Describe the internal structure of an Isobilateral (Monocotyledonous) leaf.

(N/A) $\rightarrow$ The leaf of maize is flat and long. The cells produced in the mid-leaf are of the same type. Structurally,the leaf of maize is called an Isobilateral leaf.
$\rightarrow$ When a thin section of a maize leaf is stained with safranin and viewed under a microscope,the following layers are observed: $(1)$ Epidermis,$(2)$ Mesophyll tissue,$(3)$ Vascular bundles.
$(1)$ Epidermis: In the leaf,two epidermis layers are seen.
$\rightarrow$ Upper Epidermis: It forms the uppermost or adaxial layer. It is a single-layered tissue made up of parenchymatous cells. Its outer surface is covered with a protective layer of cuticle.
$\rightarrow$ In the adaxial layer,at certain distances,Bulliform cells are arranged in long rows. These cells are in groups of $5$ to $7$.
$\rightarrow$ These cells are comparatively larger than other epidermal cells. They lack a thick protective cuticle on their outer wall,which is very thin. Generally,unicellular slanting small trichomes are located on both sides of these cells. These trichomes help in moisture absorption from the atmosphere.
$\rightarrow$ The presence of bulliform cells distinguishes the adaxial layer from the abaxial layer. In a humid atmosphere,bulliform cells absorb water,become turgid,and expose the leaf surface. In a dry atmosphere,due to water stress,they become flaccid,causing the leaves to curl inwards to minimize water loss. Thus,these cells are also called motor cells.
$(2)$ Mesophyll Tissue: Since the leaf possesses an isobilateral structure,there is no differentiation into palisade and spongy parenchyma.
$\rightarrow$ All cells are alike and possess an equal amount of chloroplasts.
$\rightarrow$ The mesophyll consists of cells that are round or oval with intercellular spaces. Air-filled sacs are seen clearly below the stomata,which are called respiratory cavities.
$(3)$ Vascular Bundles: Maize is a monocot plant with parallel venation; therefore,vascular bundles are cut vertically and are located at equal distances from each other.
$\rightarrow$ Vascular bundles are conjoint,collateral,and closed.
$\rightarrow$ Vascular bundles are of both large and small types. The xylem is located towards the adaxial layer,and the phloem is located towards the abaxial layer.
$\rightarrow$ Each vascular bundle is surrounded by a sclerenchymatous bundle sheath. This sheath extends to both the adaxial and abaxial layers,providing mechanical strength to the leaves.