A study of the vertical section of dorsiventral leaf reveals
the structure as follows:
upper and lower surface of leaf is bound by upper and lower epidermis
Upper Epidermis: contains a single layer of barrel shaped
living cells that usually have chloroplast. Cuticle forms a distinct outer
surface on the epidermis which prevents excessive transpiration and protects
internal cells against mechanical injury.
Lower epidermis: is formed from a single layer of packed
rectangular living cells .A layer of cuticle is also present on the lower
epidermis. There are a large number of pores called stomata present on the
lower surface. Each stoma contains a narrow pore surrounded by guard cells. The
guard cells possess chloroplasts and their inner conclave walls are greatly
thickened. In many leaves, guard cells are surrounded by specialised epidermal
cells known as accessory or subsidiary cells. Stomata have a dual role to play
– in exchange of gases and in transpiration. Since, in dorsiventral leaves the
stomata are present only on the lower epidermis, these leaves are also called
as hypostomatic leaves.
Mesophyll is the
ground tissue of the leaf and lies between the upper and lower epidermis
layers. It is a green parenchymatous tissue and can be differentiated into two
distinct regions – upper layer of palisade parenchyma and lower layer of spongy
occurs below the upper epidermis and contains columnar closely packed cells
forming 1-3 layers at right angle to the upper epidermis. They may have small
intercellular spaces. Since they are abundant in chloroplast, they attribute a
dark green colour to the upper surface of leaf. Palisade parenchyma is the main
photosynthetic tissue of plant.
is positioned between palisade parenchyma and lower epidermis. The cells are
unevenly shaped and have large intercellular spaces in between. These large
intercellular spaces and the smaller intercellular spaces of the palisade
parenchyma allow free movement of air within the leaf’s internal structure. Few
chloroplasts may be present in spongy parenchyma.
contains multiple vascular bundles of different sizes. Vascular bundles are
generally located in the boundary between palisade and spongy parenchyma.
Bundle sheath, a sheath formed of compactly arranged parenchyma cells surrounds
each vascular bundle. Each vascular bundle is conjoint and collateral. Xylem
lies towards the upper side of the leaf whereas phloem lies towards the lower
surface. Cambium is absent. Xylem parenchyma stores food and allows lateral
movement of sap.
A few layers of schlerenchyma may be found on
the outside of vascular bundle, mainly on the upper side just above the xylem.
They provide mechanical support.