Part – 4: Anatomy of Leaflet and Rachis
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Anatomy of Cycas Leaflet
Ø Cycas leaflet is with distinct midrib and lateral wings. Mid-rib is swollen and the wings are flattened. Margins of the leaves are revolute (curved downward) in Cycas revoluta.
Ø Anatomically, the leaf is differentiated into:
§ Upper Epidermis
§ Hypodermis
§ Mesophyll
§ Vascular Tissue
§ Lower Epidermis
Ø Upper epidermis is single layered, compactly packed parenchymatous cells
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Ø Epidermal cells are heavy cutinized (xerophytic adaptation)
Ø Hypodermis is located below the epidermis, composed of sclerenchymatous cells.
Ø Mesophyll is located below the hypodermis.
Ø Mesophyll is differentiated into upper palisade and lower spongy tissues.
Ø The palisade tissue is columnar and closely packed.
Ø Spongy tissue is loosely packed with plenty of intercellular spaces and it is placed just above the lower epidermis.
Ø Vascular bundles are located in the center portion of the midrib.
Ø Vascular bundles are surrounded by parenchymatous and multilayered bundle sheath.
Ø Bundle sheath parenchyma contains calcium oxalate crystals.
Ø Vascular bundles similar to that of the tip of rachis.
Cycas Leaflet Diagram
Ø Vascular bundles are conjoint, collateral, open and diploxylic
Ø Xylem composed of a large triangular-shaped centripetal xylem and two patches of laterally placed small centrifugal xylem.
Ø Phloem present on the abaxial side of the leaf.
Ø An inactive cambium is present between the xylem and phloem
Ø Lateral veins are completely absent in Cycas leaf.
Ø Lateral diffusion of contents takes places through a specialized tissue namely transfusion tissue and accessory transfusion tissue.
Ø Transfusion Tissue: It is the tracheid like tissue present on either side of the centripetal xylem of the midrib bundles
Ø Accessory transfusion tissue is located between palisade and spongy tissue of the leaves (2 – 4 layered) as long, colourless and tubular cells.
Ø Transfusion tissue and accessory transfusion tissue facilitate lateral conduction of water, minerals and food.
Ø The lower epidermis composed of single-layered, compactly packed and heavily cutinized parenchymatotus cells.
Ø Plenty of stomata are present on the lower epidermis.
Ø Stomata are sunken type (xerophytic adaptation)
(2). Anatomy of Cycas Leaf Rachis
Ø T.S. of rachis is circular in outline with pinnae
Ø Anatomically the rachis is differentiated into:
§ Epidermis
§ Hypodermis
§ Ground Tissue
§ Vascular Tissue
Ø Epidermis composed of compactly packed parenchymatous cells.
Ø Epidermal cells are heavily cutinized.
Ø Stomata are present on the epiderms, Stomata are sunken type.
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Ø Hypodermis is multilayered and Sclerenchymatous
Ø Few cells chlorenchymatous cells are also present in the hypodermis.
Ø The ground tissue is the largest portion of the rachis.
Ø It composed of parenchymatous loosely packed cells with plenty of intercellular spaces.
Ø Numerous mucilage ducts present are present in the cortex.
Ø Mucilage ducts are with double-layered covering.
Ø The inner layer of the duct is called epithelial layer which is the secretory layer.
Vascular Tissue
Ø Numerous vascular bundles are present
Ø Vascular bundles are arranged in an inverted omega shape
Ø Vascular bundles are conjoint, collateral and open
Ø Vascular bundles are surrounded by single-layered sclerenchymatous bundle sheath.
Ø Arrangement of xylem and phloem in the vascular bundles varies in the Basal, Middle, and Tip potion of the rachis
Ø At the base: Vascular bundles show only centrifugal xylem. Here the xylem is endarch, phloem is on the abaxial side.
Ø Middle portion: Vascular bundles shows Centrifugal xylem on the abaxial (lower) and Centripetal on the adaxial side (upper). Protoxylem is located in the centre of the centrifugal and centripetal xylem (such condition is called mesarch)
Ø Tip portion (Majority of the rachis portion): Here the vascular bundles are diploxylic (2 types of xylem). Composed of Centripetal and centrifugal xylem. The centripetal xylem is large, triangular-shaped and centrally placed. The centrifugal xylem in two patches located on both sides of the centripetal xylem. The phloem is located on the abaxial side of the rachis.
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