6. Plant tissues, 24 (1).pdf

HISTOLOGY Learning Outcomes At the end of this topic, students should be able to Define histology Explain the higher levels of body organization in plants and animals Explain the structure and functions of primary plant tissues Explain the role of plasmodesmata in intercellular communi...

HISTOLOGY Learning Outcomes At the end of this topic, students should be able to Define histology Explain the higher levels of body organization in plants and animals Explain the structure and functions of primary plant tissues Explain the role of plasmodesmata in intercellular communications INTRODUCTION Histology is the study of tissues. A tissue is a group of cells and associated intercellular substances that share a common origin and are specialized for a particular function/ functions. Study of tissue structure involves techniques of preserving, staining, sectioning, and viewing under a microscope. Among eukaryotes, multicellular animals and more advanced plants have specialized tissues. Higher levels of organization than tissues occur in plants and animals. Several tissues working together as a functional unit is called an organ. Organs form parts of even larger functional units called organ systems.  PLANT TISSUES Plant tissues are classified into Meristematic tissues that are capable of cell division found in the growing regions of the plant body, and Permanent tissues that are mature, differentiated tissues incapable of cell division. The cells of permanent tissues are modified to perform specific plant functions. They are derived from the meristematic tissue. The permanent plant tissues can be classified as Simple tissue when the tissue is composed of a single type of cell such as Parenchyma, Collenchyma, Sclerenchyma. Complex tissue when the tissue is composed of more than one type of cells. Example Xylem and Phloem tissue. SHOOT SYSTEM ROOT SYSTEM Meristematic Tissue Permanent Tissue 1. Capable of cell division 1. Lost power of cell division 2. Undifferentiated Cells 2. Differentiated cells 3. Have not attained definite form and size 3. Have attained definite form and size 4. Thin layer of cytoplasm around vacuole (if 4. Dense and abundant cytoplasm living) 5. Always living 5. May be living or dead PERMANENT TISSUES PARENCHYMA TISSUE SIMPLE TISSUES I) PARENCHYMA Shaped roughly spherical to elongated. Are living cells, cell wall material is made of cellulose, pectin, or hemicellulose. Form packing tissue; provide support in herbaceous plants. Are metabolically active, exchange gases through air spaces between cells, store food, and transport materials. Found in the outer cortex of stem and root, the central pith of stem. MODIFIED PARENCHYMA TYPES a) Epidermis, b) Mesophyll tissue, c) Endodermis, d) Pericycle, e) Companion cells MODIFIED PARENCHYMA TYPES SECONDARY GROWTH IN STEM a) Epidermis- One cell thick layer that covers the whole of the primary plant body (root, stem, leaf). Cells in the epidermis are elongated & flattened. The function of the epidermis is to protect the plant from desiccation and infection. During secondary growth, the epidermis may be ruptured and replaced by a cork layer. Epidermal cells are living; the cell wall contains a covering of cutin (lipid), which forms a layer of cuticle within the cell and on the outer surface of the cell walls. Cutin reduces water loss and prevents the entry of pathogens.  The epidermis of leaves & stems may also have special cells called guard cells, with chloroplast and a pore between them called stoma. The turgidity of the guard cells adjusts the stoma’s size. Stomata help in gaseous exchange during respiration, photosynthesis, and water vapor loss during transpiration. stoma  Epidermal cells may have hairs called trichomes and glands which provide a first line of defense against insects, pathogens, and herbivores. In roots, the hairs increase surface area for water absorption. ROOT L.S. SHOWING ROOT EPIDERMIS Root epidermis root hairs CROSS SECTION OF LEAF LEAF C.S. SHOWING UPPER AND LOWER EPIDERMIS epidermis b) Mesophyll- It is packing tissues found between the two epidermal layers of leaves containing chloroplast. Such photosynthetic parenchyma is sometimes called chlorenchyma. It is roughly spherical, irregular, or column-shaped. The upper layer of column-shaped cells forms palisade parenchyma, and the lower layer of irregular-shaped cells containing fewer chloroplast is called Dicot Leaf c.s. spongy parenchyma. Most photosynthesis is carried out by palisade