A Tissue is a group of cells that are alike in origin, structure and function. There are two principal groups: (1) Meristematic tissues and (2) Permanent tissues. Meristematic tissues comprise of self- perpetuating cells. Meristems are classified into several types on the basis of position, origin, function and activity. Many anatomists illustrated the root and shoot apical meristems on the basis of the type and arrangement and accordingly proposed many theories. The permanent tissues normally develop from apical meristem. They are classified into two types: 1)Simple permanent tissues and 2)Complex permanent tissues. Simple tissues are composed of a single type of cells only. It is of three types: (1) Parenchyma (2) Collenchyma and (3) Sclerenchyma. A complex tissue is a tissue with several types of cells but all of them function together as a single unit. It is of two types – xylem and phloem. Secretory tissues produce different types of chemicals. Some are in the form of enzymes, hormones, rubber, gum etc.
The tissues can be classified on the basis of their function, structure and location into epidermal tissue system, ground tissue system and vascular tissue system. Epidermal tissue system develops as the outermost covering of the entire plant body. It consists of epidermal cells and associated structures. All tissues except epidermis and vascular tissues constitute the ground tissue. The vascular tissue system is formed of vascular bundles.
In the primary structure, the outermost layer of the root is called piliferous layer. Cortex consists of only parenchyma cells. All the tissues present inside endodermis comprise the stele. In dicot (Example: bean) root, xylem is tetrach. Its phloem patch consists of sieve tubes, companion cells and phloem parenchyma. In monocot (Example: maize) root, xylem is polyarch.
In dicot (Example: sunflower) stem, stele is eustele type and its vascular bundles are wedge shaped, conjoint, collateral, open and endarch. In monocot stem (Example: maize) vascular bundles are scattered and skull shaped, conjoint, collateral, closed and endarch.
In dicot (Example: sunflower) and monocot (Example: grass) leaves vascular bundles are conjoint, collateral and closed.
Hydathodes discharge liquid water with various dissolved substances from the interior of the leaf to its surface. Plants that grow in salty environment are called halophiles. Salt glands typically are found in halophytes.