Two or more tissues together constitute the tissue system. The activities of the constituent tissues are responsible for the major function, although they are different in structure and origin. Sachs (1875) identified three types of tissue systems in plants.
- Epidermal Tissue System
- Ground Tissue System
- Vascular Tissue System
Epidermal Tissue System
The outer layer of cells covering the entire plant surface at young stages constitutes the epidermal tissue system. Epidermis is the outer protective covering of the primary plant body. The epidermal cell is isodiametric and compactly arranged without intercellular spaces. The cell wall is chiefly composed of cellulose, pectin and a waxy substance called 'cutin'. The outer and radial walls are more thickened and cutinised. The cuticle present on the outer surface prevents transpiration.
Epidermal cells are living and contain vacuolated protoplasts. Each epidermal cell possesses a single nucleus. The protoplasts are colourless. In monocots like grass the upper leaf epidermis shows certain enlarged colourless cells, which are filled with water. These are known as 'bulliform cells or motor cells'.
The structures present in the epidermis are:
- Stomata are minute pores present in the epidermis of land plants. Each stoma is protected by kidney shaped cells called the guard cells. The surrounding epidermal cells show variable sizes and arrangements and are referred to as subsidiary cells. Stomata are more profound in the leaf epidermis. In leaves the stomata may be present in the upper or lower epidermis. They are absent in the root and mature stems. The guard cells are thicker on the inner side and thinner on the outer side. When the guard cells absorb water the inner thicker region bends in a concave manner and causes the widening of the stomata. When they lose water the inner thick region comes closer and cause the closure of the stomata. Stomata help in the exchange of gases and perform transpiration.
- Trichomes are outgrowths present on the epidermis of many plants. They are identified into two types namely:
- Epidermal hairs
Scales are also called as pellate hairs. They can be stalked or sessile. Scales give protection from insects and pests and also help in retaining moisture.
Functions of Epidermis
- It gives protection to inner tissues of the plant.
- It prevents evaporation of water through transpiration.
- It protects the plant from injuries and pathogens.
- It helps in absorption, assimilation and gaseous exchange.
Ground Tissue System
Except the epidermis and vascular tissues, all other tissues constitute the ground tissue system. It occupies the major portion of the plant body. It originates from the ground meristem. It can be differentiated into two parts:
- Extra-stelar Ground Tissue System
- Intra-stelar Ground Tissue System
- Extra-stelar Ground Tissue System is extended between the epidermis and stele. It is generally known as cortex. It includes three zones called
- General cortex
- Medulla or PithThe central part of the stele surrounded by the vascular tissues constitutes the medulla. It is composed of thin walled parenchyma with intercellular spaces. Pith cells help in the storage of food materials and water.
- Medullary RaysThe ground tissue extending radially between the vascular bundles is called medullary ray. The medullary rays arise from primary meristems. Hence they are called as primary medullary rays.
Vascular Tissue System
Xylem and pholem constitute the vascular bundle. They are concerned with the transport of water, mineral salts and food materials. The vascular system originates from the procambium. The pholem elements differentiate from the periphery towards the centre. The first formed elements are called protophloem and the later are called metaphloem. Based on the arrangements of the xylem and phloem the vascular bundle is classified into four types.
- Collateral Vascular Bundle: The xylem and phloem are present side by side on a common radius. They are found in the stems of Gymnosperms and Angiosperms. They are described as open if the cambium is present between the xylem and phloem. They are described as closed if the cambium is absent between the xylem and phloem.
- Bicollateral Vascular Bundle: In these, the phloem is present on either side of the xylem, separated by the cambium. The vascular bundle shows outer phloem, outer cambium, xylem, inner cambium and inner phloem.
- Radial Vascular Bundle: In this type the xylem and phloem are alternating one another and are arranged on the separate radii e.g. roots.