Internal Structure of Leaves
A. Dicot Leaf
Most of the dicot leaves are dorsiventral. These leaves lie horizontally with distinct upper and lower surfaces thus having unequal illumination on two sides. It is also called bifacial leaf. A bifacial leaf is differentiated into 1. Epidermis, 2. Mesophyll tissue and 3. Vascular system.

1. Epidermis. The upper and lower surfaces of leaf are covered by upper and lower epidermis respectively.
(a) Upper epidermis
It consists of a single layer of closely fitted cells without chloroplasts. A thick cuticle is present to prevent excessive transpiration and to protect against injury. The stomata are usually absent.

2. Mesophyll
It is the ground tissue enclosed between two epidermal layers. It is made up of chlorenchyma. The cells are provided with intercellular spaces.

Detail of Leaf Section

(a) Palisade parenchyma
It lies below upper epidermis consisting of one or two layers of vertically elongated closely packed cells having intercellular spaces here and there. The cells contain numerous chloroplasts and are mainly photosynthetic in function.
(b) Spongy parenchyma
It lies between palisade parenchyma and lower epidermis. This region posses oval, rounded cells having intercellular spaces. They have less chloroplasts. The cells of this tissue around vein are compactly arranged to give mechanical strength. These cells also store food in the form of starch and proteins.