What is Tissue
- Tissue is a group of cells that are similar in structure and/or work together to achieve a particular function.
- Eg :- In human beings the cells of the muscular tissue contract and relax and help in movements.
- In plants the cells of the vascular tissue conduct water and food from one part of the plant to the other
Differences between plant and animal tissue
- Plants do not move from place to place. Most of the cells and tissues in plants provide mechanical support and strength.
- Since dead cells do not need maintenance and provide with mechanical support, thus the plant tissues are mainly composed of dead cells
- The growth of plants takes place only in some regions. So plants have tissues called meristematic tissues which divide and help in growth and permanent tissues which do not divide
- Animal tissues are composed of mainly living cells.
- Animals grow uniformly thus there is no such demarcation of dividing and non-dividing regions in animals.
- Animals have organs having specialized functions. So the organs have specialized tissues
- The growth in plants is limited to specific regions thus the meristematic tissue is located at only these regions
- The are three types of meristematic tissue apical, lateral and intercalary.
- When meristematic cells produce new cells the two cells are similar, but as they grow they become differentiated as components of other tissues.
- Apical meristem is present at the growing tips of stem and roots and increases the length of the stem and root.
- Intercalary meristem is present at the base of the leaves or internodes on twigs
- The girth of the stem or root increases due to lateral meristem.
- Cells of meristematic tissue are very active hence have very dense cytoplasm, this cellulose walls, prominent nuclei and lack vacuoles.
- The cells produced by meristematic tissue take a specific role and loose the ability to divide.
- Differentiation is the process of taking up a permanent shape, size and function.
Simple permanent tissue
- Parenchyma is a basic packing tissue, consisting of relatively unspecialised cells with thin cell walls.
- Parenchyma has live cells and are loosely packed.
- It provides support to plants as well as store food.
- It stores excretory products such as gums, raisins, tannins and crystals/
- It may also contain chlorophyll and perform photosynthesis, in such situations it is called collenchyma.
- In aquatic animals parenchyma contains large air cavities to help them float by giving buoyancy and is called aerenchyma.
- The parenchyma of stems and roots stores nutrients and water.
- Collenchyma is a permanent tissue responsible for flexibility in plants along with providing mechanical support and is present in leaf stalks below epidermis.
- The cells of collenchyma are elongated, irregularly thickened at corners and have very less intercellular space.
- Sclerenchyma is a permanent tissue which makes plant hard and stiff, and is made up of dead cells, that are long and narrow as the walls are thickened due to lignin.
- Sclerenchyma is present at veins of leaves, hard covering of seeds and vascular bundles.
- Coconut husk is made of sclerenchymatous tissue.
- There are two types of sclerenchyma fibers and sclereids.
- Sclerenchyma protects plants from the stress and strain of environmental factors like wind.
- The outermost layer of cells is called epidermis and is usually made up of single layer of cells and the entire surface of the plant is covered by epidermis.
- Epidermal cells on the aerial parts of the plant secrete a waxy, water resistant layer on outer surface to aid water loss, mechanical injury and invasion by parasitic fungi.
- Due to the protective layer that epidermal cells play, they form a continuous layer without intercellular spaces in tissue, are relatively flat and their outer and side walls are thicker than inner wall
- Stomata are pores present in epidermis of tissue enclosed by two kidney shaped cells called guard cells and are responsible for exchanging gases and transpiration.
- In desert plants epidermal tissue secretes a waxy coating of cutin on its outer surface to prevent loss of water.
- As plants grow older a strip of secondary meristem replaces the epidermal tissue of the stem and this forms the bark of the tree and cork.
Complex permanent tissue
- Complex permanent tissues are made up of more than one type of cells coordinating to perform a common function
- Xylem and phloem are a type of complex tissue.
- They are both conducting tissues and constitute a vascular bundle, which is a feature of complex plants which made their survival possible in terrestrial environment.
- Xylem consists of tracheids, vessels, xylem parenchyma and xylem fibres.
- The tracheids and vessels help to transport water and minerals from roots to all part of the plant.
- Xylem parenchyma stores food and fibres help in support.
- Xylem cells have thick walls and are dead.
- Phloem Consists of sieve tubes, companion cells, phloem parenchyma and phloem fibres.
- The sieve tubes and companion cells transports food from leaves to all parts of the plant.
- Phloem parenchyma stores food and fibres help in support
- Unlike xylem in phloem, materials can move in both directions.