A plant contains different types of tissues. These tissues perform their own functions in the parts where they are present. However, one or more tissues combinedly discharge a common function in different parts irrespective of their continuity or position. Such groups of tissues constitute a system called tissue system. For example, phloem and xylem are two different tissues with respect to their own structure and functions. But they combinedly discharge the function of conduction in all parts of the plant and, hence, they constitute the conducting tissue system or vascular tissue system.
So a tissue system can be defined as a group of tissues that perform the same general function irrespective of position or continuity in the plant body.
The term is entirely a physiological one and does not take into account the differences in the structure and arrangement of the cells.
(i) Epidermal Cells
Epidermal cells are generally rectangular and of varying sizes. These form a continuous layer and may be interrupted by stomata. Intercellular spaces are generally absent except in petals of flowers of Clarlcia and Linum. Epidermal cells are parenchymatous and living. In some members of family Gramineae, Poaceae and Cyperaceae, the epidermis contains two types of cells : long cells and short cells. The short cells are again of two types: cork cells and silica cells and they occur in pairs. In roots, the epidermis has specialised cells called trichoblast from which root hair arises.
A layer of fatty substance called cutin is found on the outer most walls of the epidermal cells in addition to the true wall layers. It is called cuticle. However, active growing regions do not contain cuticle. Sometimes, wax is also found deposited on the cuticle in form of granules. Carnauba wax is found on the leaves of Copernicia cerifera (wax palm) and is used for manufacturing of phonographs and polishes.
(ii) Bulliform or motor cells
These are thin walled larger cells than the typical epidermal cells commonly found in the members of family Poaceae and other monocotyledons. They contain large vacuoles and found mainly in the upper surface of the leaves.
They help in rolling and unrolling of the leaves due to changes in their size, depending on inflow and outflow of water from them. By this, they control the loss of water by the process of transpiration.