In an angiosperm plant body, there are different types of tissues to perform a variety of functions such as growth, protection, absorption, photosynthesis and conduction of water and minerals.
Types of plant tissues are shown in the tree diagram of Figure.
Meristematic Tissues (Meristems)
A meristematic tissue is a group of similar cells that are in a continuous state of division. These tissues are, in general, called meristems.
The meristematic cells are usually isodiametric, spherical, oval or polygonal in shape. They are compactly arranged without intercellular spaces. They have large nuclei. Metabolically, these are the most active cells. The main function of meristematic tissue is to continuously form a number of new cells.
Meristematic tissues are present in growing regions such as root tip, stem apex and buds.
The meristematic cells lose their power of division, and differentiate into permanent tissues. The cells of these tissues are either living or dead, or either thin-walled or thick-walled. Based on the nature of cells comprising a permanent tissue, primary permanent tissues are classified into three types, namely simple, complex and special or secretory.
Simple tissues are composed of similar cells, whereas complex tissues are made up of different types of cells. A simple tissue consists of only one type of cells. The common simple tissues of plant body are as follows:
- collenchyma and
- The cells of parenchyma are thin-walled living cells. They are usually isodiametric and appear oval or spherical in shape, but sometimes they appear polygonal as a result of pressure due to the neighbouring cells.
- Intercellular spaces are present between the cell walls of adjacent cells.
- Parenchyma cells play an important role in the storage of food and water.
- Parenchymatous tissue occurs practically in all the soft parts of the plant, namely roots, stems, leaves, fruits and seeds.
An example of parenchyma cells is shown in Figure.
Structure of Parenchyma Cells
- The cells are elongated in shape but appear oval or polygonal in transverse section. They are living cells.
- The cells have pointed, blunt or oblique ends. They have thickenings at the corners due to the deposition of cellulose and pectin at the corners.
- They provide mechanical strength to the part in which they occur.
- They are usually present in the hypodermis of several herbaceous dicotyledonous stems and around the veins in the leaves.
Figure shows T.S. and L.S. views of collenchyma cells.
- The sclerenchyma consists of thick-walled cells often lignified.
- It occurs in all mature parts of plants including leaves, stems, roots and bark.
- Its main function is to give mechanical support to plants.
- Sometimes special types of sclerenchyma cells called sclereids develop in various parts of the plant, such as cortex, pith, phloem, hard seeds, etc. They provide more mechanical support. Sclereids do not have any definite shape and are dead.
Figure represents sclerenchyma fibres.
Sclerenchyma Fibres: (A) Group of Fibres, (B) T.S. of Fibre, (C) Single Fibre
These tissues are of two types, namely xylem or wood and phloem or bast. Xylem and phloem are also called vascular tissues and together they constitute vascular bundles.
- Xylem is a complex permanent tissue forming a part of vascular bundle. It is primarily responsible for the conduction of water and solutes from the roots up to the top of the plant. It is composed of cellular structures, such as tracheids, vessels, xylem parenchyma and xylem fibres.
- Tracheids are long, tubular with tapering, chisel-like ends and are dead. The cell wall is lignified. Tracheids are present in all groups of vascular plants. In pteridophytes and most of the gymnosperms, tracheary elements consist exclusively of tracheids.
- The vessel members are characterised by the presence of perforation plate at their end walls They are joined end to end to form a long chain of cells called vessels.
- The vessels are shorter and broader than tracheids. These are thickened and lignified and are dead at maturity. If an end wall has several openings, it is termed as multiple perforation plate. If it has one opening, then it is known as simple perforation plate.
- Vessels are absent in pteridophytes and gymnosperms.
- Phloem is also known as bast. It is responsible for conducting food material (sugar) from the leaf downwards to the storage organs such as the roots and stems. It is a complex tissue made up of cellular structures, such as sieve tube members, companion cells, phloem fibres and phloem parenchyma.
- The sieve tube members are found in angiosperms, in which large openings (sieve pores) occur mainly on the end walls (sieve plate). A mature sieve element is living but it lacks a nucleus.
- Companion cells are specialised parenchymatous cells, which are closely associated with sieve elements in origin, position and function. These cells help the sieve tube members in translocation of food.
- Phloem fibres are elongated sclerenchymatous cells having lignified walls with simple pits. These occur frequently in the secondary phloem tissues and are meant for mechanical support
- The phloem parenchyma cells are living and are often cylindrical in shape. They mainly store food material.
The plant body is externally covered by a tissue known as epidermis.
- Usually epidermis is covered by a layer of waxy substance called cutin, is also called as cuticle.
- The cuticle prevents the excessive loss of water from the cells.
- Epidermis protects the inner tissues of the plant just like the skin in animals. Epidermis is modified in different parts to perform various functions.
- Epidermis of a leaf consists of tiny pores called stomata. Each stoma has a pair of bean-shaped cells called guard cells. The guard cells help in closing and opening of the stomata, thereby controlling the rate of transpiration.
- In insectivorous plants such as Drosera and Nepenthes, the epidermal cells in some of the leaves are modified into digestive glands.