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NOTES: CH 35 - Plant Structure & Growth In their evolutionary journey, plants adapted to the problems of a terrestrial existence as they moved from water to land ANGIOSPERMS (flowering plants) -most diverse and widespread of plants (about 275,000 species!) -characterized by FLOWERS and FRUITS (adaptations for reproduction and seed dispersal) *Angiosperms are divided into two taxonomic classes: 1) MONOCOTS 2) DICOTS Terrestrial plants faced a divided habitat: air is the source of CO2 for photosynthesis (not dissolved in water) sunlight cannot penetrate soil soil provides water and dissolved minerals to plant To accommodate this divided habitat, plants have two basic systems: 1) ROOT SYSTEM (subterranean) 2) SHOOT SYSTEM (aerial) Roots and Shoots are interdependent!!! *Roots depend on shoots for sugar and other organic nutrients *Shoots depend on roots for minerals, water and support THE ROOT SYSTEM: Root structure is well adapted to: > Anchor plants > Absorb and conduct water and nutrients > Store food Two major types of root systems: 1) TAPROOT SYSTEM: -one large, vertical root -provides firm anchorage in soil -modified taproots: carrots, turnips, sweet potatoes 2) FIBROUS ROOT SYSTEM: -mat of threadlike roots spread out below soil -provides exposure to soil water and minerals -roots concentrated in upper few cm of soil -helps to prevent soil erosion *Absorption of water is greatly enhanced by ROOT HAIRS THE SHOOT SYSTEM: • Stem morphology includes: -axillary buds = embryonic side shoots in angle of leaf and stem; usually dormant -terminal bud = bud on a shoot tip • Growth of a shoot is concentrated at the apex of the shoot (TERMINAL BUD) **the terminal bud inhibits development of axillary buds – this is called APICAL DOMINANCE LEAVES *the main photosynthetic organs of a plant *leaves of monocots and dicots differ: -monocot leaves have parallel veins -dicot leaves have a branched network of veins PLANT CELL OVERVIEW: *Recall: in addition to the structures in other eukaryotic animal cells, PLANT CELLS have unique organelles: -CELL WALL -CHLOROPLASTS -LARGE, CENTRAL VACUOLE -TONOPLAST (membrane around vacuole) Typical Plant Cell Specialized Plant Cells: 1) Parenchyma cells: -least specialized of plant cells -function in synthesizing and storing organic products Specialized Plant Cells: 2) Collenchyma cells: -usually grouped in strands to support young parts of plants -elongate as the stems and leaves grow -e.g.: make up the strings in celery Specialized Plant Cells: 3) Sclerenchyma cells: -function in support -FIBERS: long, slender, tapered cells -SCLERIDS: shorter, irregularly-shaped cells Each organ of a plant (leaf, stem, root) has three tissue systems: 1) DERMAL 2) VASCULAR 3) GROUND • DERMAL TISSUE SYSTEM (epidermis) -covers and protects plant -root hairs are extensions of epidermal cells near root tips -waxy CUTICLE secreted by epidermal cells of the leaf and stem • VASCULAR TISSUE SYTEM -xylem and phloem -functions in transport and support; continuous throughout plant Specialized Plant Tissues: ● XYLEM: water-conducting vessels ● PHLOEM: sugar / sap-conducting vessels Specialized Plant Tissues (continued): Food-conducting cells of PHLOEM include: ● at least one COMPANION CELL is connected to each phloem segment by openings called PLASMODESMATA • GROUND TISSUE SYSTEM -predominantly parenchyma -occupies space between dermal and vascular tissues -functions in: photosynthesis, storage, support PLANT GROWTH! *Plant growth begins with germination of the seed and continues for the lifespan of the plants MERISTEMS: -perpetually embryonic tissues -unspecialized and constantly dividing Two types of plant growth: 1) PRIMARY GROWTH: elongation 2) SECONDARY GROWTH: thickening of roots and shoots -APICAL MERISTEMS: located in root tips and shoot buds; involved in primary growth -LATERAL MERISTEMS: cylinders of dividing cells extending along the lengths of roots and shoots; involved in secondary growth Primary Growth of Roots: • root growth is concentrated