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NICHOLLS BIOL 156 - Plant Structure and Growth

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Plant Structure and GrowthPlant body divided into root andshootShoot consists of leaves, buds, flowers, and stemRoot consists of primary and secondary (lateral) rootsGrowth occurs at meristemsApical meristems of root and shoot allow for increase in length - primary growthlateral meristems allow for increase in girth (diameter) -secondary growthPlant growthPrimary growth occurs at root and shoot apical meristemsRoot apical meristem is covered by protective root capApical meristems give rise toprimary tissues:protoderm- gives rise to epidermisprocambium - gives rise to primary vascular tissue (primary xylem and phloem)ground meristem - gives rise to parenchymaLateral meristems produce secondarygrowth:cork cambium - produces cork cells of outer barkvascular cambium - produces secondary vascular tissue(secondary xylem and phloem)Plant tissue typesground tissue - thin walled parenchyma cells in interior of root andshoot - which function in storage, photosynthesis, and secretionoften capable of giving rise to other tissue typesdermal tissue - covers entire plant body (more detail later)vascular tissue -xylem - for conducting water and dissolved mineralsphloem - for conducting carbohydrates and other substances necessary for plant growthSecondary growth - only possible in woody plantsproduced by lateral meristemsvascular cambium produces secondary xylem to the interior of the stem and secondary phloem to theoutside of the stemcork cambium produces cork cells of bark as diameter increasesDermal tissueepidermal cells originate from protoderm - covered by waxy cuticlein green shoots, young roots, and leavesspecialized cells in the epidermis include guard cells and trichomesroot hairs are extensions of individual epidermal cellsGround tissueParenchyma - irregularly shaped thin walled living cells that fill muchof the interior of a root, shoot, or fruitCollenchyma - elongate living cells with thickened walls providesupport for plant organs like leaves and stems without secondarygrowth - e.g. strings in celerySclerenchyma - usually nonliving cells - cell walls reinforced with ligninfibers - long slender cells grouped together into strands that provide support - e.g. linen fibers from flaxaddition of fibers to wood makes wood “hardwood”sclerids - irregular shaped cells found in groups and serve to strengthen and protect - e.g. grit of pearsVascular tissueXylem : tracheids and vessel elements - both dead cells, lackingcytoplasm, attached end to end, for water and mineral transporttracheids are found in all vascular plants, vessels are onlycommon in angiospermsvessels are more efficient conductorsVascular tissuePhloem: consists of sieve tubes and companion cells, for transport ofcarbohydrates from areas of photosynthesisSieve tubes have cytoplasm, and cell membrane, but lack a nucleus,and are maintained by companion cells (modified parenchyma cells)Sieve tube elements connected through sieve plateRoot structure:Developing roots have four regionsRoot cap - parenchyma cells thatcover and protect tip of young root-secretes mucilaginous substance, alsofunctions to perceive gravityZone of cell division - root apicalmeristem - dome of cells at center ofroot tip can divide every 12 to 36hours, gives rise to protoderm,procambium and ground meristemZone of elongation - Cells become longer than wide by expansionof vacuoles, no further increase in cell size afterwardsZone of maturation - cells differentiate into specific cell typesRoot structure:epidermal cells can develop root hairs as outgrowthscortex - parenchyma cells that can function in food storageendodermis - single layer of cells with cell walls impregnated with suberin - prevents water and mineral passage between cells - suberin band around cells is the “Casparian strip”Stele - all cells interior to the endodermisPericycle - parenchyma just interior to the endodermis - can producelateral roots and the root vascular cambiumDicot roots have central core of primary xylem, arms radiatetoward pericycle and phloem between armsMonocot xylem forms ring of vascular bundles surroundingcentral cylinder of pithDicotMonocotRoot modificationsAdventitious roots - arise along stem or locations other than baseof plantAerial roots - roots that extend out into the air, unconnected to thegroundProp roots - located on lower part of stem of some monocots likecorn, grow down into ground, anchor against windPneumatophores - spongy outgrowths from underwater rootsMay extend above water, increase oxygen supply to rootsParasitic roots - penetrate host plants to parasitize themFood storage roots - branch roots of plants like sweet potatoesproduce extra parenchyma cells for carbohydrate storageButtress roots - produced by certain varieties of fig and tropicaltrees for supportStem structure:Node - the region of leaf attachmentInternode - stem area between nodesLeaves project from stem Blade: Flattened portionPetiole: Slender stalk supporting bladeAxil - angle formed by leaf attachment to the stemAxillary bud is produced in each axilAxillary buds may develop into branchesTerminal bud - produced at tip of stemTerminal buds extend length of stemDicot stems have vascular bundles arranged in ring around a coreof pith - with primary xylem to the inside and primary phloem tothe outside - vascular cambium is found between xylem andphloemMonocot stems have vascular bundles scattered throughoutground tissue - have no vascular cambium - so secondary growthis not possibleSecondary growthVascular cambium produces secondary xylem to inside and secondary phloem to outsidesecondary xylem is wood and is added in sequential layersphloem is thin-walled and is older layers are crushed as new layers are produced and stem increases in girthCork cambium arises in cortex and produces new cork to outsideModified stemsLeaf structureLeaf cellular structureUpper and lower epidermis covered by waxy cuticleEpidermal cells are nonphotosynthetic except for guard cells that surround stomataInterior cells are called mesophyll cells and are photosyntheticpalisade mesophyll tightly packed under upper epidermisspongy mesophyll composed of loose cells above lower epidermisVascular bundles or veins project from the stem into the leaves with the xylem above the phloemStomate and Guard Cells:Two guard cells surround the stomacell wall thickened along stoma border - allows opening and closing of stoma with change in cell volumeModified leavesFloral leaves (bracts) - brightly colored leaves that surround


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NICHOLLS BIOL 156 - Plant Structure and Growth

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