cells while intercellular air spaces between spongy cells allow for efficient gas exchange. c) Endodermis- Found around innermost layer of the cortex. They are elongated and flattened and contain, in addition to cellulose, deposits of suberin (a waxy substance) in the cell wall of root endodermis. It forms a selective barrier for the movement of water and mineral salts in roots. In stem, it may be one to several cells thick, contain starch grains, and form a starch sheath. The starch sheath helps in the geotropic (gravity) response of the stem. Endodermis is more conspicuous in roots than stems. d) Pericycle- roughly spherical to elongated. Found between the central vascular tissue (xylem and phloem) and endodermis. DICOT STEM C.S. SHOWING STARCH SHEATH STEM C.S. ROOT C.S. e) Companion cells- These are specialized parenchyma cells associated with sieve tubes of the phloem tissues. They have denser cytoplasm, a smaller vacuole than normal parenchyma cells. II)COLLENCHYMA They are elongated and polygonal with tapering ends. They are living tissue. It is found just below the epidermis in the outer region of the cortex. COLLENCHYMA It is a mechanical tissue that provides support. III)SCLERENCHYMA- Provide support and mechanical strength to the plant. They have two types of cells. a. Fibers- elongated and polygonal with tapering ends. They are dead cells; cell walls contain lignin in addition to cellulose, hemicellulose, and pectin. Deposition of extra cellulose is seen at the corners. Found in the outer region of the cortex, xylem, and phloem. Function is support. b. Sclereids- roughly spherical or irregular, dead cells found in the cortex, pith, phloem, shells, and stones of fruits. In both fibers and sclereids, structures called simple pits appear in the cell wall where there is no lignin deposit, and groups of plasmodesmata facilitate the intercellular exchange of materials.  SCLERENCHYMA C.S. SCLEREIDS SIMPLE PLANT TISSUES COMPLEX PLANT TISSUES The xylem and phloem constitute the plant's conducting tissue or vascular tissues. 1. Xylem Conducts water and mineral salts and provides support. It consists of 4 cell types. a. Xylem vessels Formed by the fusion of several cells end to end with no end walls. Each cell that makes up the vessel is called a vessel element. They are heavily lignified to withstand the pressure of carrying water. Dead and hollow, so minimum resistance for water flow. Perforated with holes where there used to be plasmodesmata, for sideways/radial transport. Function- Transport of water and mineral ions dissolved in water. b. Tracheids – single cells with tapering ends that overlap with adjacent tracheids. Heavily lignified, dead, with empty lumen. Water only passes sideways through the pits found on its surface via the pit membranes made of thin middle lamella and primary wall. c. Fibers – Similar to tracheids but shorter & narrower than tracheids. has pits on its walls. heavily lignified, only for support. d. Xylem parenchyma – Carry out normal metabolic activity, packing tissue. Involved in radial transport. Alive, with cellulose cell walls. May store starch. 2. Phloem Consists of 4 cell typesa. a. Sieve tube element – alive, but with only a thin cytoplasmic strand along the sides with no Golgi apparatus, nucleus, or ribosomes. Stacked on end to end, end walls are modified to form sieve tube plates with pores Transport of organic solutes such as sucrose, amino acids. b. Companion cells – Usually, one per sieve tube element. It is a fully functional cell with lots of mitochondria. Involved in translocation by unloading/loading sucrose into the sieve tube element and for metabolic support. Linked to the sieve tube element by plasmodesmata. c. Phloem fiber – cylindrical cells with tapering ends with heavily lignified walls. d. Phloem parenchyma – Live, metabolically active cells. PLASMODESMATA They are cell junctions found connecting adjacent plant cells. They are channels 20-40nm wide that pass through adjacent cell walls connecting the cytoplasm of neighboring cells. Plasma membranes of adjacent cells are in communication with each other, and they also contain cylindrical membranous structures called desmotubules, which run through the opening and connect the ER of the adjacent cells. Allows for molecules, proteins & ions to pass adjacent plant cells.  Celery C.S. stained Collenchyma Epidermis Collenchyma Phloem Phloem Xylem Xylem Sclereids in pear Stomata in Moses in the Cradle plant lower epidermis stomata Sclereids