internal structure of leaf

Aug 09, 2022, 16:45 IST

internal structure of leaf

Like the root and stem, the leaf consists of three tissue systems, the dermal system, consisting of the upper and lower epidermis, the ground tissue system, the main photosynthetic tissue which consists of mesophyll, and the vascular system, comprising of veins of various degrees.Common leaves are bifacial and are further two types, dorsiventral and isobilateral. Unifacial leaves occur in Onion and Garlic.

EPIDERMIS

The upper as well as the lower surface of the leaf is covered by a uniseriate epidermis. However, in some plants (e.g., Nerium, Ficus, etc.) the epidermis is multiseriate.

All epidermal cells of a leaf are alike. The epidermal cells are compactly arranged and their outer walls are usually thickened. The epidermis is covered by a layer of cuticle; the thickness of the cuticle varies considerably and the xerophytic species have a thicker cuticle.

In some xerophytic leaves, especially those of grasses, the epidermal cells situated in longitudinal furrows are large with thin flexible walls. These cells are said to be motor cells or bulliform cells, and they help in rolling of leaves in dry weather.

A characteristic feature of the leaf epidermis is the presence of numerous small openings, called stomata. They occur either on both sides of the leaf (leaf is said to be amphistomatic), confined to the lower surface of the leaf (leaf is known as hypostomatic) or to the upper surface as in floating leaves of aquatic plants (leaf is called epistomatic). A stoma consists of two highly specialized epidermal cells, known as guard cells, enclosing a space. In some plants (e.g., Nerium), stomata present in sunken cavities, called stomatal crypts.

MESOPHYLL

The bulk of the internal tissue of the leaf, enclosed by the upper and lower epidermis, forms mesophyll. It is composed of thin walled parenchymatous cells containing numerous chloroplasts. The mesophyll is differentiated into palisade and spongy parenchyma in dicot leaves.

The spongy parenchyma consists of irregular and loosely arranged cells, enclosing large intercellular spaces. These air spaces are connected with the substomatal chambers and maintain gaseous exchange with the outside through stomata.

The palisade parenchyma is composed of more or less cylindrical and elongated cells arranged compactly with their long axis perpendicular to the epidermis.

Vascular system

The mid rib (mid vein) in most dicotyledons consists of a single large collateral vascular bundle with an adaxial xylem and abaxial phloem.

The cells surrounding the vascular bundles in the leaf are mostly morphologically distinct from the mesophyll cells. These cells constitute the bundle sheath. In dicotyledons the vascular bundles are surrounded by thin walled parenchymatous cells that extend in the direction parallel to the veins. In monocotyledons, the vascular bundles are completely or partially surrounded by one or two bundle sheaths, each consisting of a single layer of cells.

internal structure of leaf