PLANT TISSUES Biology Department PLANT TISSUES Tissues can

PLANT TISSUES Biology Department

PLANT TISSUES Ø Tissues can be classified into : § Meristematic tissues § Permanent tissues

I. MERISTEMATIC TISSUES They are usually called meristems. They are young tissues of the embryo or the mature plant which are responsible for its growth and development since their cells have the ability to divide. The cells are : Ø small Ø thin-walled Ø usually no central vacuole Ø no specialized features.

MERISTEMATIC TISSUES Ø Meristematic tissue is located in: Primary meristems: near tips of roots and stems. This is called apical meristems. Secondary meristems: in vascular cambium and the cork (cortical) cambium. Fig. Root tip

MERISTEMATIC TISSUE ROOT TIP Islamic University -Gaza Department Biology

II. PERMANENT TISSUES: Are derived from meristeims. They are known as mature tissues. During their development, mature tissues gradually change morphologically and physiologically and become specialized for specific function in the plant body. Such are termed tissue differentiation.

PERMANENT TISSUES: Tissues are further arranged to form tissue systems : Dermal tissue consists of epidermis which may later be replaced by periderm or exoderm. Ground tissue Contains of three types of tissues: parenchyma collenchyma sclerenchyma. Vascular tissue xylem and phloem.

II. A. 1 DERMAL TISSUES: Epidermis. Locations: in the outermost layer of the primary plant body covering leaves, 1. floral parts, fruits, seeds, stems and roots (until they undergo secondary growth). Characteristics: 1. These cells essentially tabular but differences in size 2. Cells are closely fitted together. 3. In surface view, the cells may be isodiametric or elongated. 4. The cell has a central vacuole and thin peripheral cytoplasm. Ø The epidermis in generally is only one cell layer thick and forms when protoderm cells derived from the apical meristems differentiate. Functions: are diverse including desiccation resistance, gas exchange, and protection against herbivores and pathogens.

Cell types of the epidermis Pavement cells fit tightly together and secrete a waterrepellent cuticle that reduces water loss and pathogen invasion. Guard cells form stomata, (pores for gas exchange). They are generally kidney shaped cells (in Dicot epidermal tissue )in surface view, rich in cytoplasm and with prominent nucleus and contain chloroplast. (in monocot epidermal tissue dample shape cells) The epidermis of leaves often contains trichomes, various types of hairs. Ø

Epidermis

CUTICLE

HAIRS

II. A. 2 GROUND TISSUES : FOUND BETWEEN EPIDERMIS AND VASCULAR TISSUE Ø Parenchyma- is the main ground tissue • Aerenchyma - Parenchyma tissue with extensive connected air spaces. • Chlorenchyma - Parenchyma cells full of chloroplasts. Locations: Ø • • • In the primary plant body they occur as continuous masses in : The cortex of roots & stems Piths of stems and roots, and leaf mesophyll. They may also occur as vertical strands of cells in vascular tissues and also as horizontal strands (rays) in secondary vascular tissues.

PARENCHYMA Characteristics: Ø living at maturity and may become meristematically active Ø large thin walled. They are generally polyhedral in shape. Tend to have large vacuoles and many contain various secretions. Ø Cell Wall: primary or primary and secondary( may be lignified, suberized or cutinized). Functions: performs several functions such as storage, respiration, photosynthesis, assimilation, those of xylem and phloem are connected with the conduction of food and water.

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2 -COLLENCHYMA : Locations: at the periphery of the primary stem, petiole and in the outer part of the cortex. Characteristics: 1. Composed of elongated collenchymal cells which are living at maturity. 2. They are similar to parenchymal cells except that they have much thicker cell walls. 3. The thickening of the walls may be in the angles where cells are joined together or on the tangential walls or on the walls around the intercellular spaces. Cell Wall: primary only, highly hemicellulosic and pectic, not lignified. Functions: they provide support, largely for the primary plant body.

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3. SCLERENCHYMA Ø It is a simple tissue formed of: Fibers- long slender cells which occur in vascular bundles (xylem fibers) and phloem, ground tissues epidermis or bundle sheath. § They provide support and some storage. § § § Sclereids- variable in shape, often branched, may occur singly or in groups in ground tissues throughout the plant. They make up the seed coats of seeds shells of nuts, stones of drupes, and give the pear its gritty texture. Their function is primarily for protection.

SCLERENCHYMA Characteristics: 1. Composed of thick- walled lignified sclerenchymal cells which are non-living and lack protoplasts at maturity. Cell Wall: Thick, lignified secondary cell walls. Functions: enables the plant to resist stresses of stretching, being, weight and pressure without damage of other thin- walled cells. Both collenchyma and sclerenchyma are mechanical tissues of support.

SCLERENCHYMA (SCLEREIDS) STONE CELLS IN PEARS Islamic University -Gaza Department Biology

SCLERENCHYMA (FIBERS)

II. A. 3 VASCULAR TISSUES: Ø Ø Ø § § include the xylem and phloem. They are presents in the vascular plants. Xylemis the principle of water conducting tissues in vascular plants. It may also act as a mechanical support to different plant organs.

PRIMARY XYLEM: Ø Ø is the xylem formed during primary growth from procambium. It includes protoxylem and metaxylem. Metaxylem develops after the protoxylem but before secondary xylem. It is distinguished by wider vessels and tracheids SECONDARY XYLEM: is the xylem formed during secondary growth from vascular cambium.

XYLEM : Xylem is a complex tissue formed from of: Ø Vascular elements: Tracheids- § are non- living elongated cell with tapering ends long and thin. § They occur in seedless vascular plants, gymenosperms and some primitive angiosperms. Vessels§ are more complex than tracheid. § The vessel is formed of a series of longitudinal expanded cells while the tracheid is unicellular. § Their wall is more thickened than tracheid. § Vessels in contrast to tracheids are perforated at points of contact with other vessels.

XYLEM : Ø Ø § § Xylem fibers: - are derived from tracheids by an increase in wall thickness, decrease in length Xylem parenchyma: are alive cells. Their walls may be thin or lignified.

2. PHLOEM: is the principle food conducting tissue of the vascular plants. It is a complex tissues formed from of: Ø Ø Sieve elements- there are two types of sieve elements: 1. Sieve cells – § are commonly long and slender with tapering ends. Have sieve areas on their walls. They occur in most seedless vascular plants and in gymnosperms. § § 2. Sieve tube members – § § have large pores on sieve plates, usually on end walls. Sieve tube members are stacked end to form sieve tubes. They are the sugar-conducting cells of the phloem in angiosperms. Sap flows between sieve tube members through sieve plate pores (modified plasmodesmata). .

Both sieve and sieve tube members do not contain a nucleus and possess thin cellulose cell walls Ø Ø Ø Companion cells- are highly specialized parenchyma cells (the companium cell retains its nucleus). They live only as long as the associated sieve tube member is living. Parenchyma cells and fibers- are associated with storage of food. Fibers and sclerids- are common in phloem.

Epiderms collenchyma Parenchyma Scelerenchyma Fibers Phloem Xylem