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