PLANT TISSUES
Non Permanent Tissues - Meristematic Tissue Undifferentiated plant cells that are continually dividing by mitosis Large thin walled cells No vacuole Dense cytoplasm Large nucleus Found at growing tips of roots and stems to increase length of these structures apical meristem Also as cambium which causes increase in width of stems and roots Function: to produce new cells for growth, repair and replacement of damaged or lost parts or organs
Meristems Apical meristems At the tips of main and lateral shoots and in tips of roots. Responsible for primary growth and length. Lateral meristems Parallel to the stem or the root e.g. vascular cambium and cork cambium. Responsible for increase in diameter (secondary growth).
Permanent tissues Mature tissues of the plant Derived from meristematic tissue by cell enlargement and differentiation Simple tissues: same cells that perform the same function (parenchyma, collenchyma, sclerenchyma) Complex tissues: different cells that perform more than 1 function (epidermis, xylem, phloem)
Permanent Tissues These tissues are grouped into three tissue systems 1. dermal tissue: covers the plant body 2. ground tissue: perform a variety of functions and make up the bulk of the plant body 3. vascular tissue: responsible for transport in the plant
Vascular tissue Conducts liquids around the plant Xylem transports water and inorganic salts (strength and support) Phloem conducts food (sugar and organic solutes)
Four types of tissues: 1. tracheids 2. vessels 3. xylem fibres 4. xylem parenchyma Tracheids are elongated dead cells with tapered ends (not continuous) Cell walls are thickened with lignin causing the protoplasm to die leaving a lumen Water is transported from tracheid to the next through pits in the walls Xylem
Xylem Vessels are elongated tubes that extend sometimes meters in length (dead cells) Walls are lignified in different ways (strength for woody tissues) Annular rings Spiral or helical Pitted Scalariform Reticulate Long, thin, perforated, hollow tubes stacked on top of each other This allows water to flow up stems to leaves as ascending sap good water conduction Function: transport water and inorganic salts from the roots to the leaves
Xylem vessels Annular thickening: 2dry wall is laid over the primary wall in rings. Allows for stretching. Spiral/ helical thickening: 2dry wall laid down in the form of spirals. Allows vessels to stretch or bend. Pitted thickening: 2dry wall laid down everywhere except small areas that form pits. Pits allow movement of water between cells. Scalariform thickening: 2dry wall covers the entire primary wall except for unthickened parts which are left in parallel lines (looks like a ladder). Reticulate thickening: Like scalariform except unthickened parts look like a net.
Xylem Xylem fibres are dead cells with heavily thickened cell walls. They do not conduct water but strengthen and support the plant. Xylem Parenchyma is the only living tissue in mature xylem. Walls are thicker than ordinary parenchyma cells. Store starch and water and can conduct solutes for short distances.
Phloem tissue Phloem fibres and cells are found in vascular bundles in roots, stems and leaves Phloem transports nutrients made in the leaves to all parts of the plant This is known as descending sap 4 tissues: Sieve tube Companion cells Phloem fibres Phloem parenchyma
Consists of elongated cells called sieve tubes The walls of the cells are thickened with lignin Pores/ perforations in the walls form a sieve-like structure Mature sieve tubes lack nuclei and have thick slime in the vacuole The conducting elements of phloem Thin cellulose walls lined with layer of cytoplasm. Few organelles and no nuclei. Phloem tissue Mass of tubules in the centre called P-protein Cross walls between sieve cells are perforated and are called sieve plates and sieve pores, respectively
Phloem Each sieve tube has a companion cells. Smaller than sieve cells and connected to them by plasmodesmata The companion cells contain nuclei, which controls the functions of the sieve tubes Phloem fibres are strengthening tissue Cell walls are lignified No water conduction Can form protective cap Phloem parenchyma are living cells that store starch
Dermal Tissue Epidermis is the outermost layer of the plant Protection from drying out and mechanical injury These cells are: square shaped (bricks) closely packed with no air spaces Transparent (lack chlorophyll) allow sunlight to penetrate inner tissues for photosynthesis Usually only a single layer of cells Thicker outer cell walls for mechanical protection
Modified Epidermis Stem and leaf epidermal cells are modified to secrete a waxy substance called cuticle The waxy cuticle reduces the amount of water lost by the aerial parts of the plant
Modified Epidermis Guard cells are bean-shaped cells with a thick inner wall and thin outer wall They also contain chloroplasts allowing them to regulate the opening and closing of the stoma This allows substances like water vapour, CO ² and O ² to move in and out of leaves
Modified Epidermis Epidermis is different in roots No cuticle as roots need to absorb water No guard cells and stomata Cells have microscopic extensions called root hairs
Modified Epidermis Root hair cells have small outgrowths of the cell wall increases the absorptive surface area The vacuole extends into the root hair to absorb more water Small enough to fit between soil particles
Epidermal tissues
Summary of epidermal functions Protects plants against injury and desiccation Cuticle prevents water loss Guard cells regulate gas exchange and transpiration Root hairs absorb water and mineral salts from the soil
Parenchyma Most abundant plant tissue: Large thin walled living cells Centrally situated nucleus Large vacuole Many plastids i.e. chloroplasts and leucoplasts Parenchyma cells containing chloroplasts are referred to as chlorenchyma Large intercellular air spaces between cells which allows gases to circulate Found in roots, stems and leaves Main function: storage and photosynthesis, gas movement, packing tissue
Types of parenchyma Spongy parenchyma: Large round cells Loosely packed with many airspaces Chloroplasts and leucoplasts Found in roots, stems and leaves Store substances like starch and water in fleshy organs
Types of parenchyma Palisade parenchyma Elongated cells arranged vertically contain many chloroplast Cytoplasm streams to ensure all chloroplasts receive maximum sunlight Main photosynthesizing tissue in leaves Aerenchyma Large air spaces in plants growing in water logged soils
Functions of parenchyma Packing tissue Stores water and nutrients Air spaces allows for water movement, photosynthesis and respiration
Spherical cells with cell walls thickened in the corners Long living cells Larger than parenchyma cells Contain chloroplasts Usually found in young growing stems Support and strength but allows stretching To prevent collapse of vascular tissue Collenchyma
Sclerenchyma Cells with very thick cell walls Thickening due to lignin and sometimes suberin This causes the contents of the cells to die off leaving a hollow cavity or lumen Occur as stone cells or fibres Provide support and protection around vascular bundles, fruit, seeds and pips, twine, rope, hemp, linen and other textiles
Sclerenchyma Fibres Elongated cells with pointed tips Thick cells walls Mostly found in vascular tissue Tough, strong and flexible Occur in groups with interlocking ends Used to make various textiles and linen Stone cells/ sclereids Do not interlock Lignified cell walls Provide rigidity and hardness