Give an illustrated account of the external and internal structures of the leaf of mustard (Brassica campestris).

 EXTERNAL STRUCTURE OF LEAF:

 Leaves grow from nodes on stems and branches. Each leaf consists of two parts. The lower stalk like part is called the petiole and the upper green expanded portion is called lamina. Young leaves are without petioles and their margins are entire or smooth but in mature leaves the margin is wavy. There is a swollen vein in the middle of the leaf which is known as midrib. The veins of different thickenings originate from the midrib and spread in the leaf forming a network. This type of arrangement of leaves is called reticulate venation. The veins consist of xylem and phloem. This network of veins supports the leaf and keeps its lamina in an expanded position. The angle formed within the stem and leaf is called the axil. In the axils of leaves buds are present from where new branches grow. Upper and lower surfaces of the leaf are different from each other such leaves are called bifacial leaves. As the function of leaves is to prepare food for the plant during photosynthesis, all of its tissues are arranged in such a way that photosynthesis can take place easily.

 Internal Structure of Leaf:

 A T.S (transverse section) of leaf shows the following parts under a Microscope. 

Epidermis:

 This layer covers the upper and lower surfaces of the leaf. The upper layer of the leaf is called the upper epidermis. The layer on the lower surface of the leaf is called the lower epidermis. The lower epidermis contains more stomata than the upper epidermis.

 The singular of stomata is stoma.

Each stoma has a pore with two guard cells which are kidney shaped. Gases and water vapour are exchanged through stomata. 

Functions:

 (i) It provides protection to the leaf. 

(ii) it carries out transpiration in which gases and water vapour are exchanged through stomata. 

Function:

 (2) Mesophyll:

 The tissue present between the upper and lower epidermis is called mesophyll. The mesophyll cells .below the upper epidermis are longer than broad arid are closely packed. They lie vertically and are double layered. They are called the palisade mesophyll. The cells below the palisade layer are irregular in shape. The lower part has more intercellular spaces and is sponge like. It is called spongy mesophyll. Both the palisade and spongy mesophyll have chloroplasts in which chlorophyll is present.

Function:

 The mesophyll prepares food for the plants. 

Vascular Bundles:

 Midribs and other veins of the leaf contain vascular bundles, the xylem and the phloem. Xylems are present towards the upper part while phloems are present towards the lower part. 

Function:

 (i) Xylem transport water and mineral salts from the soil to all parts of the plant upwards. 

(ii) Phloems transport food prepared in the leaves to all parts of the plant upwards and downwards.