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Explain various modifications of Petiole with the help of a well-labeled diagram?

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Petiole ( = Mesopodium):

A petiole or leaf stalk is a cylindrical or sub cylindrical structure of a leaf which joins the lamina to the base. A It raises the lamina above the level of stem so as to provide it with sufficient light exposure. A leaf with a petiole is called petiolate and the one without it is called sessile e.g., wheat, rice, Calotropis, Gloriosa, etc. Petiole bearing leaves are known as petiolate e.g., Peepal, mango, guava, Gossypium, Hibiscus rosa – sinensis etc.

Usually petiole is cylindrical with a long groove on upper surface. In some petioles groove is not present so it becomes cylindrical e.g., peepal. A Petiole raises the lamina high to provide more and more light and air. Mineral elements absorbed by root go into lamina through the petiole and food synthesized in the lamina go into stem through it. Petioles are of the following types-

  1. Winged petiole: It is usually present in compound leaves. In this type petiole becomes flattened and leaf like and carry out photosynthesis like the leaf blade e.g., Citrus, orange, Feronia, sweet pea etc.
  2. Phyllode: Lamina in some compound leaves falls off soon and petiole gets modified into leaf-like structure and synthesize food, e.g., Australian acacia and Parkinsonia. Phyllode develops usually in vertical direction to get least sunlight. This decreases transpiration.
  3. Tendrillar petiole: Petioles in some weak stemmed plants is modified into tendril and help in climbing the plants e.g., Clematis, Nasturtium, Nepenthes etc.
  4. Floating or bulbous petiole: In some plants viz. Eichhornia, Trcipa etc. petiole becomes spongy. This petiole is full of air and helps in floating.

3. Lamina ( = Epipodium) or Leaf blade:

The leaf lamina is normally a flat, thin, expanded, green and conspicuous structure where all the functions of leaf are carried on. It is the site of photosynthesis, gaseous exchange, transpiration and most of the metabolic reactions of the plant. The form, proportion and structure of lamina vary widely, not only between the leaves of different species but even among the leaves produced during the seedling and post seedling phases of a single species.

The leaf lamina are provided and supported by veins and veinlets which contain vascular tissues. It is made up of a thick middle line from petiole to the apex known as mid vein. Lateral veins arise from the mid vein, which later divide into small veinlets. These make a net like structure in the lamina. Each leaf has its own shape, two margins, one apex and two surfaces. According to shape, apex, margins and venation lamina has many variations.

There may be following shapes of lamina of leaf-

  1. Acicular: In this type the lamina is like a needle, long thin and pointed e.g., Pinus.
  2. Linear: In this type, lamina is long and narrow having parallel margins e.g., wheat, rice and grass.
  3. Lanceolate: In this type, lamina is pear shaped i.e., thick at the lower side of mid-point and pointed at the ends e.g., Nerium, Bamboo and Polyalthea.
  4. Oblong: This type of lamina is rectangular i.e., long, broad and with a round apex e.g., Banana.
  5. Ovate: This type of lamina is egg or top shaped i.e., its base is wider than the apex, e.g., Banyan.
  6. Cordate: In this type lamina is heart shaped i.e., its base is broad and lobed e.g.. Betel and Tinosporci .
  7. Sagittate : This has arrow-shaped lamina i.e., lower side of lamina has pointed lobes on both sides having its direction on lower side e.g., Sagittaria and Arum.
  8.  Hastate: In this type also the lamina is arrow shaped but both of the lower lobes of lamina are outwardly directed e.g., Ipomoea and Typhonium.
  9. Reniform: In this type, lamina is kidney shaped i.e., lamina has a deep notch at the base e.g., Hydrocotyle and Malva.
  10. Lunate: In this type lamina is semicircular e.g., Passiflora and in a fern (Adiantum),
  11. Obovate: In this type lamina is like an inverted egg or top i.e., its apex is broader than base e.g.. Walnut, Primus amygdalus (Badam).
  12. Obcordate: In this type lamina is like inverted heart i.e., its apex is broad and bilobed e.g., Bauhinia and Oxalis.
  13. Spathulate: In this type lamina is like a spatula i.e., it is round and broader at the end and tapering at the base e.g., Dasy, Calandula, Lipia and Drosera.
  14.  Cuneate or tunicate: In this type lamina is like the
    hood of snake i.e., its breadth increases towards its apex e.g ,,Pistia. .
  15. Oval or elliptical: In this type also lamina is oval and its breadth is slightly less than its length e.g., Guava.
  16. Orbicular or round: In this type lamina is circular and petiole attached below the lamina near its centre due to which lamina looks like an umbrella e.g., Lily, Nasturtium A leaf is called dorsiventral if its upper surface (the surface facing the stem or adaxial surface or ventral surface) differs in structure from its lower surface {i.e., abaxial or dorsal surface). On the other hand, the leaf is called isobilateral when the two surfaces are similar. However, some cylindrical leaves have no distinction of two surface.

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