the root: structure and development
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The Root: Structure and Development. Chapter 24. Roots. Root - the descending axis of a plant, normally occurs below ground. Root Functions Anchorage. Absorption of water, minerals, inorganic ions. Storage. Conduction. Two types of root systems - PowerPoint PPT PresentationTRANSCRIPT
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The Root: Structure and Development
Chapter 24
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Roots• Root- the descending axis of a plant,
normally occurs below ground.• Root Functions
– Anchorage.– Absorption of water, minerals, inorganic
ions.– Storage.– Conduction.
• Two types of root systems– Taproot- a stout, tapering main root from
which smaller lateral roots arise; deep; gymnosperms & dicots.
– Fibrous- arise from stem similar to lateral roots; shallow; monocots.
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Root Penetration
• Root depth and distance depend on soil:– Moisture
– Temperature
– Composition
• Feeder Roots- those involved in uptake of water and minerals, occur usually in the upper 1 meter of the soil.
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• Rootcap- a thimblelike mass of parenchyma cells that covers the root and secretes mucigel.
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• The epidermis in young roots absorbs water and minerals.
• Root Hairs- tubular extensions of epidermis that facilitate absorption.
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Primary Development of the Root Tip
• Region of cell division – ~1 mm in length.– Apical meristem.
• Region of cell elongation– ~2 mm in length.– Cells elongate, functional
xylem starts to develop and phloem is mature.
• Region of maturation– ~2 mm-?? mm in length.– Root hairs are produced,
functional xylem and phloem are present.
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Eudicot versus Monocot Roots
• Eudicots– Vascular cylinder solid.– Cortical cells sometimes
retained for life, but some shed due to secondary growth.
• Monocots– Vascular cylinder in ring with
pith.– Cortical cells retained for life,
because they lack secondary growth.
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Cortex and Vascular Cylinder• Cortex- ground tissue (cells between the vascular cylinder and the
epidermis) occupies the greatest area of the root.• Vascular cylinder- xylem, phloem, and pericycle.• Pericycle- 1 or more layers of nonvascular cells surrounding the
vascular tissue of the root.• Intercellular spaces- empty spaces among cortical cells; essential for
aeration of the root cells.• Epidermis- the outermost layer of cells (the root).• Exodermis- the outermost layer of the cortex, 1 or more cell layers in
depth; casparian strip in walls.• Endodermis- the innermost layer of the cortex, lacks intercellular
spaces; casparian strip in walls.
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Casparian Strip
• Casparian Strip- a bandlike region of the primary cell wall containing suberin and lignin. Impermeable to water and ions, prevents water loss and attack by microorganisms.
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Lateral Roots Arise from the Pericycle
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Effects of Secondary Growth on the Primary Body of the Root
• Secondary Growth in roots consists of:– 1) Secondary vascular tissues (secondary xylem
and phloem) from vascular cambium.– 2) Periderm, mostly cork tissue from cork
cambium.
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Root Modifications
• Aerial Roots- roots produced from above ground structures.
• Air Roots- negatively gravitropic roots that aerate the root system.
• Fleshy Roots- specialized storage organs consisting of parenchyma and vascular tissue. Carrot, sweet potato, sugarbeet.
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Fleshy Root-an adaptation for food storage.
Sugarbeet
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Sweet Potato
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