TISSUES SCIENCE NOTES
TISSUES:
Types, Structure and Function of Plant Tissues:
The progressive evolution in plants has resulted in the increasing complexity of
structures. In higher plants, roots, stems, leaves and flowers carry out different functions.
Due to this division of labour, the cells of the plants are differentiated to form different
tissues.
The diagram given below shows the classification of different tissues:
Meristematic Tissues: The diagram given below shows the classification of different tissues:
The growth of plants occurs only in certain specific regions. This is because the dividing tissue, also known as meristematic tissue (Meristos – divisible), is located only at these points.
Meristematic tissues are made up of groups of similar and immature cells, which can divide and form new cells. Meristematic cells divide continuously and help in increasing the length and thickness of the plant. Depending upon their position, meristematic tissues can be of three types. They are as follows:
i) Apical meristems: Apical meristem is present at the growing tips of stems and roots and increases the length of the plant body.
ii) Intercalary meristems: These meristems occupy the base of leaves and the base of internodal regions in plants such as grasses (mostly in monocotyledonous plants). These help in the elongation of the internodes.
iii) Lateral meristems: This includes the meristematic tissues that occupy the lateral regions of stems and roots. They bring about increase in the width of the plant body. (e.g. cork cambium and vascular cambium).
Characteristic features of meristematic tissues:
- The meristematic cells may be round, oval, polygonal or rectangular in shape.
- Their cell walls are thin, elastic and made up of cellulose.
- They are closely arranged without any intercellular spaces.
- They have dense cytoplasm with large central nucleus.
Some cells produced by meristematic
tissues stop dividing and form permanent
tissues.
Permanent Tissues:
They have definite structure and
function. They are differentiated into
various types according to the different
functions they perform.
Permanent tissues are classified as:
i) Simple tissues
ii) Complex tissues
Simple Tissues:
A tissue made up of cells having similar
structure (one type of cells) and function is called simple tissue. They are of three
types:
a. Parenchyma
b. Collenchyma
c. Sclerenchyma
Parenchyma: The cells of the parenchyma are generally thin-walled with intercellular spaces. They are living cells. They are present in all the organs of a plant. They may be oval, spherical, rectangular or cylindrical in shape. The cell wall is made of cellulose and pectin.
Parenchyma cells serve to store and conduct food materials, water and minerals.
Collenchyma:
The cells of collenchyma are polygonal in cross-section and have unevenly thickened walls. These thickenings are due to the deposition of cellulose, hemi-cellulose and pectin.
The thickening is confined to the corners of the cells. They generally occur in the dicot stem in two or more layers below the epidermis. It is absent in the roots. It also occurs in the petiole and pedicel. Collenchyma is also a living tissue. The main function of the collenchyma is to provide strength and flexibility to the growing organs like the young stem.
