34 The Plant Body • 34.1 How Is the Plant Body Organized? • 34.2 How Are Plant Cells Unique? • 34.3 How Do Meristems Build the Plant Body? • 34.4 How Does Leaf Anatomy Support Photosynthesis?
34.1 RECAP: • Monocots & Dicots • Basic body plan: root system & shoot system • Stems & leaves = part of shoot system • Bud = embryonic shoot • 3 tissue systems: vascular, dermal, ground
97% of angiosperms are in 2 clades: • Monocots: narrow-leaved; grasses, lilies, orchids, palms • Eudicots: broad-leaved; soybeans, roses, sunflowers, maples Remaining species have structure similar to eudicots: water lilies and magnoliids.
What’s an angiosperm? vascular plants with: double fertilization, a triploid endosperm, seeds enclosed in modified leaves called carpels. Three types of vegetative organs: roots, stems, leaves. They are organized in two systems: shoot system and root system.
Shoot system: stems, leaves, flowers (Leaves...photosynthesis) Stems hold and display leaves in the sun; connection between roots and leaves. Nodes:points of attachment of leaf to stem. Regions in between are internodes.
Root system: anchors plant & provides nutrition. Extreme branching of roots provides large surface area for absorption of water and mineral nutrients.
Morphology vs. Anatomy? Structure of vegetative organs, includes overall form. Arrangement of component cells and tissues.
2 Types of Root Systems Taproot systems: single, large, deep-growing root and small side roots. The root may also function as food storage. Fibrous root systems: many thin roots of equal diameter. Have large surface area; cling to soil well.
Adventitious roots: arise above ground from stem or leaves. Cutting: piece of shoot cut or broken from the plant. Adventitious roots may arise from cuttings, and can start a new plant—vegetative reproduction (asexual). Adventitious roots also help support many plants, such as corn.
Stems & Buds (Axillary, Apical) elevate and support flowers and leaves; have buds or embryonic shoots. Stems bear leaves at the nodes, buds in the angle or axil between shoot and leaf are axillary buds. Can develop into a branch. Apical buds form at the tips—produce cells for upward growth; some may develop into flowers.
Stems can be modified: Potato tuber is an underground stem. Many desert plants have enlarged stems that store water. Runners are horizontal stems, roots grow at intervals and independent plants can arise from them.
Leaves: blade, petiole Most photosynthesis occurs here. The blade is a thin, flat organ attached to stem by the petiole. Angle may be perpendicular to sun’s rays to provide maximum area for light gathering. Some leaves change position during the day to track the sun.
Simple vs Compound Leaves Simple leaves consist of a single blade. Compound leaves have multiple blades or leaflets.
Leaves can be modified: Some function as food storage, (e.g., onion bulbs). Leaves of succulents store water. Cacti spines are modified leaves. Climbing plants have modified leaves called tendrils that wrap around other structures.
What’s a Tissue? organized group of cells with similar structure and function. Vascular, dermal, and ground tissue systems extend throughout the plant body in a concentric arrangement.
The vascular tissue system: the transport system Xylem distributes water and minerals taken up by roots to all parts of the plant. Xylem can also function in storage and support. Phloem transports carbohydrates from site of production (sources) to sites of utilization or storage (sinks).
The dermal tissue system: the outer covering The epidermis: single layer of cells or several layers. May contain specialized cells such as guard cells around stomata. Shoot and leaf epidermis secretes waxy cuticle that retards water loss. Stems and roots of woody plants have a periderm.
The ground tissue system makes up the rest of the plant. Functions in storage, support, photosynthesis, and production of defensive and attractive chemicals.
34.2 RECAP: • Several types of plant cells • Different structures & functions • Most only function when they’re alive • SOME are specialized for support or transport & don’t function until they’re dead
34.2 How Are Plant Cells Unique? Have chloroplasts or other plastids. Contain vacuoles. Have cellwalls with cellulose. Differ in composition depending on function of cell. Some plant cells function after death when cell contents have disintegrated.
How’s cytokinesis different? daughter cells are separated by the cell plate. A glue-like material is deposited in the cell plate—the middle lamella. Then the daughter cells secrete cellulose to form the primary wall.
Cell wall outside the plasma membrane. can contain proteins and enzymes. Chemical reactions --cell expansion, defense. Except where waterproofed, cell walls are permeable to water and mineral ions.
Plasmodesmata connect adjacent plant cells. Cytoplasm-filled canals, traversed by a strand of ER. Can sometimes expand to allow macromolecules, including transcription factors and RNA, and viruses, to pass. Doesn’t require crossing a plasma membrane.
Parenchyma cells: thin walls, large central vacuoles. Photosynthetic cells in leaves are parenchyma cells with many chloroplasts. Many store starch and lipids. Starch is often stored in leucoplasts. Some parenchyma cells function in support.
Collenchyma cells: thick primary cell walls, usually elongate shape. Support in leaf petioles, non-woody stems, and growing organs. Tissue with collenchyma cells is flexible.
Sclerenchyma cells: thickened secondary walls. Many die after secondary wall is laid down. Fibers: elongated cells provide rigid support; often in bundles. Sclereids may be densely packed as in nut shells, or in clumps as in stone cells in pears.
Xylem contains cells called tracheary elements—die before assuming their function. Gymnosperms have tracheids with pits in the secondary walls that allow materials to move freely.
Vessel Elements Angiosperms have vessel elements with pits. Larger diameter than tracheids; lignin in secondary cell walls; end walls break down after death, forming hollow tubes. Xylem of many angiosperms also contains tracheids.
Phloem transport cells — sieve tube elements, are living. Cells meet end-to-end; transport carbohydrates.
Plasmodesmata in end walls enlarge to form pores—form sieve plates. Membrane that encloses central vacuole (tonoplast) disappears; nucleus and some cytoplasmic components break down, thus pores in sieve plate are not clogged.
Sieve tube elements are filled with phloem sap— water, sugars, other solutes. Layer of cytoplasm stays at the periphery. Each sieve tube element has one or more companion cells linked by plasmodesmata. Retain all organelles, they are the life-support system.
34.3 RECAP: • Apical & lateral meristems • Responsible for producing primary & secondary plant bodies • primary meristems: protoderm, ground meristem, procambium • 2 lateral meristems: vascular cambium & cork cambium • Wood and bark arise from them
“Modules” Growing stem, laid down 1 after another. Each is a node w/ attached leaves, internode below, axillary buds at base of internode.
Each branch may be thought of as a separate module. Branches form 1 after another may differ in # of leaves, & # of subsequent branches. are long-lived.
Leaves are modules that are short-lived—weeks to a few years. Root systems are also branching. As root systems grow, roots may die and be replaced by new ones.
Plant Bodies All plants have a primary plant body: all non-woody parts of the plant. Growth lengthens the plant body. Monocots consist entirely of primary plant body!!! Trees & shrubs have secondary plant body: wood and bark. Tissues laid down as stems, roots thicken. Grows throughout life of plant.
Meristems are... localized regions of cell division. Can produce new cells indefinitely.