BOTANY Higher Secondary Second Year - Textbooks Online

oval, spherical, rectangular, cylindrical or stellate. The cells are usually
polyhedral with 10 to12 facets. Parenchyma is of different types and
some of them are discussed as follows.
In water plants, the parenchyma found in the cortex region possesses
well-developed large intercellular spaces called air spaces. This air filled
parenchyma tissue is called aerenchyma. It helps the plant to float in
water. eg. Nymphaea and Hydrilla. The parenchyma cells that are stored
with starch grains are called storage parenchyma. eg. stem and root
tubers. In the petioles of banana and Canna, star shaped parenchyma
cells are found. These cells are called stellate parenchyma. In green
parts of the plants, the parenchymatous cells have chloroplasts. These
cells are called chlorenchyma. Its important function is photosynthesis.
Collenchyma
Collenchyma generally occurs in the dicot stems in two or
more layers below the epidermis. These layers constitute the
hypodermis. It is absent in the roots of land plants. It also occurs in
petiole and pedicel. It gives strength to young organs. Collenchyma
is a living tissue. It consists of more or less elongated cells, which
are polygonal in cross section. The cell wall is unevenly thickened.
The thickening is confined to the corners of the cells. Besides cellulose,
the cell wall contains high amounts of hemicellulose and pectin.
Cell wall
Nucleus
Angular collenchyma
Lamellar
thickening
Lamellar collenchyma
Fig. 2.3. Types of collenchyma
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Angular wall thickening
Vacuole
Lacunate collenchyma
Lacunae