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Lecture 2: Plant cells and tissues (Parenchyma cells (Characteristics…
Lecture 2: Plant cells and tissues
Cells
It is the functional and structural unit of all living organisms, and is known as the 'building block of life'.
Tissues
Organs
Each organ is composed of several different tissues
Formed by a group of cells
composed of only one/ two kinds of cell
Permanent Tissue
Simple Permanent Tissue
Parenchyma cells
Collenchyma cells
Sclerenchyma cells
Complex Permanent Tissue
Xylem
Phloem
Parenchyma cells
Characteristics
Most abundant cell type
Cells are isodiametric (equally expanded at all sides)
May be oval, round, polygonal or elongated
Living at maturity (nucleus present)
Cells are loosely arranged with prominent intercellular spaces. With exception of the epidermis where the cells are compactly arranged, and hence , intercellualar spaces are absent.
Occurrence
Forms bulk of ground and vascular tissues
Found in the cortex of root, ground tissue in stems and mesophyll of leaves
Function
Store and assimilate food
Give mechanical strength by maintaining turgidity
Prepare food if chlorophyll is present (Chlorenchyma)
Store waste products like tanin, gum, crystal and resins
Types of Parenchyma
Chlorenchyma
Parenchyma cells filled with chloroplasts
Photosynthetic
Storage Parenchyma
Occurs principally in roots, rhizomes, bulbs, tubers, seeds or cotyledon
The accumulated compound is chiefly starch
Sugars may be stored as sucrose accumulated in the vacuole
Aerenchyma
Stores air
Found in typical parenchyma in water plants and species living in moist habitats
Have enlarged intercellular cavaties . Important for exchange of oxygen supply and gas exchange .
Water storage parenchyma
Characteristic of ground tissue of succulent plants living in arid habitats.
Water absorbed and stored in the form of a mucilage
Secretory parenchyma
Collenchyma cells
Characteristics
Cells are elongated and circular, oval or polygonal in cross section
Cell walls are unevenly thickened with cellulose at the corners against intercellular spaces
Intercellular spaces are generally absent
Cells living at maturity
Occurrence
Found in the petioles and veins of leaves
Elongating zone of young stems
Pedicels of flowers
Below the epidermis in stem
Function
Provide structral or mechanical support to the growing plant body, particularly stem
Types of Collenchyma
Angular Collenchyma: Thickening occurs at the corners of adjacent cells
Lamellar Collenchyma: Thickening on their tangential walls, which are parallel with the surface.
Sclerenchyma cells
Characteristics
Important as a supporting tissue in plants
Have secondary cell walls thickened with cellulose and usually impregnated with lignin
Dead at maturity
Sclerenchyma is elastic
Types of Sclerenchyma
Fibers
Cells are needle-shaped with pointed tips
Cells have thick walls and rather small lumen
Simple pits are also present
Abundance in vascular tissue of angiosperms. Fibers of flax, hemp, are extrememly soft and elastic and are especially well suited for the processing to textiles
Hard fibers are most commonly found in monocots like the fibers of many Gramineae, Agaves, Lilies and Musa
Sclereids
Irregular in shape
Cell walls are thick, lignified which makes the lumen very small
Simple pits (canals) are found in the thickened cell walls and link adjacent cells
Sclereids are commonly found in fruit and seeds. Causes the gritty texture of pears is due to sclereids, Found in seed coat, nut shell, stone of peaches and apricots