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Roots
Chapter 24
Root functions
 Anchorage
 Absorption
 Storage
 Conduction
Root structure
 Primary root
– First root of the plant
– Also termed the taproot
 Taproot grows down and lateral roots or branching roots form
 When taproot is prominent, root system is termed a taproot system
– Common in eudicots
 Taproot can extend great distances
Root systems
 In monocots, primary root is short lived
 Main shoot system arises from the stem
 Forms a fibrous root system
 Generally are shallower than taproot systems
Root to shoot ratio (R:S)
 Plants allocate energy to roots or shoots depending on environmental conditions
 Root to shoot ratio is a common measure in plant ecology
 Root to shoot lower in high moisture environment than in low moisture environment
 Root to shoot ratio higher in high light environment compared to low light environment
Root growth
 Roots grow continuously from apical meristem
– Promeristem is most distal and least determined part of the apical meristem
 Root cap protects apical meristem
– Parenchyma cells
 Hydrated polysaccharide and sloughed root cells form slimy sheath over root cap,
called mucigel
Gravitropism
 Roots move towards gravity
 Perception to gravity is in central column of cells in the rootcap, called the columella
Root zones
 Region of actively dividing cells is called the region of cell division
 Region of elongation is small, but responsible for most of the increase in root length
 Region of maturation is where root hairs are produced
Primary structure
 Three tissues
– Epidermis
– Cortex
– Vascular
 Simple structure
 Tissues easily distinguishable
Primary structure
 Three tissues
– Epidermis
– Cortex
– Vascular
 Simple structure
 Tissues easily distinguishable
Pith in center
Root hairs
 Root hairs are tubular extensions of epidermis
 Greatly increase surface area of roots
 Important in uptake of water and minerals
 Mucigel provides intimate contact with environment
– Layer soil bound to roots with sloughed cells and microorganisms is called the
rhizosphere
Mycorrhizae
 Mutualistic relationship between plants and fungi occuring in many vascular plants
 Fungi increase plant’s ability to capture water and essential nutrients, especially
phosphorus
 Fungi provide protection against attack by pathogenic fungi and nematodes
 Fungi receive carbohydrates and vitamins
Endomycorrhizae
 Penetrate root cells
 Most common (80%)
 Fungus penetrates cells, forming branched structures called arbuscules
– Most exchange occurs in arbuscules
 In some cases, form terminal swellings called vesicles
– Storage?
 Also called vesicular-arbuscular, V/A or VAM
 Fungal hyphae extend out to soil
Ectomycorrhizae
 Surround rather than penetrate root cells
 Prevalent in some families: Fagaceae, Salicaceae, Pinaceae
 In conifers, Hartig net eventually surrounds many of the cortical and epidermal cells
 Causes roots to thicken rather than elongate
 Mantle is a sheath of hyphae that covers the root surface
Cortex
 Contains numerous intercellular spaces that provide aeration to roots
 Substances can move through apoplast (between cells) or symplast (through
plasmodesmata)
 Innermost layer of cortex is endodermis
– Characterized by presence of Casparian strips
– All materials must pass through protoplasts of endodermis (symplast)
Exodermis
 In many angiosperms, there is an exodermis, that also has Casparian strips
Vascular cylinder
 Primary vascular tissues and nonvascular pericycle
 Secondary roots arise from pericycle
 Phloem are found between ridges of xylem
 First xylem cells are near the pericycle and the are called protoxylem poles
 The metaxylem occurs on the inner portion of the vascular cylinders
Secondary growth
 Monocots do not undergo
 Some eudicots do not undergo
 Secondary growth happens after:
– Secondary vascular tissues are formed from vascular cambium
– Periderm is formed from cork cambium
Steps of secondary growth
 1. Vascular cambium initiated by divisions of procambium cells located between
primary phloem and xylem
 2. Pericycle cells opposite protoxylem poles also divide and inner cells contribute to
vascular cambium
Steps of secondary growth
 3. After these cell divisions, vascular cambium completely surrounds the xylem core
 4. Vascular cambium forms secondary xylem on inside and secondary phloem on
outside
Steps of secondary growth
 5. Periderm formation initiated after secondary vascular tissue is made by cell
division in pericycle
 6. Cork cambium forms from outer layer of pericycle
 7. Cork is produced on outside and phelloderm on inside (cork + cork cambium +
phelloderm = periderm)
Lateral roots
 Originate in pericycle
 Occurs in zone of maturation
 Root cap forms on lateral root tip
 Two vascular cylinders are joined later
Aerial roots
 Prop roots provide support
Air roots or Pneumatophores
 Provide oxygen to roots
Fleshy roots
 Carrot
 Sweet potato
 Beet