Plant Structure and Function Chapter 21 Plant Structure and Function

Specialized Tissues in Plants Seed Plant Structure (3 principal organs) Roots Absorbs water and dissolved nutrients Anchors plants in ground Stems Support system, transport system, and defense system Leaves Main photosynthetic systems

Plant Tissue Systems Plants consist of three main tissue systems Dermal: “skin” that protects against water loss and injury Vascular: “bloodstream” that transports water and nutrients throughout the plant Ground: everything else

Dermal Tissue Consists of a single layer of epidermal cells Often covered with a thick waxy layer (cuticle) Guard cells: on underside of leaves regulate water loss and gas exchange

Vascular Tissue Xylem Phloem Tracheid: long, narrow cells with walls that are impermeable to water Vessel element: arranged end to end; cell walls at both ends lost when cell dies Phloem Sieve elements: arranged like vessel elements and ends have many small holes Companion cells: cells that surround sieve element and aid in movement of substances in and out of cell

Ground Tissue Composed of cells that lie between dermal and vascular tissues Parenchyma: thin cell walls and large central vacuoles: in leaves they are packed with chlorophyll Collenchyma: strong, flexible cell walls that help support larger plants Sclerenchyma: extremely thick, rigid cell walls

Plant Growth and Meristematic Tissue Plant growth occurs at meristems that are responsible for continuing growth throughout the plant’s life Meristematic tissue: undifferentiated (not yet become specialized), only plant tissue that produces new cells by mitosis Apical meristem: produce increased length at stems and roots Differentiation: development into specialized structures and functions

Roots Types of Roots: Taproots: primary root grows long and thick to reach water far below Earth’s surface Fibrous roots: branch to such an extent that no single root grows larger than the rest

Root Structure and Growth A mature root has an outside layer, the epidermis, and a central cylinder of vascular tissue Root hairs: produce large surface area through which water can enter the plant Vascular cylinder: phloem and xylem Root cap: protects root as it forces its way through the soil (Fig 23.7)

Root Functions Uptake of Plant Nutrients Active Transport of Nutrients Composition of soil determines plants present Active Transport of Nutrients Use ATP to pump mineral ions from the soil into the plant Movement into the Vascular Cylinder Osmosis and active transport cause water and minerals to move form epidermis to cortex Root Pressure Pressure allows for upward movement of water

Stems Stem Structure and Function: they produce leaves, branches, and flowers; hold leaves up to the sunlight: and transport substances between roots and leaves Nodes: where leaves attach Internodes: regions between the nodes Buds: undeveloped tissue that can produce new stems and leaves

Monocot and Dicot Systems Monocots Vascular bundle scattered throughout the cell Phloem faces outside of cell and xylem faces the center Dicots Vascular bundles arranged in a cylinder Pith: parenchyma cells inside ring Cortex: parenchyma cells outside of ring

Primary Growth of Stems Refers to growth occurring at ends of a plant Produced by cell divisions in the apical meristem. It takes place in all seed plants

Secondary Growth of Stems Method in which stems grow in width In conifers and dicots, secondary growth takes place in lateral meristematic tissues called the vascular and cork cambium Vascular cambium: produces vascular tissues and increases thickness of stems Cork cambium: produces outer coverings of stems

Formation of Stems Formation of Vascular Cambium: Formation of Wood: Once secondary growth begins, vascular cambium appears as thin layer between clusters of vascular tissue Formation of Wood: Heartwood: no longer conducts water (dark) Sapwood: actively conducts water (light) Formation of Bark: All of the tissues outside the vascular cambium

Leaves Leaf Structure: optimized for absorbing light and carrying out photosynthesis Blades: flattened sections that absorb sunlight Petiole: thin stalk that attaches blade to stem

Leaf Function Photosynthesis: occurs in the mesophyll in most plants (Fig 23-18) Stomata: porelike openings on underside of leaf that allow CO2 and O2 to diffuse through Guard cells: control opening and closing of stomata Transpiration: loss of H2o through leaves

Leaf Function (cont.) Gas Exchange: leaves take in CO2 and release O2 Plants keep their stomata open just enough to allow photosynthesis to take place but not so much that they lose an excessive amount of water

Transport in Plants Water Transport: the combination of root pressure, capillary action, and transpiration provides enough force to move water through the xylem tissue Capillary action: tendency of water to rise in a thin tube; works by adhesion (attraction between unlike molecules)

Transport in Plants (cont.) Transpiration: major force in water transport to topmost branches and leaves Controlling transpiration: controlled by a series of feedback mechanisms Transpiration and wilting: high transpiration rates can lead to wilting

Nutrient Transport Functions of Phloem Movement From Source to Sink Carry out the seasonal movement of sugars within the plant Movement From Source to Sink Pressure-flow hypothesis: Source: where sugars are pumped into xylem Sink: region that utilizes the sugars When nutrients are pumped into or removed from the phloem system, the change in concentration causes a movement of fluid in the same direction