Answer:
Epithelial tissues are of following types:
(a)
Simple squamous epithelium (b) Stratified squamous epithelium (c) Columnar
epithelium (d) Cuboidal epithelium
(a) Simple squamous
epithelium: They are present in cells lining blood vessels or lung alveoli
where transportation of substances occurs through selectively permeable
surface, there is a simple flat kind of epithelium.
(b) Stratified squamous
epithelium: Skin epithelial cells are arranged in many layers to prevent
wear and tear. Since, they are arranged in a pattern of layers, the epithelium
is called stratified squamous epithelium.
(c) Columnar epithelium:
Where absorption and secretion occur, as in the inner lining of the intestine,
these tall epithelial cells are present. This columnar epithelial facilitates
movement across the epithelial barrier. In the respiratory tract, the columnar
epithelial tissue also has cilia, which are hair-like projections on the outer
surfaces of epithelial cells. These cilia can move and their movement pushes
the mucus forward to clear it. This type of epithelium is thus ciliated
columnar epithelium.
(d) Cuboidal epithelium:
These form the lining of the kidney tubules and ducts of salivary glands where
these provide mechanical support. Sometimes, a portion of the epithelial tissue
folds inward and a multicellular gland is formed. This is glandular epithelium.
(a) Meristematic cells are
continuously dividing cells so they have a prominent nucleus and dense
cytoplasm. But since these cells do not store food material or waste materials,
they lack vacuole.
(b) Sclerenchyma cells have
lignified cell walls which makes them compact and leaves no intercellular
spaces.
(c) Pear has sclerenchymatous
stone cells which are granular in texture. Hence, we get the crunchy and
granular feeling while chewing a pear.
(d) The branches of a tree have
collenchyma cells which provide tensile strength to plant parts.
(e) The husk of a coconut tree
is made up of sclerenchyma cells which have lignified cell walls. Lignin makes
the cells compact and leaves no intercellular spaces.
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