Based on the manner of orientation to the main axis of plant and direction of sunlight, leaves in angiosperms can be divided into two types
- Dorsiventral leaves
Dorsiventral leaves orient themselves at an angle to the main axis and perpendicular to the direction of sunlight. Most dicots have dorsi-ventral leaves that are net-veined, including most trees, bushes, garden plants and wildflowers.
- Isobilateral leaves
Isobilateral leaves orient themselves parallel to the main axis and parallel to the direction of sunlight. Most monocots possess parallel-veined isobilateral leaves, including grasses and grasslike plants, lilies, irises, amaryllises etc.
Most leaves have certain common features like a covering of an epidermal layer on each surface. The ground tissue that occurs between the two epidermal layers is called mesophyll. Vascular bundles, commonly known as veins, are embedded in the mesophyll. The structure and characteristics of each of these layers differ greatly for dorsiventral and isobilateral leaves.
Anatomy of dorsiventral leaf of mango (mangifera indica)
The internal structure of the dorsiventral leaf shows these regions with following features:
- Vascular bundles
Fig – T.S. of dorsivental leaf (mango leaf) showing its internal tissues organization
- Epidermis – It is outermost covering of the leaf that forms the boundary between the atmosphere and underlying mesophyll.
It is present on both ventral or adaxial (upper epidermis) and dorsal or abaxial (lower epidermis) surface. Both the upper and lower epidermis are covered by a cuticular layer. It consists of uniseriate, compactly arranged thin walled parenchymatous tissue without chloroplast (but present in upper epidermis).
Usually upper epidermis is thickly cuticularised. Lower epidermis contains numerous stomata without chloroplast; while upper epidermis contains high amount of chloroplast and no stomata, thus the leaf is hypostomatic (some exceptions exist).
- Mesophyll – It is the ground tissue between the upper and lower epidermis. In dorsiventral leaf, it is differentiated into pallisade parenchyma and spongy parenchyma.
- Palisade parenchyma lies towards the upper epidermis and consists of one, two or three layers of elongated cells, densely packed with profuse intercellular spaces and chloroplasts. Due to large intercellular spaces and chloroplast it helps in gaseous exchange and photosynthesis.
- Spongy parenchyma lies towards the lower epidermis and made from loosely arranged, irregular, thin walled cells parenchymatous tissues with large intercellular spaces, air cavities and few chloroplasts, Hence, it helps in gaseous exchange in transpiration and photosynthesis.
- Vascular bundles – Numerous vascular bundles are scattered in spongy parenchyma. Each bundle is conjoint, collateral and closed.
Vascular bundle is surrounded by large parenchymatous bundle sheath or border parenchyma. Collenchyma may also be associated with bundle sheath cells. Xylem lies toward the upper epidermis and phloem toward lower epidermis. Single mid-vein vascular bundle is larger and several smaller veinlet vascular bundles are smaller. Smaller vascular bundles are freely scattered in mesophyll cells of leaf.
- Xylem consists of vessels, tracheids, wood fibres and wood parenchyma (lies toward the upper epidermis).The xylem tracheids and vessels help in conduction of water and minerals and mechanical supports while parenchyma helps in lateral transport. At maturity,the protoxylem lies towards upper epidermis and metaxylem toward lower epidermis.
- Pholem consists of seive tubes, companion cells and some cells of pholem parenchyma (lies towards lower epidermis). The phloem companion cells and seive tubes help in conduction of food materials and phloem parenchyma helps in lateral conduction and storage of food materials.
Diagnostic Features of Dorsiventral Leaf
- Hypostomatic – stomata are present only on lower epidermis
- Differentiated Mesophyll – into upper palisade parenchyma and lower spongy parenchyma
- Hypodermal Collenchyma – between bundle sheath of midrib vein and the upper and lower epidermal layers
- Conjoint, collateral Vascular bundles – with endarch xylem
- Vessels and tracheids – in metaxylem and protoxylem