Internal Structures of the Dicot Stem


·         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 

Selective-focus Photography of Orange Petaled Flower

References

Evans, W. C. (2009). Trease and Evans Pharmacognosy. India: Reed Elsevier Limited.


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