Leaves are the most visible organs of a plant.
They perform many crucial functions such as photosynthesis and transpiration, in which their internal structure plays an important role.
A dicot leaf is also known as dorsiventral as its upper and lower sides are different in structure.
The transverse section of a dicot leaf has three main parts – epidermis, mesophyll and the vascular system.
The epidermis covers the upper and lower surfaces of the leaf and is coated with the cuticle. The epidermis covering the upper surface of leaf is called the adaxial epidermis while that covering the lower surface is called the abaxial epidermis. Typically, the abaxial epidermis has more stomata compared to the adaxial epidermis, which sometimes may not have any stomata at all.
The next part of the leaf is the mesophyll, which is located between the upper and the lower epidermis. It is also called the green tissue of the leaf as it contains chloroplasts.
The mesophyll is made up of parenchyma and performs photosynthesis.
In dicot leaves, the mesophyll tissue is differentiated as palisade parenchyma and spongy parenchyma. The palisade parenchyma, located near the adaxial epidermis, is made up of elongated cells arranged vertically and parallel to each other. These cells have fewer intercellular spaces. The spongy parenchyma is located just below the palisade parenchyma and it extends up to the abaxial epidermis.
It consists of round cells which are loosely arranged and have large intercellular spaces and air cavities.
The third part of a dicot leaf is the vascular system comprising vascular bundles.
When observed under a microscope, vascular bundles can be seen in the veins and the midrib of the leaf. The size of the vascular bundles depends on the size of the vein. In dicot plants, the leaves have reticulate venation, in which the veins are of different sizes. Therefore, the vascular bundles are found irregularly scattered in the mesophyll. In the large veins, vascular bundles are usually surrounded by a bundle sheath.
Monocot leaf: It is also known as an isobilateral leaf, which means it is identical on both sides of its axis and shows parallel venation. The anatomy of a monocot leaf is similar to that of a dicot leaf. However, there are also some key differences.
Unlike a dicot leaf, where stomata are found in greater numbers in the abaxial epidermis, in a monocot leaf, stomata are present on both surfaces of the epidermis.
Moreover, the mesophyll cells are not differentiated as palisade and spongy parenchyma in a monocot leaf. The vascular bundles in a dicot leaf are irregularly scattered due to reticulate venation. In a monocot leaf, however, they are arranged in a parallel manner in the mesophyll due to parallel venation. In the leaves of monocots such as grasses, veins and cells of the upper epidermis modify themselves as large, bubble-shaped, colourless cells also known as bulliform cells.
On absorption of water, bulliform cells become turgid and expose the leaf’s surface.
Conversely, in water-stress situations, there is loss of turgor pressure, and bulliform cells become flaccid. As a result, the leaves curl inwards to minimise water loss. Hence, a lot of activity goes on in the internal structure of a leaf.
Since major food production takes place in leaves, they are aptly known as the plant’s powerhouse of energy.