12-06-2014, 10:59 AM
Tissue
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Plant Tissues:
Plant tissues are of two main types, viz. meristematic tissue and permanent tissue.
1 - Meristematic Tissue:
Plant tissues in which cells keep on dividing are called meristematic tissue. Meristematic tissues are present in those parts of plants which keep on growing. Meristematic tissues are classified on the basis of their location. They are of following types:
a. Apical Meristem: Apical meristem is present on root apex, stem apex, leaf buds and flower buds. They are responsible for growth in length, i.e. primary growth.
b. Lateral Meristem: Lateral meristem is present along the side of the stem. They are responsible for growth in girth, i.e. secondary growth.
c. Intercalary Meristem:Intercalary meristem is present at the base of leaf or internodes. They are present on either side of the node.
Since cells of meristematic tissue are highly active so they have dense cytoplasm. Vacuole is absent in these cells.
2 - Permanent Tissue:
Once the cells of meristematic tissue divide to a certain extent, they become specialized for a particular function. This process is called differentiation. Once differentiation is accomplished, the cells lose their capability to divide and the tissue becomes permanent tissue. Permanent tissues are of two types, simple permanent tissue and complex permanent tissue.
(1) Parenchyma: The cells of parenchyma have thin cell wall. They are loosely packed; with lot of intercellular spaces between them. Parenchyma makes the largest portion of a plant body. Parenchyma mainly works are packing material in plant parts. The main function of parenchyma is to provide support and to store food. In some plant parts, parenchyma has chlorophyll as well. In that case, parenchyma carries out photosynthesis and is then termed as chlorenchyma. In aquatic plants, large air cavities are present in parenchyma. This provides buoyancy to the plant, and then the parenchyma is known as aerenchyma.
(2) Collenchyma: The cell wall of collenchyma is thickened at corners. Intercellular spaces are very few. Collenchymas provides some degree of structural rigidity with flexibility. Collenchymas is present in leaf stalk; below the epidermis. Due to this, the leaf talk can easily bend but does not break.