Parenchyma is a primary, simple permanent tissue of the plants consisting of thin-walled cells. The cells of this tissue are living and more or less isodiametric in shape with intercellular spaces. The cell wall is usually thin and composed of cellulose. Different types of parenchyma are present in plants that contain chloroplasts with chlorophyll (chlorenchyma), air cavities (aerenchyma), oil, tannin, calcium oxalate (idioblast), etc. The main functions of parenchyma are the storage of food, photosynthesis, conduction of water (by xylem and phloem parenchyma), wound recovery, etc.

The permanent simple tissue consisting of slightly elongated cells whose primary walls are unevenly thickened is called collenchyma. The unevenly thick cell wall is made up of cellulose, hemicellulose and pectin materials. The thickening of the cell wall generally occurs at the corners where a number of cells join with each other. Primary pit fields may be distinguished in the collenchyma cell wall. These cells are living and containing vacuolated protoplasm. On the basis of thickening pattern, collenchyma tissues are classified into angular collenchyma (thickening are localised at the corners), lacunate collenchyma (thickening on the walls around the intercellular spaces) and plate collenchyma (thickening on tangential wall).

Complex tissues are heterogeneous in nature and are composed of different types of cell components. Xylem and phloem are the complex tissues that together form the vascular bundles. The xylem consists of four types of tissues such as tracheids, xylem vessels, xylem fibres and xylem parenchyma. Tracheids, vessels and fibres are dead and elongated cells that help in the transportation of water and minerals and provide mechanical support to the plants. Xylem parenchyma is the living, thin-walled cells that involved in water conduction and act as a storage tissue.

Phloem is a complex tissue of vascular plants that are conducting organic nutrients produced by photosynthesis. Xylem and phloem together form the vascular bundles. It is made up of 4 types of tissues such us sieve tube, companion cells, phloem fibres and phloem parenchyma. Sclereids and various kinds of secretory cells are also found in the phloem. In pteridophytes and many gymnosperms, phloem is composed of sieve cells and phloem parenchyma.

Sclerenchyma is a type of simple permanent tissue consisting of very thick hard and lignified walls with a very low percentage of water. More or less short isodiametric sclerenchyma cells are called sclereids or sclerotic cells. Due to the loss of the living protoplasm at maturity, these cells become dead and performing mechanical function. Generally, simple pits are present in the walls of sclereids. Sclereids are different types based on the difference in shape, size and nature of the cell wall. Osteosclereids, astrosclereids, Brachysclereids or stone cells and macrosclereids are the different types of sclereids.