1. The Root System
Chapter 7

2. Functions of Roots Absorption of water and minerals
Anchor plant in soil or to a surface
Storage of foods
Conduction of food and water from soil and from storage reserves into the shoot

3. Root System Becomes more complex as plant grows
Balance between shoot and root systems
Root system – supplies shoot with enough water and minerals
Shoot system – manufactures enough food for root system

4. Root System Rhizosphere Competition among plants
Contact zone between root surface and soil
Few mm thick
Bacteria and fungi nearer roots richer and more diverse than in soil farther away
Competition among plants
Competition for water, minerals, light energy by plants in close proximity to each other
Reduce competition by using different parts of soil

5. Types of Root Systems Type of System Description Examples
Fibrous root system
Several main roots that branch to form dense mass of roots
Corn (Zea mays)
Rye (Elymus cereale)
Tap root system
Large storage root
Carrots (Daucus carota)

6. Different Types of Roots
Radicle
Embryonic root
Forms primary root
Tap root system
Develops from one primary root that forms lateral roots

7. Different Types of Roots
Grasses
Seminal roots
Embryonic roots just above radicle
Emerge after radicle
Radicle and seminal roots form fibrous root system

8. Different Types of Roots
Adventitious roots
Originate on leaves and stems
Examples
Prop roots
Develop on stem just above soil
Absorb water and minerals
Support plant in soil
Example: corn
Aerial roots
Pneumatophores

9. Different Types of Roots
Aerial roots
Formed by branches of tree
Absorb water and minerals
Most important function is to hold up tree
Example: banyan tree
Pneumatophores
Small, adventitious roots
Stick up from mud
Absorb oxygen to supply submerged roots
Example: mangrove tree

10. Root Propagation
Place stem of plants such as blackberry or willow in moist soil to induce root formation
Leaves of plants such as Begonia can be rooted by soaking them in water

11. Root Tip Organized into three regions Root apical meristem (RAM)
Region of elongation
Region of maturation

12. Root Tip Root cap Protects RAM Site of gravity perception
Cells constantly sloughed off and replaced
Sloughed cells called border cells
Border cells remain alive in soil for short period of time
Provide nutrients for soil bacteria and fungi in rhizosphere

13. Root Tip RAM Regularly shaped cells Organized into two areas
Quiescent center (QC)
Small, centrally located portion
Slow rate of cell division
Seems to be activated in times of stress
May be site where hormones controlling root development are synthesized
Cells apical to QC
Divide to produce cells that form root cap

14. Root Tip Region of elongation Region of maturation Cells elongate
Site of root hair formation
Site of maturation of other cell types

15. Primary Meristems of Root
RAM differentiates into
Protoderm
Ground meristem
procambium

16. Summary of Tissues and Meristems in Roots

17. Epidermis Formed by differentiation of protoderm cells Root hairs
Epidermal cell extension into surrounding soil
Thin cell walls composed of pectin and cellulose
Sticky
soil particles cling to root hairs
Short life
Function only a few days or weeks

18. Epidermis Root hairs Not formed by all plants
Some gymnosperms (Example: firs)
Many aquatic plants
Usually one cell layer thick
In aerial roots of plants like orchids can be multilayered

19. Cortex Derived from ground meristem
Mainly composed of parenchyma cells
Endodermis
Innermost layer of cortex

20. Cortex Casparian strip
Waxy material embedded in upper, lower, and side walls of endodermal cells
Function
Guarantee minerals that reach vascular cylinder do so by first passing through at least one plasma membrane
Provides mechanism to increase concentration of needed minerals through pumps in endodermis cell membrane

21. Cortex
Exodermis
Present in many grass roots and aerial roots of orchids
Occurs at outer layer of cortex
Functions to regulate ion absorption and accumulation

22. Vascular Cylinder
Composed of tissue differentiated from procambium cells
Dicot roots
Central core of primary xylem elements organized into two or more radiating points
Monocot roots
Center of root composed of parenchyma (sometimes called pith)
Primary xylem and phloem form in a ring

23. Vascular Cylinder Protoxylem First xylem element to mature
Develops at outer points of xylem
Transports water while root is elongating
Secondary cell wall in shape of annular rings or spirals
Provides both strength and elasticity

24. Vascular Cylinder Metaxylem Last primary xylem to mature
Differentiates in center of vascular cylinder
Mature in regions of root where elongation is completed
Forms thick secondary cell walls with pits for lateral exchange of water and nutrients

25. Vascular Cylinder Xylem cell types in roots Vessel elements Tracheids
Parenchyma
Fibers

26. Vascular Cylinder Phloem Forms in areas between protoxylem arms
Protophloem
First part of vascular cylinder to become functional
Cells form at periphery of phloem
Function mainly during root elongation
Metaphloem
Develops toward inside
Functions during plant’s adult life

27. Vascular Cylinder Phloem cell types Parenchyma Fibers
Sieve-tube members
Companion cells

28. Vascular Cylinder Pericycle Outermost boundary of vascular cylinder
Remains capable of dividing for long period of time
Functions in dicot roots
Site where lateral root development is initiated
Contributes to formation of vascular cylinder
Contributes to formation of cork cambium

29. Vascular Cylinder Pericycle Function in monocot roots
Lateral root initiation

30. Lateral Roots
Chemical regulators cause pericycle cells to begin dividing at specific sites
Lateral root primordia result
Lateral root pushes through and destroys cortical cells and outer epidermis
Digestive enzymes released from lateral root primordia probably aid in process
Emerging lateral root organized into root cap and RAM

31. Lateral Roots
Site where lateral root emerges is healed by secretion of mucilage and waxy substances from cortical cells
Vascular system of lateral root connected to vascular system of main root axis

32. Secondary Vascular Tissues
Formed by activation of vascular cambium
Initiated by activation of pericycle cells and residual procambium cells located between arcs of xylem and phloem
Forms secondary xylem toward inside
Forms secondary phloem toward outside

33. Secondary Vascular Tissues
Secondary xylem
continued growth expands root
Causes splitting, sloughing off, destruction of cortex and epidermis
Pressure stimulates pericycle to divide again
Converts secondary xylem into cork cambium which forms periderm

34. Secondary Vascular Tissues
Secondary xylem
Bark on woody roots
Includes all cells from vascular cambium outward
Appears similar to bark in stems
Has thinner, smoother outer surface
Only monocot plant with secondary growth in roots is dragon’s blood tree (Dracaena draco)

35. Special Functions of Roots
Haustorial roots
Parasitic roots
Grow into vascular tissue of host stem
Obtain water and nutrients from host

36. Special Functions of Roots
Root nodules
Formed by leguminous plants
Bacteria associated with nodules fix atmospheric nitrogen to form usable by plants

37. Special Functions of Roots
Mycorrhizae
Association between roots and a fungus found in the soil
Short, forked roots
Common to about 90% of seed plants
Not essential for growth of host plant
Functions
Make roots more efficient in mineral absorption
May secrete hormones or antibiotics that reduce potential for plant diseases

38. Special Functions of Roots
Mycorrhizae
Two types
Ectotrophic mycorrhizae
Causes changes in root shape
Fungus does not penetrate root cells
Penetrates between cell walls of cortex forming sheath of fungal hyphae around root
These mycorrhizal roots lack root cap and have simple vascular cylinder
Endotrophic mycorrhizae

39. Special Functions of Roots
Endotrophic mycorrhizae
Fungus enters cortex cells

40. Special Functions of Roots
Dandelion roots
Capable of contracting
Accomplished by radial expansion of cells in root cortex
Keeps aboveground parts near soil surface