near its tip and results in roots extending through the soil • the root is covered by a ROOT CAP which protects the meristem and secretes a polysaccharide coating that lubricates the soil ahead of the growing root • the root tip contains 3 zones of cells: *ZONE OF CELL DIVISION: near tip of root; -includes apical meristem *ZONE OF CELL ELONGATION: above the zone of cell division -cells elongate to at least 10x their original length -elongation of cells here pushes the root tip through the soil *ZONE OF MATURATION: located farthest from the root tip -where new cells become specialized Primary Tissues of Roots: 1) PROTODERM: gives rise to epidermis Primary Tissues of Roots: 2) PROCAMBIUM: forms a STELE (central cylinder) where xylem and phloem develop *difference between monocots and dicots Primary Tissues of Roots: 3) GROUND MERISTEM: -gives rise to ground tissue system; -fills the CORTEX (root area between stele and epidermis); -stores food; -has ENDODERMIS (innermost layer of cortex; forms boundary between cortex and stele) *LATERAL ROOTS may sprout from the outermost layer of the stele (the PERICYCLE) as a result, the lateral root maintains its vascular connection to the stele of the main root TISSUE ORGANIZATION OF STEMS: • vascular tissue is organized into strands of VASCULAR BUNDLES that run the length of the stem; • these converge at the shoot root transition to join the root stele • each vascular bundle is surrounded by ground tissue DICOTS: -ground tissue includes PITH (inside) and CORTEX (outside) -bundles arranged in a RING with: XYLEM facing PITH PHLOEM facing CORTEX MONOCOTS: -vascular bundles scattered throughout the ground tissue of stem TISSUE ORGANIZATION OF LEAVES: • EPIDERMIS: protects against water loss and pathogens • STOMATA: pores flanked by GUARD CELLS which regulate gas exchange and allow for TRANSPIRATION (water loss) stomata are more numerous on the bottom surface of the leaf…WHY? MONOCOT = parallel veins DICOT = network veins TISSUE ORGANIZATION OF LEAVES: • the ground tissue inside the leaf is called: MESOPHYLL -consists mainly of parenchyma cells equipped to perform photosynthesis Mesophyll TISSUE ORGANIZATION OF LEAVES: *Dicots have two mesophyll regions: 1) Palisade parenchyma: upper half of leaf; cells are columnshaped 2) Spongy parenchyma: lower half of leaf; irregularly shaped cells surrounded by air spaces TISSUE ORGANIZATION OF LEAVES: **Vascular bundles from stem branch repeatedly throughout leaf blade forming VEINS this brings the photosynthetic tissue of the leaf in close contact with the xylem and phloem also forms a skeleton to support the shape of the leaf SECONDARY GROWTH: • results in thickening of stems / roots (seen in gymnosperms & some dicots) • results from two lateral meristems: 1) VASCULAR CAMBIUM: produces secondary xylem and phloem 2) CORK CAMBIUM: produces a tough, thick covering for roots and stems that replaces the epidermis VASCULAR CAMBIUM: • forms between the 1º xylem and phloem of each vascular bundle • a continuous cylinder of dividing cells: -2º xylem cells form towards the inside -2º phloem cells form towards the outside *accumulated layers of 2º xylem produces WOOD VASCULAR CAMBIUM: **forms annual growth rings due to yearly activity: 1) cambium dormancy (winter) 2) spring wood production 3) summer wood production *2º phloem does not accumulate extensively; it becomes part of bark and eventually sloughs off the tree trunk CORK CAMBIUM: • cylinder of meristematic tissue that forms in the outer cortex of the stem • cork cells form to the outside; as these cells mature, they deposit a waxy material (SUBERIN) in their walls and die • these dead cork tissues protect the stem from damage and pathogens and reduce water loss BARK = all tissues external to the vascular cambium phloem, cork cambium and cork Secondary Growth in Roots: -similar to stems -in roots, the outer layer is impermeable to water…consequently, the roots with 2º growth function to anchor the plant and to transport water and nutrients between the younger roots and the shoot system