· Internal structure of dicotyledonous stem consists of Epidermis, Cortex and Stele.
· Epidermis
§ It consists of a single layer of parenchymatous rectangular cells
§ Epidermal cells are compactly arranged without intercellular spaces
§ Epidermis is covered on the outside by cuticle made up of waxy substance
§ The cuticle checks/controls transpiration
§ Stomata may be present at irregular intervals
§ A large number of multicellular hairs occur on the epidermis
· Cortex
§ Cortex lies below the epidermis
§ The cortex is differentiated into three zones
§ The first zone (Hypodermis)
· Lies below the epidermis, consists of a few layers of collenchymacells
§ The second zone
· Lies inner to the hypodermis
· Consists of a few layers of chlorenchyma cells
· Some resin ducts also occur in this zone
§ The third zone
· Consists of parenchyma cells
· The innermost layer of the cortex is the endodermis
· Made of barrel shaped cells arranged compactly without intercellular spaces
· Rich in starch grains, hence “starch sheath”
· The Stele
§ In the central part of the stem inner to the endodermis
§ It consists of pericycle, vascular bundles and pith
§ Vascular bundles are arranged in a ring around the pith
§ Pericycle
§ Layers of cells between endodermis and vascular bundles
§ Vascular Bundles
§ Consist of xylem, phloem and cambium
§ Xylem and phloem in stem occur together to form the vascular bundles
§ These vascular bundles are wedge shaped
§ The vascular bundle is conjoint, collateral, open and endarch
§ Phloem
§ Consists of sieve tubes, companion cells and phloem parenchyma
§ Phloem conducts organic food materials from the leaves to other parts of the plant body
§ Cambium
§ Consists of brick shaped and thin walled meristematic cells
§ Form new cells during secondary growth
§ Xylem
§ Consists of xylem fibres, xylem parenchyma, vessels and tracheids
§ Vessels are thick walled and arranged in a few rows
§ Conducts water and minerals from the root to the other parts of the plant body
§ Pith (medulla)
§ It is the large central portion of the stem
§ It is composed of parenchyma cells with intercellular spaces
§ The pith extends between the vascular bundles and form primary medullary rays
§ These extensions of the pith between the vascular bundles are called
It stores food
References
Evans, W. C. (2009). Trease and Evans Pharmacognosy. India: Reed Elsevier Limited.
Leave a Reply