surface numerous digestive glands are found. The digestive agent, secreted by glands, is trypsin which helps in digesting the proteins. In the sundew (Drosera) the upper surface of the leaf is covered with glandular hairs which are sensitive to touch and capture insects.
3. Phyllode:
In some species of Australian Acacia the lamina of the leaf is absent but the petiole is so flattened as to appear leaf-like. These flattened petioles are known as phyllodes and they are so developed as to place their surfaces in the vertical plane. The normal leaf is pinnately compound and only develops in the seedling stage. The phyllode then carries all the functions of the leaf.
4. Leaf-Tendrils:
In certain plants the leaves become modified into slender, wire-like-coiled structures known as tendrils. The leaf may be partially or wholly modified into tendrils.
For example, in pea only the upper leaflets are modified into tendrils. In Naravelia and Bignonia the terminal leaflet converts into a tendril. In Gloriosa the leaf apex becomes modified into a tendril. In Nepenthes the petiole acts as a tendrillar structure.
In Lathyrus aphaca the whole leaf is being converted into a single tendril while the two foliaceous stipules act like the leaves. Tendrils are always climbing organs and are sensitive to contact with any solid body. Whenever a tendril comes in contact with a neighbouring object it coils around it and helps the plant to climb.
5. Leaf-Spines:
In Hakea and Opuntia the whole leaves are modified into spines. The morphological nature of such spines can be pointed out by the presence of a bud in their axis. In such cases the stems become green and carry on photosynthesis. In Acacia nilotica and Zizyphus the stipules are modified into spines.
The position of such spines on either side of the leaf base shows their morphological nature. In Solatium xanthocarpum, Argemone Mexicana, Aloe, Acanthus, etc., the surface and margins of leaf are covered with spines. Morphologically, they are the modified parts of the leaves.
6. Scale-Leaves:
They are thin, dry, papery, stalk-less membranous structures usually brown in colour. Sometimes scale-leaves are thick and fleshy as found in onion. In Casuarina, Tamarix, Asparagus, Ruscus, etc., the leaves are reduced to scales. In such cases the stem becomes green, flattened and leaf like to perform functions of leaf.
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Scale leaves are common on underground stems where they cover and protect the axillary buds under unfavourable conditions. The scale- leaves are also common on angiospermic parasites where they replace the green vegetative leaves.
Functions of the Leaves:
The functions of the leaves are as follows:
1. Manufacture of Carbohydrates:
The main function of the leaf is to manufacture food particularly carbohydrates. Chloroplasts found in the leaf cells, trap the solar energy which is then utilized in the synthesis of carbohydrates from carbon dioxide and water by the process of photosynthesis.
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The upper side of the leaf contains abundance of the chloroplasts and the sun rays fall directly on the upper surface and normally the manufacture of food takes place in this region of the leaf.
2. Exchange of Gases:
To facilitate the exchange of gases between the atmosphere and the plant body numerous minute openings called stomata, develop, usually on the under-surface of the leaf. The stomata remain open during day light.
In the process of respiration of all the living cells the oxygen is taken in and carbon dioxide is given out while in photosynthesis the green cells absorb carbon dioxide and give out oxygen. The respiration of the living cells goes on round the clock, while the photosynthesis takes place only in daytime.
3. Transpiration:
Although large quantities of water are absorbed by plants from the soil but only a small amount of it is utilized. The excess of water is lost from the aerial parts of plants in the form of water vapours. This is called transpiration. It occurs mostly through stomata, but sometimes it also takes place through cuticle and lenticels.
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The transpiration is necessary as it helps in the transport of water within the plant body and also regulates its temperature.
4. Storage of Food:
Fleshy leaves of succulents, such as Indian aloe, purslane and fleshy scale leaves of onion store up water and food material for the future use of the plants. Fleshy leaves of many desert plants store a large quantity of water, mucilage and food material.
5. Vegetative Propagation:
The leaves of Bryophyllum, Begonia and Kalanchoe produce buds on their margins. Bach such bud develops into a new plant.
6. Protection:
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The leaves also give necessary protection to the axillary bud. The leaves modified into thorns and spines (e.g., in Berberis, Aegle), give protection to the plants from animals.
