Chapter 24 Notes
Chapter 24 Notes BIOL 3040
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This 4 page Class Notes was uploaded by Min-Young Kim on Thursday March 10, 2016. The Class Notes belongs to BIOL 3040 at Clemson University taught by Christina Wells in Spring 2016. Since its upload, it has received 16 views. For similar materials see Biology of Plants in Biology at Clemson University.
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Date Created: 03/10/16
Chapter 24 Notes – The Root: Structure and Development -‐ First structure to emerge from germinating seed is root • Anchorage and absorption • Storage and conduction • Foods manufactured aboveground in the photosynthesizing portions of the plant move down through the phloem to the storage tissues of the root • In biennial plants, large food reserves accumulate in the storage regions of the root during the first year. Reserves are used during the second year to produce flowers, fruits, seeds • Water and minerals move through xylem to aerial parts • Hormones synthesized in meristematic regions of roots transported upward in xylem to aerial parts (stimulate growth and development) • Clonal regeneration: roots of certain eudicots produce buds that can develop into new shoots -‐ Root Systems o Primary root: first root of plant that originates in embryo o Taproot: primary root in all seed plants except monocots; grows directly downward, giving rise to lateral roots o Taproot system: root system formed from strongly developed primary root and its branches o In monocots: main root system develops from adventitious roots (arise from stem) and lateral roots give rise to fibrous root system o Developmental plasticity of root system is its response to uneven distribution of nitrogen and inorganic phosphate o Taproot systems generally penetrate deeper than fibrous root systems o The bulk of most fine roots, or feeder roots, actively engaged in the uptake of water and minerals o Many fine roots infected with mycorrhizal fungi -‐ In growing plant, balance maintained between total surface area available for manufacture of food and surface area available for absorption of water and minerals -‐ Growth of many roots is continuous process that stops only under such adverse conditions as drought and low temperatures -‐ Root cap: thimble-‐like mass of living parenchyma cells that protects apical meristem behind it and aids root in its penetration of the soil o As root grows longer, root cap pushed forward, large amounts of mucilage (highly hydrated polysaccharide) lubricates root during passage through soil o Border cells: root cap cells that are programmed to separate from the root cap and from each other as they reach the root cap periphery. As border cells are released, new cells are added to the root cap § Functions: protection of apical meristem from infection, maintenance of intimate root soil contact, mobilization of essential elements for uptake by roots, short-‐term protection from drying out (desiccation), and specific attraction/repulsion of bacteria; decrease frictional resistance for root o Rootcap senses, processes, and transmits signals to meristem and elongation region of root, controls direction § Central column of cells, columella, lateral rootcap -‐ Root apical meristem – region of actively dividing cells o Promeristem – least differentiated part of apical meristem; composed of initials and immediate derivatives o Two main types of apical organization: § “Closed type”: rootcap, vascular cylinder, cortex have own initials § “Open type”: all regions arise from one group of initials o Most cell divisions occur a short distance beyond quiescent initials § Quiescent center: relatively inactive region of apical meristem; does not include initials of rootcap o Region of cell division = apical meristem o Region of elongation – increase in length of root, only near root tip o Region of maturation – differentiation; primary tissues mature; root hairs produced o Processes overlap o Protophloem sieve tubes – first-‐formed primary phloem elements; reach maturity nearer root tip than protoxylem elements -‐ In most roots, vascular tissues form a solid cylinder, but many roots have pith or pithlike region in center -‐ The epidermis in young roots absorbs water and minerals o Root hairs uptake water and minerals; tubular extensions of epidermal cells, increase surface and absorptive area; generally short lived -‐ Cortex occupies greatest area of primary body o Cortical tissue contains numerous intercellular spaces – air spaces essential for aeration of root cells o Aerenchyma – term applied to parenchyma tissue with large and abundant intercellular spaces -‐ Endodermis characterized by presence of Casparian strips (wall thickening; bandlike portion of primary wall and middle lamella impregnated with suberin and sometimes lignin) in anticlinal walls o Suberin and lignin infiltrate spaces in wall usually occupied by water o Apoplastic movement of water and solutes across endodermis is blocked by strips -‐ In older roots where cortex is retained, suberin lamella is eventually deposited internally over all wall surfaces of endodermis. This is followed by deposition of cellulose, which may become lignified -‐ Passage cells: endodermal cells remaining thin-‐walled and retaining Casparian strips -‐ Roots of angiosperms have second compact layer of cells with Casparian strips called exodermis. Followed by deposition of suberin lamella and cellulosic layer; reduces water loss and provides defense against attack by microorganisms -‐ Vascular cylinder of root consists of primary vascular tissues and one or more layers of nonvascular cells that constitute the pericycle, completely surrounds vascular tissues o Pericycle originates from procambium; young root composed of parenchyma cells with primary walls, but may develop secondary walls o Lateral roots arise in pericycle o Center of vascular cylinder occupied by solid core of primary xylem from which projections extend toward pericycle o Number of ridges of primary xylem differs from species to species § Proto-‐primary xylem elements located next to pericyle, and tips called protoxylem poles. § Metaxylem is part of primary xylem that differentiates after protoxylem and occupies inner portions of ridges and center of vascular cylinder -‐ Roots of some angiosperms have pith or pithlike region -‐ Secondary growth in roots and stems consists of: o Secondary vascular tissues from a vascular cambium o Periderm (mostly cork tissue) from a cork cambium -‐ Formation of vascular cambium is initiated by divisions of procambial cells that remain meristematic between primary xylem and primary phloem in portions of root that are no longer elongating. o Pericycle cells divide; inner sister cells contribute to vascular cambium o Vascular cambium opposite phloem strands produce secondary xylem toward inside; primary phloem separated from primary xylem o Repeated divisions toward inside and outside; increases in width of secondary xylem and phloem, primary phloem almost obliterated o Periderm replaces epidermis; cork cambium arises in outer pericycle, producing cork toward outer surface and phelloderm toward inner surface o Cork, cork cambium, phelloderm make up periderm o Lenticels are regions of periderm that allow gas exchange between root and soil atmosphere -‐ At end of first year’s growth, these tissues are found in woody root (outside to inside): possible remnants of epidermis and cortex; periderm; pericycle; primary phloem (if present fibers), secondary phloem; vascular cambium; secondary xylem; primary xylem -‐ Endogenous: lateral roots originate deep within parent root -‐ Root primordium: young lateral root; pushes through cortex, secreting enzymes -‐ Prop roots: aerial roots of some plants used for support, produced aboveground; stem-‐borne -‐ Still roots: produced from stem and branches of many tropical trees -‐ Air roots, or pneumatophores: negative gravitropic extensions that grow out of mud and provide adequate aeration via numerous lenticels and broad cortex of aerenchyma tissue -‐ Most roots are storage organs; some are fleshy because of storage parenchyma (found in carrot, sweet potato, sugar beet)
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