Roots and Soil

In this chapter you will learn about roots, beginning with the functions, and continuing with the development of roots from a seed. You will learn about the function and structure of the root cap, region of cell division, region of elongation, and region of maturation (with its tissues). The endodermis and pericycle are also discussed.  Specialized roots (food-storage roots, water-storage roots, propagative roots, pneumatophores, aerial roots, contractile roots, buttress roots, parasitic roots, mycorrhizae) are given brief treatment. This is followed by some observations on the economic importance of roots. After a brief examination of soil horizons, the chapter continues with a discussion of the development of soil, its texture, composition, structure, and water holding capacity. (Slide 29)

At the end of this chapter the successful student will be able to

• List and describe the function of roots
• Describe how roots develop
• Identify and describe regions of the roots. 
• Identify the cross section regions of a root monocot and a root dicot
• List and describe the function of specialized roots
• List at least 5 practical human uses for roots
• Describe the development of good agricultural soil form raw materials
• Describe various forms of soil particles and soil waters in terms of specific location and availability to plants.
• Assignments
• Glossary

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List and describe the function of roots

• Anchorage
• Absorption
• Storage of food and water

See text p.63

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Describe how roots develop

• When a seed germinates, the embryo’s radicle grows out and develops into the first root.
  • May develop into thick taproot with branch roots.
    • Dicotyledonous Plants
  • May develop adventitious roots that develop a fibrous root system.
    • Monocotyledonous Plants

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Identify and describe regions of the roots

• Root Cap - Thimble-shaped mass of parenchyma cells covering each root tip.
  • Protects tissue from damage.
  • Function in gravity perception.
• Region of Cell Division - Composed of apical meristem in the center of the root tip.
  • Most cell division occurs at the edge of the inverted cup-shaped zone.
• 
• Region of Elongation - Cells become several times their original length.
  • Vacuoles merge
• Region of Maturation - Most cells differentiate into various distinctive cell types.
  • Root hairs form.
    • Absorb water and minerals and adhere tightly to soil particles.
      • Thin cuticle
  • 
  • Cortex cells mostly store food.
    • Contain endodermis
      • Cell walls impregnated with suberin bands, Casparian Strips.
        • Forces all water and dissolved substances entering and leaving the central core to pass through plasma membranes of the endodermal cells.
  • Vascular Cylinder lies at the inside of the endodermis.
  • Pericycle lies directly against the inner boundary of the endodermis.
    • Lateral Roots
  • In both roots and stems, growth may be determinate (stops at a certain size) or indeterminate (new tissues added indefinitely).

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Identify the cross section regions of a root monocot and a root dicot

Monocot:

Dicot

If you want to know more about the differences between monocot and dicot plants, you should look at the materials at:  http://www2.volstate.edu/msd/BIO/1020/lab11seedplants.htm

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List and describe the function of specialized roots

• Food Storage Roots
  • Sweet Potatoes
• Water Storage Roots
  • Pumpkin Family
• Propagative Roots
  • Adventitious Buds develop into suckers.
    • Fruit Trees
• Pneumatophores
  • Spongy roots that extend above the water’s surface and enhance gas exchange between the atmosphere and subsurface roots.
• Aerial Roots
  • Orchids
• Contractile Roots
  • Pull plant deeper into the soil.
    • Lilly Bulbs.
• Buttress Roots
  • Stability - Tropical Trees.
• Parasitic Roots
  • Have no chlorophyll and are dependent on chlorophyll-bearing plants for nutrition.
    • Dodder
• Mycorrhizae form a mutualistic association with plant roots.
  Fungus is able to absorb and concentrate phosphorus much better than it can be absorbed by the root hairs.

  Particularly susceptible to acid rain.
• 
• Root Nodules:  Few species of bacteria produce enzymes that can convert nitrogen into nitrates and other nitrogenous substances readily absorbed by roots.
  Legume Family (Fabaceae)

  Root nodules contain large numbers of nitrogen-fixing bacteria.
  • This image is from http://distans.livstek.lth.se:2080/rootnodules.htm.  Click the image if you would like to see  more images and learn more about root nodules.

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List at least 5 practical human uses for roots

• Food storage
  • sugar, beets, turnip, rutabagas, parsnip, radish, carrot
• Spices
  • Sassafras, sarsaparilla, licorice,
• Dyes
  • reds, browns, coffee bean
• Drugs
  • gentian, reserpine (tranquilizer)
• Insecticide
  • rotenone

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Describe the development of good agricultural soil form raw materials

• Soil is formed through the interaction of climate, parent material, topography, vegetation, and living organisms.
  • Solid portion of soil consists of minerals and organic matter.
  • Pore spaces occur between solid particles.
    • Filled with air or water.
  • Divided into soil horizons
• A Horizon - Topsoil
  • Dark, rich soil
• B Horizon - Subsoil
  • More clay, lighter in color
• C Horizon - Parent Material
  • Not broken down into smaller particles.
• Climate
  • Deserts experience little weathering due to low rainfall.
  • Grasslands have moderate rainfall and well-developed soils.
  • Rainforests have excessive rain and nutrients are quickly leached from the soil.
• Living Organisms and Organic Composition
  • In upper 30 cm of a good agricultural soil, living organisms constitute about one-thousandth of the total soil weight.
  • Bacteria and fungi in the soil decompose organic material.
    • Humus, partially decomposed organic matter, gives soil a dark color.
• Topography
  • Steep areas may erode via wind or water.
  • Flat areas may be flooded, and thus contain little available oxygen.
• Soil Texture and Composition
  • Best agricultural loams are composed of 40% silt, 40% sand and 20% clay.
    • Coarse soils drain water too quickly
    • Dense soils have poor drainage.

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Various forms of soil particles and soil waters

• Soil Structure
  • Arrangement of soil particles into aggregates.
    • Productive agricultural soils are granular with pore spaces occupying between 40-60% of the total soil volume.
      • Particle size is more important than total volume.

• Soil Mineral Components
  Stones	> 76 mm
  Gravel	76 mm - 2.0 mm
  Very Coarse Sand	2.0 mm - 1.0 mm
  Coarse Sand	1.0 mm - 0.5 mm
  Medium Sand	0.5 mm - 0.25 mm
  Fine Sand	0.25 mm - 0.10 mm
  Very Fine Sand	0.10 mm - 0.05 mm
  Silt	0.05 mm - 0.002 mm
  Clay	< 0.002 mm

• Soil Water
  • Hygroscopic Water - Physically bound to soil particles and is unavailable to plants.
  • Gravitational Water - Drains out of pore spaces after a rain.
  • Capillary Water - Water held against the force of gravity in soil pores
• Field Capacity - Water remaining in the soil after drainage by gravity.
• Permanent Wilting Point - Rate of water absorption insufficient for plant needs.
• Available Water - Soil water between field capacity and the permanent wilting point.
• Soil pH
  Alkalinity causes some minerals to become less available.

  Add nitrogenous fertilizers.
  Acidity may inhibit growth of nitrogen-fixing bacteria.

  Add calcium or magnesium compounds.

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Last modified: October 08, 2004 by Cynthia Herbrandson  © Copyright 1999, Kellogg Community College. All rights reserved.