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