9. Morphology of Flowering Plants - part 03 - LEAF

 

9. Morphology of Flowering Plants - part 03 - LEAF



Leaf :
  • Leaves are the most important appendages as they carry out photosynthesis and also help to remove excess amount of water from plant body. 
  • Leaf develops from leaf primordium. 
  • Leaf is dorsiventrally flattened lateral appendage of stem.  It is produced at nodal region. 
  • Leaf is thin, expanded and green due to presence of photosynthetic pigments. It shows exogenous origin. 
  • Axil of leaf shows presence of axillary bud. 
  • Leaf shows limited growth, does not show apical bud or a growing point.

1. Typical leaf structure : 

  • It shows presence of three main parts-
  1. Leaf base or Hypopodium,
  2. Petiole or Mesopodium and 
  3. Leaf lamina/ blade or epipodium.

Leaf base : 
  • The point by which leaf remains attached to stem is known as leaf base.
  • The nature of leaf base is varies in different plants. 
  • It may be pulvinus (swollen), sheathing or ligulate etc.
  • In some plants leaves possess a pair of lateral outgrowths called as stipules. 
  • The leaf with stipule is said to be stipulate and without stipule is exstipulate. 
  • Stipules are normally green protective structure.
Petiole or mesopodium : 
  • The part of leaf which connect leaf lamina with the leaf base is known as petiole of leaf. 
  • A leaf with petiole is petiolate and a leaf without petiole is termed as sessile leaf.
  • Petiole helps lamina to get exposed to light and also helps in conduction.
Lamina or epipodium : 
  • Large expanded, flat and green part of leaf. 
  • The lamina surface plays important role in photosynthesis, gaseous exchange and transpiration. 
  • The leaf is either dorsiventral or isobilateral. 
  • Dorsiventral leaf is common in Dicots and isobilateral in Monocots.
  • Centric or cylindrical leaves are those in which both the surfaces of leaf cannot be distinguished distinctly. 
  • Leaf lamina varies greately in shpe, margin and apex.
2. Leaf venation : 
  • Arrangement of veins and veinlets in leaf lamina is known as venation.
  • Veins are responsible for conduction of water and minerals as well as food. 
  • The structural framework of the lamina is developed by veins.

3. Types of leaf : 

  • Based on incision of lamina leaves are of two main types. i.e. 
  1. simple and
  2. compound. 
  • The leaf with entire lamina is called simple leaf 
  • Leaf in which lamina is divided into number of leaf lets called as compound leaf.

Types of Compound leaf :
a. Pinnately compound : 
  • Leaflets are present laterally on a common axis called rachis, which represents the midrib of the leaf.
b. Palmately compound : 

  • In which all the leaflets are attached at tip of petiole.
4. Modification of leaves : 

  • Apart from photosynthesis leaf also performs transpiration gaseous exchange and perception of light for flowering. 
  • However leaves may undergo modifications to perform several other functions. 
  • As per the modification their are different types of leaves shown below.
a. Leaf spines: 
  • Sometimes entire leaf is modified into spines (Opuntia) or margin of leaf becomes spiny (Agave) or stipule modifies into spine (Acacia) to check the rate of transpiration or to protect plant from grazing. 
  • E.g. Zizyphus etc.
b. Leaf tendril:

  • In some weak stems for providing addtitional support; leaf, leaflet or other part modifies to produce thin, green, wiry, coiled structure called as leaf tendril. 
  • It helps in climbing.












c. Leaf hooks: 




  •  In plants like Bignonia unguiscati (Cat’s nail) the terminal three leaflet get modified into three stiff curve and pointed hooks used to cling over bark of tree.





d. Phyllode: 

  • When petiole of leaf becomes flat, green and leaf like it is called as phyllode.
  • In Acacia auriculoformis the normal leaf is bipinnately compound and falls off soon.
  • The petiole modifies itself into phyllode. It is xerophytic adaptation.
5. Phyllotaxy : 

  • Arrangement of leaves on the stem and branches in a specific manner is known as phyllotaxy. 
  • It enable leaf to get sufficient light.
  • Alternate Single leaf from each node E.g. Mango
  • Whorled Many leaves from each node E.g. Nerium
  • Opposite decussate - A pair of leaf from each node and the consecutive pair at right angle E.g. Calotropis
  • Opposite superposed - A pair of leaf from each node and the consecutive pair is arranged just above. E.g. Jamun

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