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by bwhitt

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How Roots Develop
Radicle develops into the first root.
Two development patterns taproot system
One thick central root
–Dicotyledonous Plants Adventitious roots
–that develop a fibrous root system.
Many roots of same size -Monocotyledonous Plants
Root Structure
Root Cap
parenchyma cells covering each root tip.
Protects tissue from damage.
Function in gravity perception.
Region of Cell Division
apical meristem
Region of Elongation
Cells increase size
Vacuoles merge
Region of Maturation
Cell (tissue) differentiation
Root hairs form.
–Absorb water and minerals –adhere tightly to soil particles.
–(Thin cuticle)
Tissues in Mature Roots
Four noted:
-Cortex Cell
-Vascular Cylinder

-Growth of these may be determinate (stops at certain point) or indeterminate (new tissue added indefinitely).
Tissues in Mature Roots: Epidermis
-contains the root hairs
Tissues in Mature Roots: Cortex Cell
-mostly stores food
-Parenchyma cells
Tissues in Mature Cells: Endodermis
-Casparian Strip
-walls impregnated with suberin (water proofing)
-substances must enter and leave vascular cylinder through cytoplasm
-Rare in stems
Tissues in Mature Roots: Vascular Cylinder
-Pericycle lies directly against the inner boundary of the endodermis.
-lateral roots
-vascular cambium (secondary growth)
Specialized Roots: Food Storage Roots
-Enlarged to store carbohydrates
-Parenchyma cells
- ex: Sweet Potatoes
Specialized Roots: Water Storage Roots
-Pumpkin Family
Specialized Roots: Propagative Roots
-Adventitious Buds develop into suckers
-ex: Fruit Trees
Specialized Roots: Pneumatophores
-Spongy roots that extend above the water's surface and enhance gas exchange between the atmosphere and subsurface roots.
Specialized Roots: Aerial Roots
-Prop roots
-Photosynthetic roots
Specialized Roots: Contractile Roots
-Pull plant deeper into the soil
-ex: Lilly Bulbs
Specialized Roots: Buttress Roots
-ex: Tropical Trees
Specialized Roots: Parasitic Roots
-their plants have no chlorophyll
-dependent on chlorophyll-bearing plants for nutrition
-ex: Dodder
-Fungi 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.
Formed through the interaction of:
parent material
living organisms.
Solid portion are minerals and organic matter.
spaces occur between solid particles.
Filled with air or water.
Divided into soil horizons
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.
Influences on soil: Climate
–little weathering
low rainfall.
moderate rainfall
–well-developed soils.
excessive rain
nutrients leached
Influence on soil: Living Organisms
-Living organisms constitute about 1/1000th of the total soil weight
-Bacteria and fungi decompose organic material.
-Humus: partially decomposed organic matter
Influences on soil: Topography
-Steep areas may erode via wind or water
-Flat areas may be flooded, and thus contain little available oxygen.
Influences on soil: Soil texture and Composition
-Best agricultural loams are composed of 40% silt, 40% sand, and 20% clay.
-Coarse soilds grain water too quickly
-Dense soils have poor drainage
Influence on soil: Soil Structure
-Arrangement of soil particles in 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
Field Capactiy
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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