Tissue: A groups of cells which is meant for a specific task is called tissue.
Tissue and Division of Labour
In complex organisms, different tasks are carried out by different organs and organ systems. This is necessary to ensure division of labour. In other words, it is necessary to ensure different parts of the body perform different tasks. Tissues are the first step towards division of labour in complex organisms.
1: (b) Tissue, 2: (d) Vacuoles, 3: (c) Leaf bud, 4: (b) Lateral meristem, 5: (a) Xylem, 6: (b) Parenchyma, 7: (c) Water lily, 8: (d) Collenchyma, 9: (b) Sclerenchyma, 10: (c) Sclerenchyma, 11: (a) Parenchyma, 12: (d) Vascular Bundle, 13: (c) Sclerenchyma, 14: (b) Two, 15: (c) Both a and b, 16: (a) Xylem, 17: (b) Xylem, 18: (c) Both a and b, 19: (d) Phloem, 20: (c) Xylem parenchyma
In simple organisms, a cell or a small group of cell can be responsible for all the biological functions. In complex organisms, there is a need for more efficiency for different tasks.
Plant tissues are less complex
Plants have a sedentary life because they do not need to move in search of food or mate. So, plants may not need so many specialised tissues and much complexity.
Animal tissues are more complex
But an animal has to move in search of food and mate. So complex animals need more specialised tissues and complexity.
Plant tissues are of two main types, viz. meristematic tissue and permanent tissue.
Plant tissues in which cells keep on dividing are called meristematic tissue. Cells of meristematic tissue have dense cytoplasm and vacuoles are absent. Vacuoles are absent because these cells do not need to store substances. 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:
- Apical Meristem: Apical meristem is present on root apex, stem apex, leaf buds and flower buds. They are responsible for growth in length. Growth in length of a plant part is called primary growth.
- Lateral Meristem: Lateral meristem is present along the side of the stem. They are responsible for growth in girth. Growth in girth is called secondary growth.
- Intercalary Meristem: Intercalary meristem is present at the base of leaf or internodes. They are present on either side of the node.
Once the cells of meristematic tissue divide to a certain extent, they become specialized for a particular function. This process of attaining specialisation 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.
Simple Permanent Tissue
Simple permanent tissue is composed of similar types of cells. Simple permanent tissues are of three types, viz. parenchyma, collenchyma and sclerenchyma.
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 as packing material in plant parts. The main function of parenchyma is to provide support and to store food. Parenchyma can be of special types, which are as follows:
- Chlorenchyma: In some plant parts, parenchyma has chlorophyll as well. In that case, parenchyma carries out photosynthesis. In this case, parenchyma termed as chlorenchyma.
- Aerenchyma: In aquatic plants, large air cavities are present in parenchyma. This provides buoyancy to the plant. In this case, parenchyma is known as aerenchyma.
The cell wall of collenchyma is thickened at corners. Intercellular spaces are very few. Collenchyma provides some degree of structural rigidity along with flexibility. Collenchyma is present in leaf stalk, and present just below the epidermis. Due to this, the leaf stalk can easily bend but does not break.
The cell wall in sclerenchyma is highly thickened all around. The cells are dead and intercellular space is absent. Sclerenchyma provides structural rigidity to plant parts. Bark is composed of sclerenchyma. Another example of sclerenchyma can be seen in the coconut husk. Hard covering of fruits and nuts are composed of sclerenchyma. While eating a pear, you will get a gritty feeling in your mouth. This comes due to sclerenchymatous tissue.
The epidermis of leaves has small pores called stomata. A stoma is a composed of two guard cells which regulate the opening and closing of stoma. Stomata facilitates exchange of gases and transpiration.
Complex Permanent Tissue
Complex permanent tissue is composed of different types of cells. Complex permanent tissues are of two types, viz. xylem and phloem. Xylem and phloem together make the vascular bundle in plants.
Xylem is composed of trachieds, vessels, xylem parenchyma and xylem fibres. The cells of xylem are dead, except the cells of xylem parenchyma. Trachieds and vessels are tubular structures and thus they provide a channel for conduction of water and minerals. Xylem fibre provides structural support to the tissue. Xylem parenchyma stores food.
Phloem is composed of sieve tubes, companion cells, phloem fibre and phloem parenchyma. Sieve tubes are tubular cells with perforated walls. Sieve tubes are the conducting elements of phloem. It means conduction or transportation of materials takes place through sieve tubes. Phloem is responsible for translocation of food in plants. The transport of food in phloem is a two way movement.