Form 4 Biology Chapter 2 (b) Cell OrganisationCell Organization in Plants1. There are two main types of tissues in plants: meristematic tissues and permanent tissues.2. Meristematic tissues consist of small cells which have thin walls, large nuclei, dense cytoplasm and no vacuoles.3. Meristematic tissue comprises of young, actively dividing cells which have not undergone differentiation.4. The main plant meristems are at the apex of shoot and the tip of the root.5. Permanent tissues comprise of more mature cells that are either undergoing or have undergone differentiation, and have lost the ability to divide.6. There are three types of permanent tissues:a) Epidermal tissuesb) Ground tissuesc) Vascular tissues

A) Epidermal tissue1. Epidermal tissue is the outermost layer that covers the stem, leaves, and roots of young plants. It forms a protective layer to protect the inner tissue.2. The epidermis is a layer of elongated, flattened cells, one-cell thick, which have large vacuoles, and covers the whole of the primary plant body.3. The cells secrete cutin which forms a layer of waxy cuticle on the outer surface of the epidermis. The waxy cuticle reduces water loss through evaporation from the plant, and also prevents the entry of pathogens.4. The epidermis of leaves have specialized cells which contain chloroplasts and are distributed among epidermal cells.5. These are pairs of guard cells which encloses a stoma.6. The epidermis is transparent and without chloroplasts, it allows light to reach the mesophyll layers of leaves for photosynthesis.7. Stomatal pores in the epidermis allow gaseous exchange.8. Root epidermal cells have long projections called root hairs to increases the surface area for water absorption.

B) Ground tissueParenchyma tissue

1. Parenchyma is the most common plant tissue that forms the bulk of the packing tissue within plants.2. Parenchyma cells are living cells, usually spherical and thin-walled, which have one large vacuole with the cytoplasm pushed to the cells periphery.3. Parenchyma cells have thin primary cell walls containing cellulose, hemicellulose and pectin. The walls are permeable to water, and permit the passage of solutes.4. Parenchyma cells are arranged loosely with many intercellular air spaces to allow exchange of gases to take place between the plant tissues and the external environment.5. The most important function of the parenchyma cells of roots and stems is the storage of food and water.6. When tightly packed and turgid, parenchyma cells give support and shape to herbaceous plants.7. The parenchyma tissues can differentiate to form specialized cells which carry out specific functions.8. The cells include the mesophyll, endodermis, pericycle and secretory cells.

Collenchyma tissue

1. Collenchyma is a simple tissue with loving cells.2. Collenchyma cells are polygonal, elongated and have unevenly thickened cell walls.

3. The thickened areas are usually at the corner of the cell walls.4. Thickening materials include cellulose and pectin.5. Air spaces between collenchyma cells are very small or non-existent.6. Collenchyma cells are not lignified.7. Collenchyma tissue provides herbaceous plants with mechanical strength and flexibility.8. It allows the cells to expand and be stretched in young plants and in non-woody plants for growth.9. Collenchyma tissue is usually found in herbaceous plants, located below the epidermis of the leaves.10. Some of the collenchyma cells contain chloroplasts which carry out photosynthesis.

Sclerenchyma tissue1. The main functions of sclerenchyma cells are for supporting and strengthening plant parts. Some sclerenchyma tissues are also protective in nature.2. Sclerenchyma cells usually die after maturity, have secondary cell walls that are thickened uniformly and are lignified.3. The thickened cell walls are very hard and impermeable to water, solute and gases.4. Sclerenchyma fibres are found below the epidermis of the stems or roots, usually bundles5. It acts as supporting tissue. Collectively, the tightly packed sclerenchyma fibres with thick lignified walls provide the plant with mechanical strength and rigidity

C) Vascular TissueXylem1. Xylem consists of tracheids and xylem vessels. These are long tubes joined together end to end from the roots right up to the shoots.2. The cell walls of the xylem are thickened with lignin which prevents food substances from entering the cells. Consequently, the xylem tissue dies upon reaching maturity.3. When the cytoplasm disintegrates, a hollow tube is left behind. This feature allows water and mineral to flow easily through the xylem vessels.4. Xylem conducts water and minerals from the roots to the leaves.5. It provides support and mechanical strength to the plant.

Phloem1. Phloem tissue consists of parenchyma cells, sieve tubes and companion cells.2. The conducting cells of phloem are the sieve tubes which have pores at both ends, called sieve plates.3. Sieve tubes obtain nutrients and energy from the adjacent companion cells4. Phloem transports organic substances such as carbohydrates and amino acids from the leaves to storage organ and to growing parts of plants.

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