A group of cells that perform a specific function is called tissue.
As in animals, the cells of plants are grouped into tissues with characteristic functions such as photosynthesis, transport etc. There are two major categories of tissues in plants i.e. simple tissues and compound (complex) tissues.
The tissues which are made of single type of cells, are called simple tissues. They are of two types i.e. meristematic tissues and permanent tissues.
A- MERISTEMATIC TISSUES
These tissues are composed of cells, which have the ability to divide. The cells are thin walled, have large nucleus and small or no vacuoles. They do not have inter-cellular spaces among them. Two main types of meristematic tissues are recognized in plants.
- Apical meristems are located at the apices (tips) of roots and shoot. When they divide they cause increase in the length of plant. Such growth is called primary growth.
- Lateral meristems are located on the lateral sides of roots and shoot. By dividing, they are responsible for increase in growth of plant parts. This growth is called secondary growth. They are further of two types i.e. vascular cambium(located between xylem and phloem) and cork cambium (in the outer lateral sides of plant).
B- PERMANENT TISSUES
Permanent tissues originate from meristematic tissue. The cells of these tissues do not have the ability to divide. They are further classified into following types:
- Epidermal Tissues
Epidermal tissues are composed of a single layer of cells and they cover plant body. They act as a barrier between environment and internal plant tissues. In roots, they are also responsible for the absorption of water and minerals. On stem and leaves they secrete cutin (the coating of cutin is called cuticle) which prevents evaporation.
Epidermal tissues also have some specialized structure that perform specific functions; for example root hairs andstomata.
- Ground Tissues
Ground tissues are simple tissues made up of parenchyma cells. Parenchyma cells are the most abundant cells in plants. Overall they are spherical but flat at point of contact. They have thin primary cell walls and have large vacuoles for storage of food. In leaves, they are called mesophyll and are the sites of photosynthesis. In other parts, they are the sites of respiration and protein synthesis.
- Support Tissues
These tissues provide strength and flexibility to plants. They are further of two types.
- Collenchyma Tissue
They are found in cortex (beneath epidermis) of young stems and in the midribs of leaves and in petals of flowers. They are made of elongated cells with unevenly thickened primary cell walls. They are flexible and function to support the organs in which they are found.
COMPOUND (COMPLEX) TISSUES
A plant tissue composed of more than one type of cell is called a compound or complex tissue. Xylem and phloem tissues, found only in vascular plants, are examples of compound tissues.
- Xylem Tissue
Xylem tissue is responsible for the transport of water and dissolved substances from roots to the aerial parts. Due to the presence of lignin, the secondary walls of its cells are thick and rigid. That is why xylem tissue also provides support to plant body. Two types of cell are found in xylem tissue i.e. vessel elements and tracheids. Vessel elements or cells have thick secondary cell walls. They lack end walls and join together to form long tubes. Tracheids are slender cells with overlapping ends.
- Phloem Tissue
Phloem tissue is responsible for the conduction of dissolved organic matter (food) between different parts of plant body. Phloem tissue contains sieve tube cells and companion cells.Sieve tube cells are long and their end walls have small pores. Many sieve tube cells join to form long sieve tubes.Companion cells make proteins for sieve tube cells.