UT Arlington BIOL BIOL 3427 - ch7 (11 pages)

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ch7



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ch7

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Pages:
11
School:
University of Texas at Arlington
Course:
Biol Biol 3427 - Plant Science
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Chapter 7 ROOTS FUNCTIONS OF THE ROOT 1 2 3 4 Anchorage Absorption and conduction of water and minerals Storage of food Production of some hormones and secondary metabolites The cylindrical shape of the root allows all sides to have the same absorptive capacity The cylindrical shape also facilitates the penetration of the root through the soil EXTERNAL STRUCTURE OF THE ROOTS Taproot system Produced by gymnosperms eudicots and magnoliids The primary root originates in the embryo derived from the embryonic radicle It grows downward and is called then a taproot It produces lateral roots branch roots This system generally penetrates deep into the soil Fibrous root system It is found in monocots The primary root originates from the embryonic radicle It is short lived Adventitious roots develop from the stem This system is usually shallow Many eudicots are perennial and undergo secondary growth resulting in an increased quantity of functional tissue The taproot system allows an increase in the absorptive capacity of the roots to satisfy the demands of the above ground tissues Monocots do not have secondary growth and after their tracheary elements mature their conducting capacity cannot increase Extra leaves could not be supplied with water and minerals and the demands of the secondary growth of a taproot system could not be satisfied The fibrous root system is the most practical for monocots Some monocots add new tissue through the growth of stolons The stolons produce adventitious roots at regular intervals to supply the new growth with an adequate supply of water and nutrients Feeder roots are those laterals that are actively engaged in absorption of water and minerals Most of the absorption of water and minerals takes place in the upper 15 cm of soil the area richest in organic material Desert plants can have very long taproots that penetrate deep into the soil A mesquite bush near Tucson AZ had roots 53 3 m or 175 feet deep into the soil Acacia and Tamarix trees found in Egypt grow roots 30 m deep into the soil Plants maintain a balance between its shoot and root systems In young plants the absorbing surface of roots far exceeds that of photosynthesizing parts As the plant ages the root to shoot ratio decreases Damage to the roots decreases the shoot production Damage to the shoot decreases the availability of food and hormones for the roots STRUCTURE OF INDIVIDUAL ROOTS Root growth is a continuous process that stops only during very adverse conditions of low temperature and heavy drought Roots follow the path of least resistance through the soil The tip of the root is covered by a rootcap that produces mucigel The rootcap is a thimble like mass of living parenchyma cells that covers the apical meristem behind it and helps the root to penetrate the soil Behind the rootcap and root apical meristem is a zone of elongation behind it is the root hair zone or zone of maturation INTERNAL STRUCTURE OF ROOTS Root cap As the root grows it pushed the rootcap forward and the peripheral cells are sloughed off The sloughed off cells and the cap is covered with a slimy mucigel that lubricates the path of the root through the soil New cells produced by the apical meristem are added to the cap to replace the ones that have been sloughed off As cells are pushed closer to the edge of the cap their ER becomes less conspicuous starch grains are digested and the dictyosomes secrete copious amounts of mucigel by exocytosis the middle lamella breaks down and releases the cells The life of these root cap cells is estimated to be between 4 or 5 days The mucigel is a hydrated polysaccharide secreted by the rootcap cells The mucigel accumulates in the Golgi vesicles that release the mucigel into the cell wall and eventually leaks to the outside The rootcap also plays a role in gravitropism The central column of cells or columella contains starch grains or amyloplasts which probably act as gravity sensors Root apical meristem Meristematic initials are the dividing cells of the meristem Longitudinal files or lineages of cells emanate from the apical meristem of the root The meristematic initials and its derivatives are polyhedral cells with large nuclei and dense cytoplasm The activity of the meristematic initials decreases as the plant develops and divisions become infrequent This region is called the quiescent region of the meristem Cell divisions occur in cells a bordering the quiescent initials The quiescent region is the origin of initials The quiescent region helps to repopulate the bordering meristematic regions surrounding it in case of injury The quiescent center plays an essential role in organization and development of roots Zone of elongation The region of cell elongation is immediately behind the region of cell division It usually s a few millimeters in length The elongation of cells results in most of the increase in root length There is no elongation beyond this area Only one small portion of the root is pushed through the soil at a given time Differentiation begins in this region The tissues in this zone are quite permeable Minerals penetrate deep into the root through the apoplast simply by diffusing along the thin fully hydrated well walls and intercellular spaces Zone of Maturation In the region of maturation or differentiation is where most of the primary tissues mature Root hairs are produced here This region is also called the root hair zone There is gradual transition from one zone to the next Some cells may begin to elongate and differentiate in the region of cell division whereas others do it the region of elongation The protoderm ground meristem and procambium can be distinguished in very close proximity to the apical meristem These will differentiate into epidermis cortex and primary vascular tissues respectively PRIMARY STRUCTURE There are three tissue systems in the root epidermis dermal tissue system cortex ground tissue system and vascular bundle vascular tissue system Epidermis Absorption of water and minerals is facilitated by root hairs A 4 month old rye plant was estimated to have 14 billion root hairs with an absorbing surface of 401 m2 and an end to end length of over 10 000 km Root hairs are short lived They are found in the region of maturation Production of root hairs occurs in the region of elongation at a rate that matches the rate at which the older root hairs are dying off at the upper end of the of the root hair zone The walls of the epidermal cells offer little


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