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UWL BIO 203 - Plant Structure and Secondary Growth
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BIO 203 1st Edition Lecture 18 Outline of Last Lecture I. Pros and cons of growing biggerII. Plant bodiesIII. Turgor pressureIV. Sclerenchyma cellsV. How do trees hold themselves up when they grow sideways?VI. Three tissue systemsVII. Bud primordia and axillary budsOutline of Current Lecture I. What is secondary growth?II. Which plants do and don’t do true secondary growth?III. Axial and radial systemsIV. Secondary growthV. Rays, periderms and barkVI. Gymnosperm woodVII. Anthophyte woodVIII. Annual RingsCurrent LectureI. What is secondary growth?a. Woody growth: durable tissues with chemical composition that make them rigid and resistant to decayb. Increase in girth as opposed to length (elongation = primary growth)c. Accomplished through the activities of two secondary or lateral meristemsII. Which plants do and don’t do true secondary growth?a. Do:i. Herbaceous plantsii. Tree ferns and palmsiii. Lack the ability to produce true wood, which is formed in distinctive concentric layersb. Don’t:i. All the seed plants except monocotsii. Ginkgo, cycads, Gnetales, Pinaceae and Cupressophytesiii. Shrubs, trees, and woody vine Anthophyta of all sortsThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.III. Axial and Radial systemsa. Secondary growth produces cells running in two directionsb. Axial system: vertically-running cells (majority of the conducting cells)i. Big vertical bunch of strawsc. Radial system: horizontally-running cells i. A few straws stuck horizontally through the big bunchIV. Secondary Growtha. Requires two lateral, secondary meristems: vascular cambium and cork cambiumb. Vascular Cambiumi. A cylinder of meristematic tissue that runs through the original vascular bundlesii. Develops in a stem that has reached a certain ageiii. Just like in the original vascular bundles, xylem is produced towards center, phloem towards edgeiv. Accumulation of xylem pushes phloem outward, destroying the cells (quite thin-walled)v. Only thick-walled 10 phloem fibers remain intactV. Rays, periderms and barka. Raysi. Rays within wood provide horizontal transport and storageii. Living cellsiii. Phloem rays are triangular – filling in the spaces created as circumference increases iv. Xylem rays are narrowerb. Peridermsi. Kill off older phloemii. Periderms are produced by a second lateral meristem, the cork cambium, which we will not focus onc. Barki. Living and dead secondary phloemii. Outer bark = all dead phloem cellsiii. Inner bark – living phloem1. Removal of inner bark all the way around a tree (= girdling) will killit VI. Gymnosperm Wooda. Relatively simpleb. Xylem has one type of conducting cells: tracheidsi. Narrowii. Dead at functional maturityc. Xylem also has resin ductsi. Tubes lined with living cells that secrete resin (“sap” or “pitch”) into ductii. Protects from insects and decayd. Ray cells also present: living cells, run radially, primarily for storagee. Cross sectioni. Cut at right angle to the long axis (like chopping a tree down)ii. Tracheids appear squarish iii. Resin ductsiv. Rays are lines of thin-walled cellsv. Only one cell wide in gymnospermsf. Longitudinal sectioni. Radial section is a cut along a radius of the stemii. Through the midline, parallel to raysiii. Rays appear as horizontal sets of cells, tracheids run verticallyVII. Anthophyte wooda. Two types of conducting cells:i. Tracheids – similar to those in conifersii. Vessel elements – thick secondary walls; larger diameter; dead when functionalb. Fiber cells i. “Hardwoods” have lots of fibersc. Rays are taller and wider than in gymnospermsVIII. Annual Ringsa. Visible rings mark the end of a year’s growth in species in seasonal environmentsb. Result from differences in cell size throughout the yearc. Early wood (spring)i. Larger cells, thinner wallsd. Late wood (fall)i. Smaller cells, thicker wallse. The width of each year’s growth ring also creates a record of the growth conditions that tree experiencedf. If you know how a given species responds to climatic variation (precipitation? temperature?) the tree rings can tell you about past


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