Plant Tissue Systems

·         Tissue systems are groups of tissues performing a similar function irrespective of its position in the plant body

·         Tissues systems are
:           §  Dermal (Epidermal) tissue system,

§  vascular tissue system and
§  Fundamental (Ground) tissue system
§  Epidermal Tissue System:
§  In leaves there are guard cells
§  Chloroplasts: present only in the guard cells of the epidermis
§  Other epidermal cells usually do not have chloroplasts
§  Outer wall of epidermis is usually covered by cuticle          
§  The Stoma
·         Stoma is a minute pore surrounded by 2 guard cells
·         The stomata occur mainly in the epidermis of leaves
·         Trichomes
o   Are unicellular or multicellular appendages originating from the epidermal cells
o   Trichomes may be branched or unbranched

§  Functions of Epidermal Tissue System
§  Excessive loss of water due to the presence of cuticle
§  Epidermis protects the underlying tissues
§  Stomata are involved in transpiration and gaseous exchange
§  Trichomes help dispersal of seeds and fruits
§  Root hairs absorb water and mineral salts from the soil

·         Vascular Tissue System
§  Consists of xylem and phloem
§  Elements of xylem and phloem are always organized in groups called vascular bundles
§  In dicot stems, the vascular bundle consists of cambial tissue in between xylem and phloem (i.e. Open vascular bundle)
§  In monocot stem, cambium is absent in the vascular bundle (i.e. closed vascular bundle)
§  In roots, xylem and phloem are arranged in an alternate manner on different radii, this is called radial arrangement
§  In stems and leaves, xylem and phloem are arranged at the same radius and form a vascular bundle together (i.e. Conjoint vascular bundle)
§  Conjoint vascular bundles may be;
§  Collateral
·         Xylem and phloem on same radius, phloem is outward
§  Bicollateral

·         Phloem on outer and inner sides of xylem
§  Concentric vascular bundle
§  Either phloem or xylem surrounds the other completely
§  It is amphicribral if the phloem completely surrounds the xylem
§  It is amphivasal if the xylem completely surrounds the phloem

·         Ground or Fundamental Tissue System
§  Form the main body of the plants (bulk of plant)
    • Includes all the tissues except epidermis and vascular bundles
    • In monocot stem, ground tissue system is a continuous mass of parenchymatous tissue in which vascular bundles are found scattered i.e. ground tissue is not differentiated into cortex, endodermis, pericyclic and pith.
    • In the leaves, ground tissue is made of chlorenchyma tissues forming the mesophyll
    • These cells are arranged in a single layer without intercellular spaces
    • Cells in the pith generally store starch, fatty substances, tannins, phenols, calcium oxalate crystals, etc.
  • Secretory Tissues
    • Cells or organizations of cells that produce a variety of secretions
    • Secreted substance may remain deposited within secretory cell itself or may be excreted (released from the cell)
    • The substance maybe excreted to the surface of the plant or into intercellular cavities or canals
    • Some of the secretions are waste products e.g. resins, rubber, tannins and various crystals
    • Other secretions are useful in plants e.g. enzymes and hormones.
    • Secretory structures may be single cells or group of cells (glands)
    • Oil cells occur in ginger, pepper, mace, cardamoms, cinnamon and cassia
    • The latex cells (produce latex) are found e.g. in Cannabis sativa
References
Evans, W. C. (2009). Trease and Evans Pharmacognosy. India: Reed Elsevier Limited.


Close-Up Photography of Leaves With Droplets

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