11th Class Biology Morphology of Flowering Plants (Root, Stem And Leaf) Venation

Venation

Category : 11th Class

The arrangement of veins in the lamina of a leaf is called venation. The veins are the hard structures consisting of xylem and pholem. The veins give mechanical strength and shape to the lamina. Angiosperms exhibit two types of venation.

(1) Reticulate venation

In this type, the lateral veins divide and redivide to form many veinlets. These veinlets are arranged in a net like fashion or reticulum. Reticulate venation is the characteristic feature of dicotyledons. But exceptionally some monocotyledons also show reticulate venation. e.g., Smilax, Alocasia and Dioscorea etc. Reticulate venation is of two types :

(i) Unicostate or Pinnate venation : This type of venation is characterized by the presence of a single strong midrib that extends upto the apex of lamina. The midrib produce lateral veins on either side which divide repeatedly. e.g., Ficus and Mangifera.

(ii) Multicostate or Palmate venation : Here more than one prominent veins start from the base of the lamina and proceed upwards.

The lateral veinlets, arising from main veins, form network. Multicostate venation is of two types :

(a) Convergent : When the prominent veins converge towards the apex of lamina. e.g., Zizyphus and Cinnamonum camphora (kapoor), etc.

(b) Divergent : When the prominent veins spread out towards the margins. e.g., Papaya, Ricinus, Cucurbita etc.

(2) Parallel or Striate venation

In this type, veins and veinlets run parallel to each other. Parallel venation is the characteristic feature of monocotyledons.

Exceptionally few dicots show parallel venation, e.g., Calophyllum and Eryngium. It is of two types :

(i) Unicostate or Pinnate venation : The leaf lamina possesses single prominent vein which gives rise to a large number of lateral veins. All the lateral veins run parallel towards margin. e.g., Banana, Canna, Curcuma etc.

(ii) Multicostate or Palmate venation : The leaf lamina possesses several prominent veins which run parallel to each other. It is of two types :

(a) Convergent : The prominent veins run parallel to each other and converge at the apex. e.g., Sugarcane, Maize, Wheat, Bambooes and Grasses.

(b) Divergent : All the prominent veins of leaf lamina spread out towards the margin. e.g., Fan palm.

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