Describe the internal structure of a dorsiventral (dicotyledonous) leaf.

(N/A) The internal structure of a dorsiventral leaf (e.g.,sunflower leaf) consists of three main parts: $(1)$ Epidermis,$(2)$ Mesophyll tissue,and $(3)$ Vascular bundles.
$(1)$ Epidermis: The leaf has two layers of epidermis:
$(i)$ Adaxial (upper) epidermis: $A$ single layer of parenchymatous cells covered by a cuticle. It usually has fewer stomata.
$(ii)$ Abaxial (lower) epidermis: $A$ single layer of parenchymatous cells. It bears a large number of stomata for gaseous exchange.
$(2)$ Mesophyll Tissue: The tissue between the upper and lower epidermis is called mesophyll,which performs photosynthesis. It is differentiated into two types:
$(i)$ Palisade parenchyma: Adaxially placed,consisting of elongated cells arranged vertically and parallel to each other. They contain numerous chloroplasts.
$(ii)$ Spongy parenchyma: Located below the palisade cells,these are oval or round and loosely arranged with large intercellular spaces and air cavities.
$(3)$ Vascular Bundles: These are conjoint,collateral,and closed. They are surrounded by a layer of thick-walled bundle sheath cells. In the vascular bundle,xylem is located towards the adaxial side (upper surface) and phloem is located towards the abaxial side (lower surface). The size of the vascular bundles depends on the size of the veins.