Transverse Section of Leaflet of Cycas

Transverse Section of Leaflet of Cycas

Cycas is a perennial, slow-growing evergreen gymnospermic plant. It looks like a palm tree. Its main plant body is sporophytic, diploid, dominant and can be differentiated into three parts – roots, stem and leaves.leaves are also produced in a crown at the apex of the stem. A single foliage leaf is pinnately compound. It is unipinnate and paripinnate. Each leaf has 80-100 pairs of leaflets which are arranged on both the sides of adaxial groove of the rachis in opposite or alternate manner. The leaflets are tough, thick, and leathery. A transverse section of the leaflet of Cycas shows the following structures :

(1) Cuticle: The upper surface of the leaf is covered with a thick cuticle. It serves to check excessive transpiration.

(2) Epidermis: The leaflet is surrounded by an upper and lower epidermis. the single-layered epidermis is made up of thick-walled parenchymatous cells. both the lower and upper epidermis are covered externally by a thick layer of cuticle. Stomata are sunken and found only on the lower side of the leaflet.

(3) Hypodermis: Hypodermis is a single-layered thick-walled sclerenchymatous hypodermis that lies below the upper epidermis. In the midrib region, it is 3 to 4 layers thick. On the lower side, it forms only 1 to several layers in the region of the midrib. It protects the plant from excessive transpiration and overheating and provides mechanical support.

(4) Mesophyll tissue: A well-developed mesophyll tissue is present in the leaflet. It is differentiated into palisade tissue and spongy parenchyma. Palisade tissue is present in the form of a continuous layer below the sclerenchymatous hypodermis. Spongy parenchyma are present only in the wings directly above the lower epidermis. It is made up of loosely arranged oval cells filled with chloroplast. These cells have many intercellular spaces filled with air.

(5) Accessory fusion tissue: Between the palisade and spongy parenchyma there is a three of four - layered tissue consisting of long and colourless cells extending transversely from the midrib to the margins. This constitutes the accessory transfusion tissue. It probably serves as a lateral conducting channel for water.

(6) Transfusion tissue: Groups of tracheidal cells, separated by some parenchymatous cells, or directly in contact with the centripetal xylem, the bundle sheath are present in the leaflet. It is called primary transfusion tissue. The cells of this tissue are short and wide with are reticulate or bordered pitted walls. A zone is present on either side of the midrib between the palisade and spongy layers. It is three-layered and is composed of elongated colourless cells. These cells run paralled to the leaf surface from the midrib to the margin. This zone is called accessory transfusion tissue or secondary transfusion tissue or hydrostereom or radial parenchyma. On either side of the leaflet it is connected with the primary transfusion tissue present around centripetal xylem of the vascular bundle. Primary and secondary transfusion tissue help in the lateral conduction of water. The presence of transfusion tissue is to compensate for the unbranched condition of the midrib and it probably serves as a later conducting channel of water.

(7) Vascular Bundle: A single large vascular bundle is present in the midrib region of the leaflet. It is surrounded by a single layer of sclerenchymatous cells, known as a bundle sheath. It is a single, conjoint, collateral vascular bundle in the center. The xylem consists of tracheids only and is found towards the upper surface. The phloem consists of sieve tubes and phloem parenchyma and lies towards the upper surface. The vascular bundle is mesarch and has both centripetal and centrifugal xylem.